diff --git a/sky130/Makefile.in b/sky130/Makefile.in
index b4f0bd6..9fabc6c 100644
--- a/sky130/Makefile.in
+++ b/sky130/Makefile.in
@@ -313,6 +313,7 @@
 	(cd ${MAGICTOP_STAGING_A} ; ln -s ${REV_DIR} current)
 	cp -rp custom/scripts/seal_ring_generator ${MAGIC_STAGING_A}/.
 	cp custom/scripts/generate_fill.py ${MAGIC_STAGING_A}/.
+	cp custom/scripts/check_density.py ${MAGIC_STAGING_A}/.
 	${CPP} ${SKY130A_DEFS} magic/${TECH}.tech > ${MAGIC_STAGING_A}/${SKY130A}.tech
 	${CPP} ${SKY130A_DEFS} magic/${TECH}gds.tech > ${MAGIC_STAGING_A}/${SKY130A}-GDS.tech
 	${CPP} ${SKY130A_DEFS} magic/${TECH}.magicrc > ${MAGIC_STAGING_A}/${SKY130A}.magicrc
diff --git a/sky130/custom/scripts/check_density.py b/sky130/custom/scripts/check_density.py
new file mode 100755
index 0000000..0df2423
--- /dev/null
+++ b/sky130/custom/scripts/check_density.py
@@ -0,0 +1,516 @@
+#!/usr/bin/env python3
+# SPDX-FileCopyrightText: 2020 Efabless Corporation
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# SPDX-License-Identifier: Apache-2.0
+
+#
+# check_density.py ---
+#
+#    Run density checks on a design (from GDS, after running fill generation).
+#
+
+import sys
+import os
+import re
+import select
+import subprocess
+
+def usage():
+    print("Usage:")
+    print("check_density.py [<gds_file_name>] [-keep]")
+    print("")
+    print("where:")
+    print("   <gds_file_name> is the path to the .gds file to be checked.")
+    print("")
+    print("  If '-keep' is specified, then keep the check script.")
+    return 0
+
+
+if __name__ == '__main__':
+
+    optionlist = []
+    arguments = []
+
+    debugmode = False
+    keepmode = False
+
+    for option in sys.argv[1:]:
+        if option.find('-', 0) == 0:
+            optionlist.append(option)
+        else:
+            arguments.append(option)
+
+    if len(arguments) != 1:
+        print("Wrong number of arguments given to check_density.py.")
+        usage()
+        sys.exit(0)
+
+    gds_filepath = arguments[0]
+
+    gdspath = os.path.split(gds_filepath)[0]
+    if gdspath == '':
+        gdspath = os.getcwd()
+
+    if os.path.splitext(gds_filepath)[1] != '.gds':
+        if os.path.splitext(gds_filepath)[1] == '':
+            gds_filepath += '.gds'
+        else:
+            print('Error:  Project is not a GDS file!')
+            sys.exit(1)
+
+    # Check for valid path to the GDS file
+
+    if not os.path.isdir(gdspath):
+        print('Error:  Project path "' + gds_filepath + '" does not exist or is not readable.')
+        sys.exit(1)
+
+    # Check for valid path to the layout directory (NOTE:  Should check for
+    # additional argument or open_pdks install if not in a standard project space;
+    # this needs to be done.)
+
+    gdsroot = os.path.split(gdspath)[0]
+    magpath = gdsroot + '/mag'
+    if not os.path.isdir(magpath):
+        print('Error:  Layout path "' + magpath + '" does not exist or is not readable.')
+        sys.exit(1)
+
+    if '-debug' in optionlist:
+        debugmode = True
+    if '-keep' in optionlist:
+        keepmode = True
+
+    # NOTE:  There should be some attempt to find the installed PDK magicrc file
+    # if there is no mag/ directory.
+    rcfile = magpath + '/.magicrc'
+
+    with open(magpath + '/check_density.tcl', 'w') as ofile:
+        print('#!/bin/env wish', file=ofile)
+        print('crashbackups stop', file=ofile)
+        print('drc off', file=ofile)
+        print('snap internal', file=ofile)
+
+        print('set starttime [orig_clock format [orig_clock seconds] -format "%D %T"]', file=ofile)
+        print('puts stdout "Started reading GDS: $starttime"', file=ofile)
+        print('', file=ofile)
+        print('flush stdout', file=ofile)
+        print('update idletasks', file=ofile)
+
+        # Read GDS file
+        print('gds readonly true', file=ofile)
+        print('gds rescale false', file=ofile)
+        print('gds read ' + gds_filepath, file=ofile)
+        print('', file=ofile)
+
+        print('set midtime [orig_clock format [orig_clock seconds] -format "%D %T"]', file=ofile)
+        print('puts stdout "Starting density checks: $midtime"', file=ofile)
+        print('', file=ofile)
+        print('flush stdout', file=ofile)
+        print('update idletasks', file=ofile)
+
+        # Get step box dimensions (700um for size and 70um for step)
+        print('box values 0 0 0 0', file=ofile)
+        # print('box size 700um 700um', file=ofile)
+        # print('set stepbox [box values]', file=ofile)
+        # print('set stepwidth [lindex $stepbox 2]', file=ofile)
+        # print('set stepheight [lindex $stepbox 3]', file=ofile)
+
+        print('box size 70um 70um', file=ofile)
+        print('set stepbox [box values]', file=ofile)
+        print('set stepsizex [lindex $stepbox 2]', file=ofile)
+        print('set stepsizey [lindex $stepbox 3]', file=ofile)
+
+        print('select top cell', file=ofile)
+        print('expand', file=ofile)
+        print('set fullbox [box values]', file=ofile)
+        print('set xmax [lindex $fullbox 2]', file=ofile)
+        print('set xmin [lindex $fullbox 0]', file=ofile)
+        print('set fullwidth [expr {$xmax - $xmin}]', file=ofile)
+        print('set xtiles [expr {int(ceil(($fullwidth + 0.0) / $stepsizex))}]', file=ofile)
+        print('set ymax [lindex $fullbox 3]', file=ofile)
+        print('set ymin [lindex $fullbox 1]', file=ofile)
+        print('set fullheight [expr {$ymax - $ymin}]', file=ofile)
+        print('set ytiles [expr {int(ceil(($fullheight + 0.0) / $stepsizey))}]', file=ofile)
+        print('box size $stepsizex $stepsizey', file=ofile)
+        print('set xbase [lindex $fullbox 0]', file=ofile)
+        print('set ybase [lindex $fullbox 1]', file=ofile)
+        print('', file=ofile)
+
+        print('puts stdout "XTILES: $xtiles"', file=ofile)
+        print('puts stdout "YTILES: $ytiles"', file=ofile)
+        print('', file=ofile)
+
+        # Need to know what fraction of a full tile is the last row and column
+        print('set xfrac [expr {($xtiles * $stepsizex - $fullwidth + 0.0) / $stepsizex}]', file=ofile)
+        print('set yfrac [expr {($ytiles * $stepsizey - $fullheight + 0.0) / $stepsizey}]', file=ofile)
+        print('puts stdout "XFRAC: $xfrac"', file=ofile)
+        print('puts stdout "YFRAC: $yfrac"', file=ofile)
+
+        print('cif ostyle density', file=ofile)
+
+        # Process density at steps.  For efficiency, this is done in 70x70 um
+        # areas, dumped to a file, and then aggregated into the 700x700 areas.
+
+        print('for {set y 0} {$y < $ytiles} {incr y} {', file=ofile)
+        print('    for {set x 0} {$x < $xtiles} {incr x} {', file=ofile)
+        print('        set xlo [expr $xbase + $x * $stepsizex]', file=ofile)
+        print('        set ylo [expr $ybase + $y * $stepsizey]', file=ofile)
+        print('        set xhi [expr $xlo + $stepsizex]', file=ofile)
+        print('        set yhi [expr $ylo + $stepsizey]', file=ofile)
+        print('        box values $xlo $ylo $xhi $yhi', file=ofile)
+
+        # Flatten this area
+        print('        flatten -dobbox -nolabels tile', file=ofile)
+        print('        load tile', file=ofile)
+        print('        select top cell', file=ofile)
+
+        # Run density check for each layer
+        print('        puts stdout "Density results for tile x=$x y=$y"', file=ofile)
+
+        print('        set fdens  [cif list cover fom_all]', file=ofile)
+        print('        set pdens  [cif list cover poly_all]', file=ofile)
+        print('        set ldens  [cif list cover li_all]', file=ofile)
+        print('        set m1dens [cif list cover m1_all]', file=ofile)
+        print('        set m2dens [cif list cover m2_all]', file=ofile)
+        print('        set m3dens [cif list cover m3_all]', file=ofile)
+        print('        set m4dens [cif list cover m4_all]', file=ofile)
+        print('        set m5dens [cif list cover m5_all]', file=ofile)
+        print('        puts stdout "FOM: $fdens"', file=ofile)
+        print('        puts stdout "POLY: $pdens"', file=ofile)
+        print('        puts stdout "LI1: $ldens"', file=ofile)
+        print('        puts stdout "MET1: $m1dens"', file=ofile)
+        print('        puts stdout "MET2: $m2dens"', file=ofile)
+        print('        puts stdout "MET3: $m3dens"', file=ofile)
+        print('        puts stdout "MET4: $m4dens"', file=ofile)
+        print('        puts stdout "MET5: $m5dens"', file=ofile)
+        print('        flush stdout', file=ofile)
+        print('        update idletasks', file=ofile)
+
+        print('        load ' + project_with_id, file=ofile)
+        print('        cellname delete tile', file=ofile)
+
+        print('    }', file=ofile)
+        print('}', file=ofile)
+
+        print('set endtime [orig_clock format [orig_clock seconds] -format "%D %T"]', file=ofile)
+        print('puts stdout "Ended: $endtime"', file=ofile)
+        print('', file=ofile)
+
+
+    myenv = os.environ.copy()
+    myenv['MAGTYPE'] = 'mag'
+
+    print('Running density checks on file ' + gds_filepath, flush=True)
+
+    mproc = subprocess.Popen(['magic', '-dnull', '-noconsole',
+		'-rcfile', rcfile, magpath + '/check_density.tcl'],
+		stdin = subprocess.DEVNULL,
+		stdout = subprocess.PIPE,
+		stderr = subprocess.PIPE,
+		cwd = magpath,
+		env = myenv,
+		universal_newlines = True)
+
+    # Use signal to poll the process and generate any output as it arrives
+
+    fomfill  = []
+    polyfill = []
+    lifill   = []
+    met1fill = []
+    met2fill = []
+    met3fill = []
+    met4fill = []
+    met5fill = []
+    xtiles = 0
+    ytiles = 0
+    xfrac = 0.0
+    yfrac = 0.0
+
+    while mproc:
+        status = mproc.poll()
+        if status != None:
+            try:
+                output = mproc.communicate(timeout=1)
+            except ValueError:
+                print('Magic forced stop, status ' + str(status))
+                sys.exit(1)
+            else:
+                outlines = output[0]
+                errlines = output[1]
+                for line in outlines.splitlines():
+                    print(line)
+                    dpair = line.split(':')
+                    if len(dpair) == 2:
+                        layer = dpair[0]
+                        try:
+                            density = float(dpair[1].strip())
+                        except:
+                            continue
+                        if layer == 'FOM':
+                            fomfill.append(density)
+                        elif layer == 'POLY':
+                            polyfill.append(density)
+                        elif layer == 'LI1':
+                            lifill.append(density)
+                        elif layer == 'MET1':
+                            met1fill.append(density)
+                        elif layer == 'MET2':
+                            met2fill.append(density)
+                        elif layer == 'MET3':
+                            met3fill.append(density)
+                        elif layer == 'MET4':
+                            met4fill.append(density)
+                        elif layer == 'MET5':
+                            met5fill.append(density)
+                        elif layer == 'XTILES':
+                            xtiles = int(dpair[1].strip())
+                        elif layer == 'YTILES':
+                            ytiles = int(dpair[1].strip())
+                        elif layer == 'XFRAC':
+                            xfrac = float(dpair[1].strip())
+                        elif layer == 'YFRAC':
+                            yfrac = float(dpair[1].strip())
+
+                for line in errlines.splitlines():
+                    print(line)
+                print('Magic exited with status ' + str(status))
+                if int(status) != 0:
+                    sys.exit(int(status))
+                else:
+                    break
+        else:
+            n = 0
+            while True:
+                n += 1
+                if n > 100:
+                    n = 0
+                    status = mproc.poll()
+                    if status != None:
+                        break
+                sresult = select.select([mproc.stdout, mproc.stderr], [], [], 0)[0]
+                if mproc.stdout in sresult:
+                    outstring = mproc.stdout.readline().strip()
+                    print(outstring)
+                elif mproc.stderr in sresult:
+                    outstring = mproc.stderr.readline().strip()
+                    print(outstring)
+                else:
+                    break
+
+    total_tiles = (ytiles - 9) * (xtiles - 9)
+
+    print('')
+    print('Density results (total tiles = ' + str(total_tiles) + '):')
+
+    # Full areas are 10 x 10 tiles = 100.  But the right and top sides are
+    # not full tiles, so the full area must be prorated.
+
+    sideadjust = 90.0 + (10.0 * xfrac)
+    topadjust = 90.0 + (10.0 * yfrac)
+    corneradjust = 81.0 + (9.0 * xfrac) + (9.0 * yfrac) + (xfrac * yfrac)
+
+    print('')
+    print('FOM Density:')
+    for y in range(0, ytiles - 9):
+        if y == ytiles - 10:
+            atotal = topadjust
+        else:
+            atotal = 100.0
+        for x in range(0, xtiles - 9):
+            if x == xtiles - 10:
+                if y == ytiles - 10:
+                    atotal = corneradjust
+                else:
+                    atotal = sideadjust
+            fomaccum = 0
+            for w in range(y, y + 10):
+                base = xtiles * w + x
+                fomaccum += sum(fomfill[base : base + 10])
+                    
+            print('Tile (' + str(x) + ', ' + str(y) + '):   ' + str(fomaccum / atotal))
+            if fomaccum < 33.0:
+                print('***Error:  FOM Density < 33%')
+            elif fomaccum > 57.0:
+                print('***Error:  FOM Density > 57%')
+
+    print('')
+    print('POLY Density:')
+    for y in range(0, ytiles - 9):
+        if y == ytiles - 10:
+            atotal = topadjust
+        else:
+            atotal = 100.0
+        for x in range(0, xtiles - 9):
+            if x == xtiles - 10:
+                if y == ytiles - 10:
+                    atotal = corneradjust
+                else:
+                    atotal = sideadjust
+            polyaccum = 0
+            for w in range(y, y + 10):
+                base = xtiles * w + x
+                polyaccum += sum(polyfill[base : base + 10])
+                    
+            print('Tile (' + str(x) + ', ' + str(y) + '):   ' + str(polyaccum / atotal))
+
+    print('')
+    print('LI Density:')
+    for y in range(0, ytiles - 9):
+        if y == ytiles - 10:
+            atotal = topadjust
+        else:
+            atotal = 100.0
+        for x in range(0, xtiles - 9):
+            if x == xtiles - 10:
+                if y == ytiles - 10:
+                    atotal = corneradjust
+                else:
+                    atotal = sideadjust
+            liaccum = 0
+            for w in range(y, y + 10):
+                base = xtiles * w + x
+                liaccum += sum(lifill[base : base + 10])
+                    
+            print('Tile (' + str(x) + ', ' + str(y) + '):   ' + str(liaccum / atotal))
+            if liaccum < 35.0:
+                print('***Error:  LI Density < 35%')
+            elif liaccum > 70.0:
+                print('***Error:  LI Density > 70%')
+
+    print('')
+    print('MET1 Density:')
+    for y in range(0, ytiles - 9):
+        if y == ytiles - 10:
+            atotal = topadjust
+        else:
+            atotal = 100.0
+        for x in range(0, xtiles - 9):
+            if x == xtiles - 10:
+                if y == ytiles - 10:
+                    atotal = corneradjust
+                else:
+                    atotal = sideadjust
+            met1accum = 0
+            for w in range(y, y + 10):
+                base = xtiles * w + x
+                met1accum += sum(met1fill[base : base + 10])
+                    
+            print('Tile (' + str(x) + ', ' + str(y) + '):   ' + str(met1accum / atotal))
+            if met1accum < 35.0:
+                print('***Error:  MET1 Density < 35%')
+            elif met1accum > 70.0:
+                print('***Error:  MET1 Density > 70%')
+
+    print('')
+    print('MET2 Density:')
+    for y in range(0, ytiles - 9):
+        if y == ytiles - 10:
+            atotal = topadjust
+        else:
+            atotal = 100.0
+        for x in range(0, xtiles - 9):
+            if x == xtiles - 10:
+                if y == ytiles - 10:
+                    atotal = corneradjust
+                else:
+                    atotal = sideadjust
+            met2accum = 0
+            for w in range(y, y + 10):
+                base = xtiles * w + x
+                met2accum += sum(met2fill[base : base + 10])
+                    
+            print('Tile (' + str(x) + ', ' + str(y) + '):   ' + str(met2accum / atotal))
+            if met2accum < 35.0:
+                print('***Error:  MET2 Density < 35%')
+            elif met2accum > 70.0:
+                print('***Error:  MET2 Density > 70%')
+
+    print('')
+    print('MET3 Density:')
+    for y in range(0, ytiles - 9):
+        if y == ytiles - 10:
+            atotal = topadjust
+        else:
+            atotal = 100.0
+        for x in range(0, xtiles - 9):
+            if x == xtiles - 10:
+                if y == ytiles - 10:
+                    atotal = corneradjust
+                else:
+                    atotal = sideadjust
+            met3accum = 0
+            for w in range(y, y + 10):
+                base = xtiles * w + x
+                met3accum += sum(met3fill[base : base + 10])
+                    
+            print('Tile (' + str(x) + ', ' + str(y) + '):   ' + str(met3accum / atotal))
+            if met3accum < 35.0:
+                print('***Error:  MET3 Density < 35%')
+            elif met3accum > 70.0:
+                print('***Error:  MET3 Density > 70%')
+
+    print('')
+    print('MET4 Density:')
+    for y in range(0, ytiles - 9):
+        if y == ytiles - 10:
+            atotal = topadjust
+        else:
+            atotal = 100.0
+        for x in range(0, xtiles - 9):
+            if x == xtiles - 10:
+                if y == ytiles - 10:
+                    atotal = corneradjust
+                else:
+                    atotal = sideadjust
+            met4accum = 0
+            for w in range(y, y + 10):
+                base = xtiles * w + x
+                met4accum += sum(met4fill[base : base + 10])
+                    
+            print('Tile (' + str(x) + ', ' + str(y) + '):   ' + str(met4accum / atotal))
+            if met4accum < 35.0:
+                print('***Error:  MET4 Density < 35%')
+            elif met4accum > 70.0:
+                print('***Error:  MET4 Density > 70%')
+
+    print('')
+    print('MET5 Density:')
+    for y in range(0, ytiles - 9):
+        if y == ytiles - 10:
+            atotal = topadjust
+        else:
+            atotal = 100.0
+        for x in range(0, xtiles - 9):
+            if x == xtiles - 10:
+                if y == ytiles - 10:
+                    atotal = corneradjust
+                else:
+                    atotal = sideadjust
+            met5accum = 0
+            for w in range(y, y + 10):
+                base = xtiles * w + x
+                met5accum += sum(met5fill[base : base + 10])
+                    
+            print('Tile (' + str(x) + ', ' + str(y) + '):   ' + str(met5accum / atotal))
+            if met5accum < 45.0:
+                print('***Error:  MET5 Density < 45%')
+            elif met5accum > 86.0:
+                print('***Error:  MET5 Density > 80%')
+
+    if not keepmode:
+        os.remove(magpath + '/check_density.tcl')
+
+    print('')
+    print('Done!')
+    sys.exit(0)
+
diff --git a/sky130/custom/scripts/generate_fill.py b/sky130/custom/scripts/generate_fill.py
index cb481d9..1f9254f 100755
--- a/sky130/custom/scripts/generate_fill.py
+++ b/sky130/custom/scripts/generate_fill.py
@@ -113,11 +113,11 @@
         print('set xmax [lindex $fullbox 2]', file=ofile)
         print('set xmin [lindex $fullbox 0]', file=ofile)
         print('set fullwidth [expr {$xmax - $xmin}]', file=ofile)
-        print('set xtiles [expr {int(ceil($fullwidth / $stepwidth))}]', file=ofile)
+        print('set xtiles [expr {int(ceil(($fullwidth + 0.0) / $stepwidth))}]', file=ofile)
         print('set ymax [lindex $fullbox 3]', file=ofile)
         print('set ymin [lindex $fullbox 1]', file=ofile)
         print('set fullheight [expr {$ymax - $ymin}]', file=ofile)
-        print('set ytiles [expr {int(ceil($fullheight / $stepheight))}]', file=ofile)
+        print('set ytiles [expr {int(ceil(($fullheight + 0.0) / $stepheight))}]', file=ofile)
         print('box size $stepwidth $stepheight', file=ofile)
         print('set xbase [lindex $fullbox 0]', file=ofile)
         print('set ybase [lindex $fullbox 1]', file=ofile)
@@ -130,6 +130,8 @@
         print('        set ylo [expr $ybase + $y * $stepheight]', file=ofile)
         print('        set xhi [expr $xlo + $stepwidth]', file=ofile)
         print('        set yhi [expr $ylo + $stepheight]', file=ofile)
+        print('        if {$xhi > $fullwidth} {set xhi $fullwidth}', file=ofile)
+        print('        if {$yhi > $fullheight} {set yhi $fullheight}', file=ofile)
         print('        box values $xlo $ylo $xhi $yhi', file=ofile)
         # The flattened area must be larger than the fill tile by >1.5um
         print('        box grow c 1.6um', file=ofile)
@@ -145,6 +147,8 @@
         print('        property GDS_FILE ""', file=ofile)
         # Set boundary using comment layer, to the size of the step box
 	# This corresponds to the "topbox" rule in the wafflefill(tiled) style
+        print('        select top cell', file=ofile)
+        print('        erase comment', file=ofile)
         print('        box values $xlo $ylo $xhi $yhi', file=ofile)
         print('        paint comment', file=ofile)
         print('        puts stdout "Writing GDS. . . "', file=ofile)