diff --git a/CHANGELOG.md b/CHANGELOG.md
index ee662c9..9212725 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 3.1.31 Jan 13, 2020
+
+Added `sam-coverage-depth.py`.
+
 ## 3.1.30 Jan 13, 2020
 
 Added `--minGenomeLength` to `make-protein-database.py`.
diff --git a/bin/sam-coverage-depth.py b/bin/sam-coverage-depth.py
new file mode 100755
index 0000000..7516f61
--- /dev/null
+++ b/bin/sam-coverage-depth.py
@@ -0,0 +1,122 @@
+#!/usr/bin/env python
+
+from __future__ import print_function
+
+import sys
+import argparse
+from collections import Counter
+from numpy import std
+
+from dark.filter import (
+    addFASTAFilteringCommandLineOptions, parseFASTAFilteringCommandLineOptions)
+from dark.reads import Reads
+from dark.sam import samfile, SAMFilter, samReferences, UnknownReference
+from dark.utils import pct
+
+
+def baseCountsToStr(counts):
+    """
+    Convert base counts to a string.
+
+    @param counts: A C{Counter} instance.
+    @return: A C{str} representation of nucleotide counts.
+    """
+    return ' '.join([
+        ('%s:%d' % (base, counts[base])) for base in sorted(counts)])
+
+
+parser = argparse.ArgumentParser(
+    formatter_class=argparse.ArgumentDefaultsHelpFormatter,
+    description=('Print SAM/BAM file coverage statistics by offset. '
+                 'Output lines show the offset.'))
+
+addFASTAFilteringCommandLineOptions(parser)
+SAMFilter.addFilteringOptions(parser, samfileIsPositional=True)
+
+parser.add_argument(
+    '--noOffsets', default=False, action='store_true',
+    help='Do not print per-offset details of base counts.')
+
+parser.add_argument(
+    '--noStats', default=False, action='store_true',
+    help='Do not print final average and standard deviation statistics.')
+
+args = parser.parse_args()
+
+if args.noOffsets and args.noStats:
+    print('You have used both --noOffsets and --noStats, so there is no '
+          'output!', file=sys.stderr)
+    sys.exit(1)
+
+# We don't have a file of reads, we just want a read filter that we
+# can use to filter the SAM file query sequences.
+reads = parseFASTAFilteringCommandLineOptions(args, Reads())
+samFilter = SAMFilter.parseFilteringOptions(args, reads.filterRead)
+
+printOffsets = not args.noOffsets
+printStats = not args.noStats
+
+if samFilter.referenceIds and len(samFilter.referenceIds) > 1:
+    print('Only one reference id can be given. To calculate coverage for more '
+          'than one reference, run this script multiple times.',
+          file=sys.stderr)
+    sys.exit(1)
+
+try:
+    referenceLengths = samFilter.referenceLengths()
+except UnknownReference:
+    referenceId = samFilter.referenceIds.pop()
+    referenceIds = samReferences(args.samfile)
+    print('Reference %r does not appear in SAM file %s. Known '
+          'references are: %s.' % (
+              referenceId, args.samfile, ', '.join(sorted(referenceIds))),
+          file=sys.stderr)
+    sys.exit(1)
+
+if printStats:
+    counts = []
+
+with samfile(args.samfile) as sam:
+
+    if samFilter.referenceIds:
+        # No need to check if the given reference id is in referenceLengths
+        # because the samFilter.referenceLengths call above catched that.
+        referenceId = samFilter.referenceIds.pop()
+    else:
+        if len(referenceLengths) == 1:
+            referenceId = list(referenceLengths)[0]
+        else:
+            print('SAM file %r contains %d references (%s). Only one '
+                  'reference id can be analyzed at a time. Please use '
+                  '--referenceId to specify the one you want examined.' % (
+                      args.samfile, len(referenceLengths),
+                      ', '.join(sorted(referenceLengths))), file=sys.stderr)
+            sys.exit(1)
+
+    for column in sam.pileup(reference=referenceId):
+        bases = Counter()
+        for read in column.pileups:
+            if (not read.is_del and not read.is_refskip and
+                    samFilter.filterAlignment(read.alignment)):
+                base = read.alignment.query_sequence[read.query_position]
+                bases[base] += 1
+
+        baseCount = sum(bases.values())
+
+        if printStats:
+            counts.append(baseCount)
+
+        if printOffsets:
+            print('%d: %d %s' % (column.reference_pos + 1, baseCount,
+                                 baseCountsToStr(bases)))
+
+if printStats:
+    referenceLength = referenceLengths[referenceId]
+    print('Reference id: %s' % referenceId)
+    print('Reference length: %d' % referenceLength)
+    print('Bases covered: %s' % pct(len(counts), referenceLength))
+    print('Min coverage depth: %d' % (
+        0 if len(counts) < referenceLength else min(counts)))
+    print('Max coverage depth: %d' % max(counts))
+    print('Mean coverage depth: %.3f' % (sum(counts) / referenceLength))
+    print('Coverage depth s.d.: %.3f' % std(counts))
diff --git a/dark/__init__.py b/dark/__init__.py
index 324a302..da9777a 100644
--- a/dark/__init__.py
+++ b/dark/__init__.py
@@ -7,4 +7,4 @@ if sys.version_info < (2, 7):
 # will not be found by the version() function in ../setup.py
 #
 # Remember to update ../CHANGELOG.md describing what's new in each version.
-__version__ = '3.1.30'
+__version__ = '3.1.31'
diff --git a/dark/sam.py b/dark/sam.py
index b668b4f..07a5e08 100644
--- a/dark/sam.py
+++ b/dark/sam.py
@@ -334,13 +334,10 @@ class SAMFilter(object):
             SAM/BAM file is read.
         @return: A C{SAMFilter} instance.
         """
-        referenceIds = (set(chain.from_iterable(args.referenceId))
-                        if args.referenceId else None)
-
         return cls(
             args.samfile,
             filterRead=filterRead,
-            referenceIds=referenceIds,
+            referenceIds=set(args.referenceId) if args.referenceId else None,
             storeQueryIds=storeQueryIds,
             dropUnmapped=args.dropUnmapped,
             dropSecondary=args.dropSecondary,
diff --git a/dark/utils.py b/dark/utils.py
index 0059330..1ffa57f 100644
--- a/dark/utils.py
+++ b/dark/utils.py
@@ -259,6 +259,22 @@ def countPrint(mesg, count, len1, len2=None):
                         count, len2, percentage(count, len2)))
 
 
+def pct(a, b):
+    """
+    Format a string showing two integers and what percentage the first
+    is of the second.
+
+    @param a: An C{int}, the numerator.
+    @param b: An C{int}, the denominator.
+    """
+    assert 0 <= a <= b
+    if b:
+        return ('%d/%d (%.3f%%)' %
+                (a, b, (a / b if b else 0.0) * 100.0))
+    else:
+        return '0/0 (0.000%)'
+
+
 @contextmanager
 def cd(newdir):
     """

diff --git a/test/test_utils.py b/test/test_utils.py
index f7d7ed6..5656906 100644
--- a/test/test_utils.py
+++ b/test/test_utils.py
@@ -14,8 +14,8 @@ except ImportError:
 from .mocking import mockOpen
 
 from dark.utils import (
-    numericallySortFilenames, median, asHandle, parseRangeString, StringIO,
-    baseCountsToStr, nucleotidesToStr, countPrint)
+    numericallySortFilenames, median, asHandle, parseRangeString, pct,
+    StringIO, baseCountsToStr, nucleotidesToStr, countPrint)
 
 
 class TestNumericallySortFilenames(TestCase):
@@ -382,3 +382,36 @@ class TestCountPrint(TestCase):
             ' 2/8 (25.00%) of sequence 2',
             countPrint('Count is', count, len1, len2)
         )
+
+
+class TestPct(TestCase):
+    """
+    Test the pct function.
+    """
+    def testZeroNumerator(self):
+        """
+        The pct function must produce the correct result if the numerator is
+        zero.
+        """
+        self.assertEqual('0/10 (0.000%)', pct(0, 10))
+
+    def testZeroDenominator(self):
+        """
+        The pct function must produce the correct result if the denominator is
+        zero.
+        """
+        self.assertEqual('0/0 (0.000%)', pct(0, 0))
+
+    def testOneHalf(self):
+        """
+        The pct function must produce the correct result if the numerator is
+        one half of the denominator.
+        """
+        self.assertEqual('5/10 (50.000%)', pct(5, 10))
+
+    def testOneSeventh(self):
+        """
+        The pct function must produce the correct result if the numerator is
+        one seventh of the denominator.
+        """
+        self.assertEqual('2/14 (14.286%)', pct(2, 14))

Add a script to print per-offset coverage and overall coverage stats from a SAM/BAM file
test/test_utils.py::TestNumericallySortFilenames::testBasename test/test_utils.py::TestNumericallySortFilenames::testNoNames test/test_utils.py::TestNumericallySortFilenames::testOneNonNumericName test/test_utils.py::TestNumericallySortFilenames::testOneNumericName test/test_utils.py::TestNumericallySortFilenames::testSeveralNames test/test_utils.py::TestNumericallySortFilenames::testSeveralNamesWithUnequalPrefixLengths test/test_utils.py::TestMedian::testEmptyArgRaises test/test_utils.py::TestMedian::testMedianOfFive test/test_utils.py::TestMedian::testMedianOfFour test/test_utils.py::TestMedian::testMedianOfOne test/test_utils.py::TestMedian::testMedianOfThree test/test_utils.py::TestMedian::testMedianOfTwo test/test_utils.py::TestAsHandle::testOpenFile test/test_utils.py::TestAsHandle::testStr test/test_utils.py::TestParseRangeString::testEmptyString test/test_utils.py::TestParseRangeString::testSingleNumber test/test_utils.py::TestParseRangeString::testSingleNumberSpaceAfter test/test_utils.py::TestParseRangeString::testSingleNumberSpaceBefore test/test_utils.py::TestParseRangeString::testSingleNumberSpaceBeforeAndAfter test/test_utils.py::TestParseRangeString::testSingleRange test/test_utils.py::TestParseRangeString::testSingleRangeWithSpaceAfterHyphen test/test_utils.py::TestParseRangeString::testSingleRangeWithSpaceBeforeAfterHyphen test/test_utils.py::TestParseRangeString::testSingleRangeWithSpaceBeforeHyphen test/test_utils.py::TestParseRangeString::testTwoOverlappingRanges test/test_utils.py::TestParseRangeString::testTwoRanges test/test_utils.py::TestParseRangeString::testTwoRangesAndANumber test/test_utils.py::TestParseRangeString::testTwoRangesAndTwoNumbers test/test_utils.py::TestParseRangeString::testZeroConversion test/test_utils.py::TestStringIO::testContextManager test/test_utils.py::TestStringIO::testInitializedRead test/test_utils.py::TestStringIO::testInitiallyEmpty test/test_utils.py::TestStringIO::testWriteRead test/test_utils.py::TestBaseCountsToStr::testSimple test/test_utils.py::TestNucleotidesToStr::testSimple test/test_utils.py::TestCountPrint::testSimple test/test_utils.py::TestCountPrint::testTwoSequences test/test_utils.py::TestPct::testOneHalf test/test_utils.py::TestPct::testOneSeventh test/test_utils.py::TestPct::testZeroDenominator test/test_utils.py::TestPct::testZeroNumerator