diff --git a/misc/storage-overhead.py b/misc/storage-overhead.py
new file mode 100644
index 000000000..76d8b97d5
--- /dev/null
+++ b/misc/storage-overhead.py
@@ -0,0 +1,85 @@
+#! /usr/bin/python
+
+import sys, math
+from cStringIO import StringIO
+from allmydata import upload, uri, encode, storage
+from allmydata.util import mathutil
+
+def roundup(size, blocksize=4096):
+    return blocksize * mathutil.div_ceil(size, blocksize)
+
+
+class BigFakeString:
+    def __init__(self, length):
+        self.length = length
+        self.fp = 0
+    def seek(self, offset, whence=0):
+        if whence == 0:
+            self.fp = offset
+        elif whence == 1:
+            self.fp += offset
+        elif whence == 2:
+            self.fp = self.length - offset
+    def tell(self):
+        return self.fp
+
+def calc(filesize, params=(3,7,10), segsize=encode.Encoder.MAX_SEGMENT_SIZE):
+    num_shares = params[2]
+    if filesize <= upload.Uploader.URI_LIT_SIZE_THRESHOLD:
+        urisize = len(uri.pack_lit("A"*filesize))
+        sharesize = 0
+        sharespace = 0
+    else:
+        u = upload.FileUploader(None)
+        u.set_params(params)
+        # unfortunately, Encoder doesn't currently lend itself to answering
+        # this question without measuring a filesize, so we have to give it a
+        # fake one
+        data = BigFakeString(filesize)
+        u.set_filehandle(data)
+        u.set_encryption_key("a"*16)
+        sharesize, blocksize = u.setup_encoder()
+        # how much overhead?
+        #  0x20 bytes of offsets
+        #  0x04 bytes of extension length
+        #  0x1ad bytes of extension (=429)
+        # total is 465 bytes
+        num_segments = mathutil.div_ceil(filesize, segsize)
+        num_share_hashes = int(math.log(mathutil.next_power_of_k(num_shares, 2),
+                                    2)) + 1
+        sharesize = storage.allocated_size(sharesize, num_segments,
+                                           num_share_hashes,
+                                           429)
+        sharespace = num_shares * roundup(sharesize)
+        urisize = len(uri.pack_uri(storage_index="a"*32,
+                                   key="a"*16,
+                                   uri_extension_hash="a"*32,
+                                   needed_shares=params[0],
+                                   total_shares=params[2],
+                                   size=filesize))
+
+    return urisize, sharesize, sharespace
+
+def main():
+    filesize = int(sys.argv[1])
+    urisize, sharesize, sharespace = calc(filesize)
+    print "urisize:", urisize
+    print "sharesize:  %10d" % sharesize
+    print "sharespace: %10d" % sharespace
+    print "desired expansion: %1.1f" % (1.0 * 10 / 3)
+    print "effective expansion: %1.1f" % (1.0 * sharespace / filesize)
+
+def chart():
+    filesize = 2
+    while filesize < 2**20:
+        urisize, sharesize, sharespace = calc(int(filesize))
+        expansion = 1.0 * sharespace / int(filesize)
+        print "%d,%d,%d,%1.2f" % (int(filesize), urisize, sharespace, expansion)
+        filesize  = filesize * 2**0.5
+
+if __name__ == '__main__':
+    if sys.argv[1] == "chart":
+        chart()
+    else:
+        main()
+