diff --git a/Lib/fontTools/fontBuilder.py b/Lib/fontTools/fontBuilder.py
index c9aed9261..17806c5c8 100644
--- a/Lib/fontTools/fontBuilder.py
+++ b/Lib/fontTools/fontBuilder.py
@@ -177,6 +177,16 @@ _OS2Defaults = dict(
 class FontBuilder(object):
 
     def __init__(self, unitsPerEm=None, font=None, isTTF=True):
+        """Initialize a FontBuilder instance.
+
+        If the `font` argument is not given, a new `TTFont` will be
+        constructed, and `unitsPerEm` must be given. If `isTTF` is True,
+        the font will be a glyf-based TTF; if `isTTF` is False it will be
+        a CFF-based OTF.
+
+        If `font` is given, it must be a `TTFont` instance and `unitsPerEm`
+        must _not_ be given. The `isTTF` argument will be ignored.
+        """
         if font is None:
             self.font = TTFont(recalcTimestamp=False)
             self.isTTF = isTTF
@@ -302,13 +312,15 @@ class FontBuilder(object):
         self.font["CFF "] = newTable("CFF ")
         self.font["CFF "].cff = fontSet
 
-    def setupGlyf(self, glyphs):
+    def setupGlyf(self, glyphs, calcGlyphBounds=True):
         assert self.isTTF
         self.font["loca"] = newTable("loca")
         self.font["glyf"] = newTable("glyf")
         self.font["glyf"].glyphs = glyphs
         if hasattr(self.font, "glyphOrder"):
             self.font["glyf"].glyphOrder = self.font.glyphOrder
+        if calcGlyphBounds:
+            self.calcGlyphBounds()
 
     def setupFvar(self, axes, instances):
         addFvar(self.font, axes, instances)
diff --git a/Tests/fontBuilder/fontBuilder_test.py b/Tests/fontBuilder/fontBuilder_test.py
index 89cfcf865..b7bbb7c1a 100644
--- a/Tests/fontBuilder/fontBuilder_test.py
+++ b/Tests/fontBuilder/fontBuilder_test.py
@@ -60,7 +60,6 @@ def test_build_ttf(tmpdir):
     glyph = pen.glyph()
     glyphs = {".notdef": glyph, "A": glyph, "a": glyph, ".null": glyph}
     fb.setupGlyf(glyphs)
-    fb.calcGlyphBounds()
     metrics = {}
     glyphTable = fb.font["glyf"]
     for gn, advanceWidth in advanceWidths.items():