diff --git a/slip39/layout/components.py b/slip39/layout/components.py
index 61f770a..e35a97b 100644
--- a/slip39/layout/components.py
+++ b/slip39/layout/components.py
@@ -213,6 +213,13 @@ def element( self ):
 Coordinate			= namedtuple( 'Coordinate', ('x', 'y') )
 
 
+# A SLIP-39 mnemonic cell is eg. "12 advocate": up to MNEM_CHARS (2 digits, a space, an 8-letter word)
+# of monospaced font, each MONO_WIDTH of the font size wide; fit within MNEM_FIT of the column width.
+MNEM_CHARS			= 11
+MONO_WIDTH			= .6
+MNEM_FIT			= .9
+
+
 def layout_card(
     card_size: Coordinate,
     card_margin: int,
@@ -314,6 +321,17 @@ def layout_card(
     assert num_mnemonics in MNEM_ROWS_COLS, \
         f"Invalid SLIP-39 mnemonic word count: {num_mnemonics}"
     rows,cols			= MNEM_ROWS_COLS[num_mnemonics]
+
+    def mnem_size( region, size_ratio ):
+        """The mnemonic font size (points): from the height of each row, unless that is too wide for
+        the longest "NN word" (MNEM_CHARS mono characters) to fit in the width of each column."""
+        return min(
+            region.h / rows * PT_IN * size_ratio,
+            region.w / cols * PT_IN * MNEM_FIT / ( MNEM_CHARS * MONO_WIDTH ),
+        )
+
+    mnem_fontsize		= mnem_size( card_mnemonics, 9/16 )
+    mnem_fontsize_big		= mnem_size( card_mnemonics_big, 12/16 )
     for r in range( rows ):
         for c in range( cols ):
             card_mnemonics.add_region_proportional(
@@ -325,7 +343,7 @@ def layout_card(
                     y2		= (r+1)/rows,
                     font	= 'mono',
                     bold	= True,
-                    size_ratio	= 9/16,
+                    size	= mnem_fontsize,
                 )
             )
             card_mnemonics_big.add_region_proportional(
@@ -337,7 +355,7 @@ def layout_card(
                     y2		= (r+1)/rows,
                     font	= 'mono',
                     bold	= True,
-                    size_ratio	= 12/16,
+                    size	= mnem_fontsize_big,
                 )
             )
 
diff --git a/slip39/output_test.py b/slip39/output_test.py
index 903c0b4..68f9065 100644
--- a/slip39/output_test.py
+++ b/slip39/output_test.py
@@ -4,7 +4,8 @@
 from fpdf		import FPDF, FlexTemplate
 
 from .layout		import Region, Text, Image, Box, Coordinate
-from .defaults		import MM_IN
+from .layout.components	import layout_card, MNEM_CHARS, MONO_WIDTH
+from .defaults		import MM_IN, PT_IN, CARD_SIZES, MNEM_ROWS_COLS
 
 
 def test_Region():
@@ -73,3 +74,15 @@ def test_Region():
     tpl.render( offsetx = card_size.x * MM_IN, offsety = card_size.y * MM_IN )
 
     #pdf.output( "test.pdf" ) # To view results in test.pdf, uncomment
+
+
+def test_card_mnemonics_fit():
+    """The longest mnemonic cell ("12 advocate") must fit within its column, for every card size."""
+    for card_size,((card_h,card_w),card_margin) in CARD_SIZES.items():
+        for num_mnemonics in MNEM_ROWS_COLS:
+            card			= layout_card( Coordinate( y=card_h, x=card_w ), card_margin, num_mnemonics )
+            for e in card.elements():
+                if e['name'].startswith( 'mnem-' ):
+                    cell_pt		= ( e['x2'] - e['x1'] ) / MM_IN * PT_IN
+                    assert e['size'] * MONO_WIDTH * MNEM_CHARS <= cell_pt, \
+                        f"{card_size} card, {num_mnemonics} words: {e['name']} {e['size']:.2f}pt too big for a {cell_pt:.1f}pt column"