Convert between 5/8, 3/8, 1/2 inch, and 1 cm seam allowances — with fabric offset per edge.
Pattern → Target
0.625 in → 0.375 in
Difference per edge
0.250 in (0.64 cm)
Total fabric change
Across all seamed edges.
1.000 in
Cutting offset
Cut piece smaller by this much per edge.
0.250 in
US commercial patterns default to 5/8 in; European and independent patterns often use 3/8 in or 1 cm. When you swap between them you have to compensate on the cut — trim extra off each edge if you’re going smaller, add if you’re going bigger — or your finished piece will come out the wrong size.
Handy equivalents: 5/8 in ≈ 1.6 cm, 1/2 in ≈ 1.27 cm, 3/8 in ≈ 0.95 cm, 1 cm ≈ 3/8 in.
diff_per_edge = pattern_allowance − target_allowance
total_diff = diff_per_edge × seamed_edges
cut offset = diff_per_edge (positive = cut smaller)