Buttonhole Length Calculator
Calculate the correct buttonhole length from your button's diameter and thickness.
Reviewed by the ToolNestr Editorial Team — July 2026
How buttonhole length is calculated
A button doesn't pass through a buttonhole as a flat disc — it has to tip on edge and push through, so the buttonhole needs to accommodate both the button's diameter and its thickness (or dome height) together, plus a small amount of ease so the button moves through smoothly without excessive tightness or looseness. This calculator adds diameter and thickness together, then adds a standard 1/8-inch ease allowance.
A flat, thin button needs a buttonhole barely longer than its diameter, while a thick, domed, or shank button needs a noticeably longer buttonhole — using diameter alone for a thick button typically results in a buttonhole too short to actually close.
This follows the standard diameter-plus-thickness-plus-ease rule used across home-sewing and tailoring references for buttonhole sizing.
Worked example
A button measures 0.75 inch in diameter and 0.2 inch thick.
Real-world context
Coat and jacket buttons tend to be thicker and more domed than shirt buttons, which is exactly the scenario where using diameter alone produces a noticeably too-short buttonhole — a coat button that measures the same diameter as a shirt button can need a meaningfully longer buttonhole once its extra thickness is accounted for.
Vintage and antique buttons salvaged for a new project often have unusual shapes and thicknesses compared to modern mass-produced buttons, making a direct measurement (rather than assuming a standard button proportion) especially important to avoid a mismatched buttonhole.
Common misconceptions
"Buttonhole length only needs to match button diameter." Incomplete. Diameter alone ignores the button's thickness, which is exactly what causes a buttonhole to come out too short for a thick or domed button.
"A slightly too-large buttonhole is always safer than too small." Not necessarily. An oversized buttonhole lets the button slip through on its own during wear, causing the garment to come undone — matching size closely, with only the standard small ease, gives the most secure and reliable closure.
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Frequently asked questions
Why does button thickness matter, not just diameter?
A button has to pass through the buttonhole edge-first, so a thick or domed button needs a longer buttonhole than a flat button of the same diameter, since the button's full "profile" (diameter plus thickness) has to clear the buttonhole opening.
How do I measure button thickness?
Stand the button on its edge and measure its height from a flat surface, or measure across the button from front to back through its thickest point (including any dome or shank).
What if my button has a shank instead of holes?
Shank buttons still need this same diameter-plus-thickness calculation, though the "thickness" for a shank button includes the shank's added height, which can make the buttonhole noticeably longer than for a flat sew-through button of the same diameter.
Should I test the buttonhole size before doing all of them?
Yes — sew one test buttonhole on a scrap of the actual project fabric and check the button passes through smoothly but isn't loose, since fabric thickness and interfacing can shift the ideal length slightly from the pure calculation.