Getting the buttonhole size right for your button is the difference between a fastening that closes in one movement and one that either won't go through or pops open every time you move. Most problems come from cutting the buttonhole to the diameter alone, but a button also has thickness and shape. Below you'll find the simple formula, adjustments for different button types, a millimetre chart and a test method that saves you from unpicking.
The basic formula
The standard buttonhole length is:
Buttonhole length = button diameter + button thickness + 1.5 to 2 mm ease
Diameter tells you how wide the button is, but it also has to tilt through the slit, and that takes room for its thickness. The ease allows for the fabric around the opening and keeps fastening comfortable. For thin, flat buttons an even simpler rule is often used: diameter + 3 mm. So a 20 mm button gets a 23 mm buttonhole.
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SHELL 07 BROWN – Brown mother-of-pearl shirt button with 4 holes, thin rim, luxurious and naturalHow to measure your button
- Diameter: measure across the widest point with callipers or a ruler. If you know the size in ligne, multiply by 0.635 (24 L is about 15 mm).
- Thickness: measure at the highest point: the centre on a domed button, the rim on a rimmed one.
- Odd shapes: wrap a thin strip of paper around the button across its widest and thickest part, mark the circumference and halve it. Add 2 mm. This works for ball, square and novelty buttons.
Chart: buttonhole size by button
| Button diameter | Typical thickness | Flat button | Domed or thick button |
|---|---|---|---|
| 10 mm (16 L) | 2 mm | 13 mm | 14 to 15 mm |
| 11.5 mm (18 L) | 2 to 2.5 mm | 15 mm | 16 mm |
| 12.5 mm (20 L) | 2.5 mm | 16 mm | 17 to 18 mm |
| 15 mm (24 L) | 3 mm | 19 mm | 20 to 21 mm |
| 18 mm (28 L) | 3 mm | 22 mm | 23 to 24 mm |
| 20 mm (32 L) | 3.5 mm | 24 to 25 mm | 26 to 27 mm |
| 22 mm (36 L) | 4 mm | 27 mm | 28 to 30 mm |
| 25 mm (40 L) | 4 mm | 30 mm | 31 to 33 mm |
| 28 mm (45 L) | 4.5 mm | 33 mm | 35 to 37 mm |
Thicknesses are indicative. Always measure your own button.
Adjustments by button type
Domed and thick buttons
For strongly domed or chunky buttons, add twice the thickness plus about 2 mm: diameter + 2 x thickness + 2 mm. This applies to many coconut and wooden coat buttons that are 4 to 5 mm thick.
Shank buttons
With a shank button, only the head passes through the buttonhole; the shank stays beneath the slit. Measure the diameter and thickness of the head, ignoring the shank. If the head is round and domed, use the paper-strip method.
Natural buttons
Corozo, coconut and wood vary slightly from piece to piece, especially in thickness. Measure three or four and go with the largest. Check sizes across our natural buttons, or browse all buttons for classic styles.
Heavy fabrics
On wool coating, denim or a double layer with interfacing, add another 1 mm: the fabric's own thickness eats into the opening.
Knits and stretch fabrics
Jersey stretches and the buttonhole grows over time. Stabilise the area with a light interfacing and make the buttonhole to the formula without extra ease.
Always test first
- Take a scrap of the same fabric, with the same interfacing and number of layers.
- Sew a buttonhole at the calculated length.
- Cut it open and pass the button through ten times.
- The button should go through without force but stay put when you pull the fabric sideways.
- Too tight? Add 1 mm. Too loose? Take off 1 mm and test again.
Tip: many sewing machines have an automatic buttonhole foot that holds the button. It reads the diameter but doesn't always account for thickness, so test thick buttons on a scrap before you sew into the garment.
A worked example
Say you're making a linen overshirt with 18 mm coconut buttons. You measure the thickness at 3.5 mm in the centre; the button is slightly domed. The basic formula gives 18 + 3.5 + 2 = 23.5 mm. Interfaced linen is medium thickness, so no extra is needed. You test at 24 mm and the button passes easily but stays put. Put the same button on a double-layer wool coat and you'd add another 1 mm, landing at 25 mm.
Position and spacing
Placement matters as much as length. On a horizontal buttonhole, the slit starts about 2 to 3 mm beyond the centre line toward the garment edge, because the button settles at the end point. On a vertical buttonhole, the centre of the slit sits on the centre line. Spacing between buttons is typically 7 to 9 cm on a shirt and 10 to 12 cm on a coat.
Troubleshooting
- The button won't go through: the buttonhole is short or the button is thicker than expected. Extend it by 1 to 2 mm with hand stitches at the end.
- The button keeps coming undone: the buttonhole is too long. Close it up with a few hand stitches at one end.
- The fabric frays around the buttonhole: no interfacing, or the stitch is too open. Add interfacing and tighten the zigzag density.
- The button drags on the buttonhole: it's sewn flat to the fabric. Make a thread shank equal to the fabric thickness.
FAQ
Horizontal or vertical buttonholes?
Vertical buttonholes are used on shirt plackets, where the band is narrow. Horizontal ones go on jackets, coats and cuffs, because the strain runs crosswise and the button settles at the end of the slit.
Does the formula work for hand-worked buttonholes?
Yes. On a keyhole buttonhole, the round eye at the end takes the shank and the formula stays the same.
Which thread should I use?
Polyester sewing thread matched to the fabric. See our sewing threads.
If you need buttons in exact sizes for a production run or a brand, Bul-tex manufactures and supplies trims and takes custom orders. Get in touch through the contact page.
