Button Sizing & Ligne Scale (ASTM D5171)
Direct Answer
Button dimensions are globally measured using the historic French Ligne unit (symbol 'L'), where 1 Ligne equals exactly 0.635 mm (1/40 inch or 0.025 in). Sized from 14L (collar button-downs) and 18L (dress shirt plackets) to 32L (suit coat fronts) and 40L (winter overcoats), the Ligne scale defines standard buttonhole cutting lengths.
Source: ASTM D5171-15 (Standard Test Method for Impact Resistance of Buttons)
Specs
| Governing Standards | ASTM D5171 / International Ligne Button Standard |
| Base Unit Definition | 1 Ligne (1L) = 0.635 mm = 1/40 inch (0.025") |
| 14L Shirt Collar / Gauntlet | Diameter: 8.89 mm (0.350") | Button-down collars and sleeve placket gauntlets |
| 16L Shirt Cuff Button | Diameter: 10.16 mm (0.400") | Dress shirt sleeve cuffs |
| 18L Standard Shirt Front | Diameter: 11.43 mm (0.450") | Standard dress shirt center front placket |
| 24L Suit Sleeve Button | Diameter: 15.24 mm (0.600") | Tailored suit jacket sleeve cuffs and formal vests |
| 30L / 32L Suit Front Button | Diameter: 19.05 / 20.32 mm (0.75 / 0.80") | Suit jacket and blazer front closures |
| 40L Overcoat Button | Diameter: 25.40 mm (1.000") | Heavy trench coats, pea coats, and wool overcoats |
The 1/40-Inch Ligne Mathematical Formula
The Ligne (pronounced lean) originated in 18th-century French watchmaking and button manufacturing. The diameter in millimeters is calculated directly by multiplying the Ligne number by $0.635$: $$\text{Diameter (mm)} = \text{Lignes} \times 0.635\text{ mm}$$ $$\text{Diameter (inches)} = \frac{\text{Lignes}}{40}$$
Examples:
- $18\text{L}$ (Shirt Front): $18 \times 0.635 = 11.43\text{ mm}$ ($18/40 = 0.45\text{ in}$)
- $32\text{L}$ (Suit Front): $32 \times 0.635 = 20.32\text{ mm}$ ($32/40 = 0.80\text{ in}$)
- $40\text{L}$ (Overcoat): $40 \times 0.635 = 25.40\text{ mm}$ ($40/40 = 1.00\text{ in}$)
Buttonhole Length Sizing Rule
To ensure a button passes smoothly through a fabric buttonhole without tearing or popping open during wear, tailors use the standard calculation: $$\text{Buttonhole Length} = \text{Button Diameter} + \text{Button Thickness} + 1/16\text{ inch } (1.5\text{ mm})$$
A flat $18\text{L}$ mother-of-pearl shirt button ($11.4\text{ mm}$ diameter, $2.0\text{ mm}$ thick) requires a $15.0\text{ mm}$ buttonhole slit.