Nearest marks
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Shop estimate. Measure your stock and cut a test on scrap.
Sixteenths, from the same rounding
| Mark | Decimal inches | Millimetres |
|---|---|---|
| 1/16 | 0.0625 | 1.59 |
| 1/8 | 0.1250 | 3.17 |
| 3/16 | 0.1875 | 4.76 |
| 1/4 | 0.2500 | 6.35 |
| 5/16 | 0.3125 | 7.94 |
| 3/8 | 0.3750 | 9.52 |
| 7/16 | 0.4375 | 11.11 |
| 1/2 | 0.5000 | 12.70 |
| 9/16 | 0.5625 | 14.29 |
| 5/8 | 0.6250 | 15.88 |
| 11/16 | 0.6875 | 17.46 |
| 3/4 | 0.7500 | 19.05 |
| 13/16 | 0.8125 | 20.64 |
| 7/8 | 0.8750 | 22.22 |
| 15/16 | 0.9375 | 23.81 |
| 1 | 1.0000 | 25.40 |
How to use it
- Type a fraction, a mixed number, or a decimal. 3/4, 1-1/2, 3 5/16, and 0.75 are all inches when you do not add a unit.
- Add a suffix when you mean it: mm, cm, m, in, ft, a double prime, or a single prime.
- Read decimal inches, millimetres, and the nearest sixteenth, thirty-second, and sixty-fourth.
- If two marks are the same distance, the rounding follows ordinary half-up rounding on the positive side of the rule.
The formula
Millimetres = inches × 25.4. The nearest mark multiplies the length by the denominator, rounds, and simplifies.
- 25.4. The inch is defined as exactly 25.4 millimetres.
- 1/16. The usual furniture rule. Finer marks are there when a joint needs them.
- Error. How far the rounded mark sits from the length you typed, in inches.
Worked example
Type 20 mm. The nearest sixteenth of an inch is 13/16, and the nearer thirty-second is 25/32. Bare fractions are always read as inches, so 13/16 and 13/16 in are the same length. A bare decimal follows the unit you have not overridden with a suffix.
13/16 inch is also the thickness many hardwood dealers aim for when they surface 4/4 stock on two faces. The lumber-size page treats that as a trade habit, not a promise. Measure the board.
What it will not decide
- It does not accumulate a cut list. Rounding every part to the nearest sixteenth and then adding them up can walk off the finished size.
- Denominators only go to 64. That covers a machinist’s scale and a cabinet rule.
- A bare fraction is inches. Twenty millimetres typed as 20/1 would be twenty inches. Type 20 mm.
Questions
Which way does a halfway mark round?
Toward the larger positive mark. Halfway between 0 and 1/16 becomes 1/16. Shop lengths here are positive, so you will not see a negative tape.
Is 13/16 the same as 20 mm?
No. 13/16 inch is 20.64 mm. Twenty millimetres is closer to 25/32 inch than to 13/16. The page shows the error so you can decide which mark to cut to.
Can I type feet?
Yes. 8 ft, 8 feet, and 8′ are 96 inches. A bare 8 is 8 inches in inch mode and 8 millimetres in metric mode.