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Fraction to Decimal Chart (Inches to Decimal & MM)
Quick answer: Divide the top by the bottom. 3/8 = 0.375, 5/16 = 0.3125, 7/16 = 0.4375. Every inch fraction has a power-of-two denominator, so every one of them converts to a decimal that terminates — you will never get a repeating decimal off a tape measure.
The full 64ths chart below gives six decimal places and the exact millimetre value for every fraction of an inch. Rows shaded blue are the ones marked on a standard tape measure; the rest only appear on a machinist's rule.
Inch fraction to decimal chart — all 64ths
Read across: the 64ths column is the raw count of marks, the second column is that fraction reduced, then the decimal inch and the exact millimetre equivalent.
| 64ths | Reduced | Decimal (in) | Millimetres |
|---|---|---|---|
| 1/64 | 1/64 | 0.015625 | 0.3969 |
| 2/64 | 1/32 | 0.031250 | 0.7937 |
| 3/64 | 3/64 | 0.046875 | 1.1906 |
| 4/64 | 1/16 | 0.062500 | 1.5875 |
| 5/64 | 5/64 | 0.078125 | 1.9844 |
| 6/64 | 3/32 | 0.093750 | 2.3812 |
| 7/64 | 7/64 | 0.109375 | 2.7781 |
| 8/64 | 1/8 | 0.125000 | 3.1750 |
| 9/64 | 9/64 | 0.140625 | 3.5719 |
| 10/64 | 5/32 | 0.156250 | 3.9688 |
| 11/64 | 11/64 | 0.171875 | 4.3656 |
| 12/64 | 3/16 | 0.187500 | 4.7625 |
| 13/64 | 13/64 | 0.203125 | 5.1594 |
| 14/64 | 7/32 | 0.218750 | 5.5562 |
| 15/64 | 15/64 | 0.234375 | 5.9531 |
| 16/64 | 1/4 | 0.250000 | 6.3500 |
| 17/64 | 17/64 | 0.265625 | 6.7469 |
| 18/64 | 9/32 | 0.281250 | 7.1437 |
| 19/64 | 19/64 | 0.296875 | 7.5406 |
| 20/64 | 5/16 | 0.312500 | 7.9375 |
| 21/64 | 21/64 | 0.328125 | 8.3344 |
| 22/64 | 11/32 | 0.343750 | 8.7312 |
| 23/64 | 23/64 | 0.359375 | 9.1281 |
| 24/64 | 3/8 | 0.375000 | 9.5250 |
| 25/64 | 25/64 | 0.390625 | 9.9219 |
| 26/64 | 13/32 | 0.406250 | 10.3187 |
| 27/64 | 27/64 | 0.421875 | 10.7156 |
| 28/64 | 7/16 | 0.437500 | 11.1125 |
| 29/64 | 29/64 | 0.453125 | 11.5094 |
| 30/64 | 15/32 | 0.468750 | 11.9062 |
| 31/64 | 31/64 | 0.484375 | 12.3031 |
| 32/64 | 1/2 | 0.500000 | 12.7000 |
| 33/64 | 33/64 | 0.515625 | 13.0969 |
| 34/64 | 17/32 | 0.531250 | 13.4937 |
| 35/64 | 35/64 | 0.546875 | 13.8906 |
| 36/64 | 9/16 | 0.562500 | 14.2875 |
| 37/64 | 37/64 | 0.578125 | 14.6844 |
| 38/64 | 19/32 | 0.593750 | 15.0812 |
| 39/64 | 39/64 | 0.609375 | 15.4781 |
| 40/64 | 5/8 | 0.625000 | 15.8750 |
| 41/64 | 41/64 | 0.640625 | 16.2719 |
| 42/64 | 21/32 | 0.656250 | 16.6687 |
| 43/64 | 43/64 | 0.671875 | 17.0656 |
| 44/64 | 11/16 | 0.687500 | 17.4625 |
| 45/64 | 45/64 | 0.703125 | 17.8594 |
| 46/64 | 23/32 | 0.718750 | 18.2562 |
| 47/64 | 47/64 | 0.734375 | 18.6531 |
| 48/64 | 3/4 | 0.750000 | 19.0500 |
| 49/64 | 49/64 | 0.765625 | 19.4469 |
| 50/64 | 25/32 | 0.781250 | 19.8438 |
| 51/64 | 51/64 | 0.796875 | 20.2406 |
| 52/64 | 13/16 | 0.812500 | 20.6375 |
| 53/64 | 53/64 | 0.828125 | 21.0344 |
| 54/64 | 27/32 | 0.843750 | 21.4312 |
| 55/64 | 55/64 | 0.859375 | 21.8281 |
| 56/64 | 7/8 | 0.875000 | 22.2250 |
| 57/64 | 57/64 | 0.890625 | 22.6219 |
| 58/64 | 29/32 | 0.906250 | 23.0187 |
| 59/64 | 59/64 | 0.921875 | 23.4156 |
| 60/64 | 15/16 | 0.937500 | 23.8125 |
| 61/64 | 61/64 | 0.953125 | 24.2094 |
| 62/64 | 31/32 | 0.968750 | 24.6062 |
| 63/64 | 63/64 | 0.984375 | 25.0031 |
| 64/64 | 1 | 1.000000 | 25.4000 |
Shaded rows are the graduations found on a standard tape measure (halves through eighths). Millimetre values are exact, since 1 inch is defined as exactly 25.4 mm.
The ones worth memorising
Nine numbers cover most of what anyone actually needs on site:
| Fraction | Decimal | mm | Where it shows up |
|---|---|---|---|
| 1/16 | 0.0625 | 1.5875 | The smallest mark on most tapes |
| 1/8 | 0.125 | 3.175 | Standard tile grout line, plywood underlayment |
| 3/16 | 0.1875 | 4.7625 | Common lag and anchor pilot sizes |
| 1/4 | 0.25 | 6.35 | Everything |
| 5/16 | 0.3125 | 7.9375 | Lag bolts, deck ledger fasteners |
| 3/8 | 0.375 | 9.525 | Drywall, rebar #3, air hose ID |
| 7/16 | 0.4375 | 11.1125 | OSB sheathing thickness |
| 1/2 | 0.5 | 12.7 | Drywall, rebar #4, plywood |
| 3/4 | 0.75 | 19.05 | Subfloor plywood, hardwood, rebar #6 |
Converting a decimal back to a fraction
Going the other way is one multiplication:
0.6875 × 64 = 44 → 44/64 → 11/16
0.42 × 64 = 26.88 → round to 27 → 27/64 (0.4219, about 2 thou over)
If the result lands within a couple of thousandths of a sixteenth, use the sixteenth — a number that reduces to a clean fraction is almost always what the drawing meant, and the remainder is rounding, not intent.
Reading the fraction off a tape measure
The mark heights encode the fraction, which is faster than counting once you trust it:
- Longest mark with a number — a full inch.
- Next longest, unnumbered, mid-way — 1/2".
- Shorter again — 1/4" and 3/4".
- Shorter still — the eighths.
- Shortest marks — the sixteenths.
So a mark two shorts past the 3/4 line is 3/4 + 2/16 = 7/8" = 0.875". If you need to check a tape or a printed drawing against a known scale, our printable ruler prints at exact size in both inches and millimetres.
General fraction to decimal chart
Not everything is a power of two. This table covers the common denominators outside the inch system — thirds, fifths, ninths and twelfths — where the decimal repeats rather than terminating.
| Fraction | Decimal | Percent |
|---|---|---|
| 1/16 | 0.0625 | 6.25% |
| 1/12 | 0.08333… | 8.333% |
| 1/10 | 0.1 | 10% |
| 1/9 | 0.1111… | 11.11% |
| 1/8 | 0.125 | 12.5% |
| 1/6 | 0.1666… | 16.67% |
| 3/16 | 0.1875 | 18.75% |
| 1/5 | 0.2 | 20% |
| 2/9 | 0.2222… | 22.22% |
| 1/4 | 0.25 | 25% |
| 3/10 | 0.3 | 30% |
| 5/16 | 0.3125 | 31.25% |
| 1/3 | 0.333… | 33.33% |
| 3/8 | 0.375 | 37.5% |
| 2/5 | 0.4 | 40% |
| 5/12 | 0.41666… | 41.67% |
| 7/16 | 0.4375 | 43.75% |
| 4/9 | 0.4444… | 44.44% |
| 1/2 | 0.5 | 50% |
| 5/9 | 0.5555… | 55.56% |
| 9/16 | 0.5625 | 56.25% |
| 7/12 | 0.58333… | 58.33% |
| 3/5 | 0.6 | 60% |
| 5/8 | 0.625 | 62.5% |
| 2/3 | 0.666… | 66.67% |
| 11/16 | 0.6875 | 68.75% |
| 7/10 | 0.7 | 70% |
| 3/4 | 0.75 | 75% |
| 7/9 | 0.7777… | 77.78% |
| 4/5 | 0.8 | 80% |
| 13/16 | 0.8125 | 81.25% |
| 5/6 | 0.8333… | 83.33% |
| 7/8 | 0.875 | 87.5% |
| 8/9 | 0.8888… | 88.89% |
| 9/10 | 0.9 | 90% |
| 11/12 | 0.91666… | 91.67% |
| 15/16 | 0.9375 | 93.75% |
An ellipsis means the digits repeat forever. 1/3 is 0.333… exactly, never 0.333 — which is why thirds cause rounding drift when you chain several of them together.
Where the conversion actually bites
Ordering material by decimal when the drawing is in fractions
Suppliers quote sheet goods in decimals (0.703" for nominal 23/32" subfloor, for instance) while plans call them out as fractions. They are not always the same number: nominal 3/4" plywood is usually 23/32" (0.719"), not 0.750". A 1/32" gap per sheet across a floor adds up.
Feeding a CNC or laser from a hand drawing
Machines take decimals. Round 11/16 to 0.69 and you are 2.5 thou under; over ten features that is a visible cumulative error.
Working from a mixed metric and imperial set of parts
13 mm is 0.5118" — close enough to 1/2" (0.5") to look identical and far enough off to strip a fastener. The millimetre column above is there so you can check before you commit.
Frequently asked questions
What is 3/8 as a decimal?
0.375. In millimetres that is 9.525 mm. It is one of the few inch fractions that terminates cleanly at three places, which is why 0.375 is so often written on drawings instead of the fraction.
What is 5/16 as a decimal?
0.3125, or 7.9375 mm. Note it is not 0.31 — rounding a 16th to two decimal places loses about 2.5 thousandths of an inch, which matters for machining and does not matter for framing.
How do I convert a fraction to a decimal?
Divide the top number by the bottom number. 7/16 means 7 ÷ 16 = 0.4375. Every inch fraction has a denominator that is a power of two, so they all terminate — you never get a repeating decimal from a tape measure.
How do I convert a decimal back to a fraction of an inch?
Multiply the decimal by 64 and round to the nearest whole number — that is your numerator over 64, which you then reduce. 0.6875 × 64 = 44, so 44/64 = 11/16.
Why do inch fractions go 2, 4, 8, 16, 32, 64?
Because a ruler is made by repeatedly halving. Each mark subdivides the one before it, so the denominators can only ever be powers of two. That is also why every inch fraction converts to a decimal that stops rather than repeating.