How do I turn a decimal into a tape fraction?

Multiply the part after the decimal point by 16 and round to the nearest whole number — that is how many sixteenths you have. 0.6875 × 16 = 11, so 2.6875 in is 2 11/16. If the count is even, reduce it: 8 sixteenths is 1/2, 12 sixteenths is 3/4. The strip at the top of this page does it for you, and the chart lists all sixteen.

How many millimetres are in an inch?

Exactly 25.4 — that is the definition, not a rounding. So 1/16 in is 1.5875 mm, 1/8 in is 3.175 mm, and 3/4 in is 19.05 mm. Going the other way, divide millimetres by 25.4 to get decimal inches, then read the nearest sixteenth off the chart.

How do I add fractions on the tape?

Put everything into the smallest mark you are using — sixteenths, usually — add the counts, then reduce and carry. 2 11/16 + 3/8: 3/8 is 6/16, so 11 + 6 = 17 sixteenths, which is one whole and 1/16. The answer is 3 1/16. Subtraction works the same way, borrowing a whole inch (16 sixteenths) when the top fraction is smaller.

What is 0.5 feet in inches?

Six inches — decimal feet are tenths of a foot, not inches, and the two get mixed up constantly. Multiply the decimal part by 12: 14.142 ft is 14 ft and 0.142 × 12 = 1.7 in, so 14 ft 1 11/16 in on the tape. A 9.1 typed into a calculator as inches is nine and a tenth inches, not nine foot one.

Units & fractions reference

The numbers a crew reaches for without a calculator, on one printable sheet: the tape in sixteenths with the decimal and the millimetres beside each mark, the exact factors between the units, how to add on the tape, and the shortcuts that turn an area straight into yards. Nothing here is a code value — it is all definitions and arithmetic.

The tape, one line at a time

Every sixteenth from 1/16 to a full inch, reduced the way the tape prints it. The decimal is what a drawing or a calculator shows; the millimetres are exact (1 in = 25.4 mm by definition). The bold marks are the ones with the longer lines on the blade: the eighths, quarters and the half.

Quick referencesixteenths · decimal · mm

1/16: 0.0625 in · 1.59 mm
1/8: 0.1250 in · 3.17 mm
3/16: 0.1875 in · 4.76 mm
1/4: 0.2500 in · 6.35 mm
5/16: 0.3125 in · 7.94 mm
3/8: 0.3750 in · 9.52 mm
7/16: 0.4375 in · 11.11 mm
1/2: 0.5000 in · 12.70 mm
9/16: 0.5625 in · 14.29 mm
5/8: 0.6250 in · 15.88 mm
11/16: 0.6875 in · 17.46 mm
3/4: 0.7500 in · 19.05 mm
13/16: 0.8125 in · 20.64 mm
7/8: 0.8750 in · 22.22 mm
15/16: 0.9375 in · 23.81 mm
1 in: 1.0000 in · 25.40 mm

Reading the marks

The lines on a tape are ranked by length. The longest line is the inch. The next longest, halfway between, is the half. Shorter again, splitting each half, are the quarters. Then the eighths, and the shortest lines are the sixteenths. To read a mark, find the nearest long line you can name and count the short ones past it: one short line past the half is 9/16, one past the quarter is 5/16, one past 3/4 is 13/16.

Say it the way the trade says it — two and eleven sixteenths, never two point six-eight-seven-five — and write it with a space between the whole and the fraction: 2 11/16. A dash is fine too (2-11/16); every calculator on this site reads both. Metric has no fractions to carry — 68 mm and 22 mm is 90 mm — which is why the chart above gives every mark in millimetres too.

Adding and taking away on the tape

Work in sixteenths. Turn every fraction into a count of sixteenths (an eighth is 2, a quarter is 4, a half is 8, three-quarters is 12), add or subtract the counts, then carry or borrow a whole inch as 16.

Adding. 2 11/16 + 3/8: the 3/8 is 6 sixteenths, 11 + 6 = 17, which is one inch and 1 sixteenth. Answer: 3 1/16. A run of pieces — 5 3/8, 7 1/4, 12 5/8 — is 24 whole inches and 6 + 4 + 10 = 20 sixteenths, so 24 + 1 4/16 = 25 1/4.

Taking away. 6 1/4 − 2 7/8: 1/4 is 4 sixteenths and 7/8 is 14, so borrow: 6 1/4 becomes 5 20/16, and 20 − 14 = 6 sixteenths, 5 − 2 = 3. Answer: 3 3/8. The board-thickness case: a 1 1/2 in stud each side of a 36 in opening gives a header of 36 + 1 1/2 + 1 1/2 = 39, and the opening left between two 3 1/2 in walls 40 in apart is 40 − 7 = 33.

Halving. Half of 2 11/16 is not a tape mark in sixteenths — 11 is odd. Drop to thirty-seconds: 2 11/16 is 2 22/32, half is 1 11/32, which the tape shows as just over 1 5/16. For a centreline that is close enough to mark; for a cut that has to match, hold the two pieces together instead of measuring twice.

Decimal feet ↔ feet and inches

A total-station reading or a long diagonal comes out in decimal feet. The digits after the point are tenths of a foot, not inches: 14.142 ft is 14 ft and 0.142 × 12 = 1.704 in, so 14′ 1 11/16″. Going the other way, inches ÷ 12 gives the decimal: 9′ 1″ is 9 + 1 ÷ 12 = 9.083 ft — and 109 in. Type a feet-inches value into any tool here as 9' 1" and it reads it as 109.

Quick referencedecimal feet

0.1 ft: 1 3/16 in (1.2)
0.25 ft: 3 in
0.333 ft: 4 in
0.5 ft: 6 in
0.667 ft: 8 in
0.75 ft: 9 in
0.01 ft: 1/8 in (0.12) — a hundredth of a foot is an eighth

Length

Quick referenceexact by definition

1 inch: 25.4 mm · 2.54 cm
1 foot: 12 in · 304.8 mm · 0.3048 m
1 yard: 3 ft · 36 in · 0.9144 m
1 metre: 39.370 in · 3.2808 ft · 1.0936 yd
1 millimetre: 0.03937 in — between 1/32 and 3/64; 25 mm is 63/64
1 centimetre: 0.3937 in — just under 13/32
1 mile: 5,280 ft · 1,760 yd · 1.609344 km

Area

Quick referencethe length factor, squared

1 square foot: 144 in² · 0.092903 m²
1 square yard: 9 ft² · 0.836127 m²
1 square metre: 10.764 ft² · 1.196 yd² · 10,000 cm²
1 acre: 43,560 ft² · 4,840 yd² · 0.404686 ha
1 hectare: 10,000 m² · 2.4711 acres
A 4 × 8 sheet: 32 ft² · a 1220 × 2440 sheet is 2.977 m²

Volume

Quick referencethe length factor, cubed

1 cubic foot: 1,728 in³ · 28.3168 L · 7.4805 US gal · 0.028317 m³
1 cubic yard: 27 ft³ · 0.764555 m³ · 201.97 US gal
1 cubic metre: 35.315 ft³ · 1.30795 yd³ · 1,000 L
1 litre: 1,000 cm³ · 0.26417 US gal · 0.21997 imp gal · 61.024 in³
1 US gallon: 231 in³ · 3.78541 L · 0.13368 ft³ · 0.83267 imp gal
1 imperial gallon: 4.54609 L · 1.20095 US gal
1 board foot: 144 in³ — 1 in × 12 in × 12 in

Weight

Quick referencemass, the way the yard sells it

1 pound: 0.453592 kg · 16 oz
1 kilogram: 2.20462 lb
1 short ton (US): 2,000 lb · 0.907185 t
1 tonne (metric): 1,000 kg · 2,204.6 lb · 1.10231 short tons
1 long ton (UK): 2,240 lb · 1.01605 t
Water: 62.43 lb/ft³ · 8.345 lb/US gal · 1 kg/L

Pressure & the rest

Quick referencepsi, kPa, bar

1 psi: 6.89476 kPa · 0.0689476 bar · 2.0360 in Hg
1 bar: 100 kPa · 14.5038 psi
1 kPa: 0.145038 psi · 0.01 bar
1 atmosphere: 101.325 kPa · 14.696 psi · 1.01325 bar
Head of water: 1 psi ≈ 2.31 ft (0.704 m) of water column
Temperature: °F = °C × 1.8 + 32 · °C = (°F − 32) ÷ 1.8
Torque: 1 ft·lb = 12 in·lb = 1.35582 N·m

Takeoff shortcuts

These fall straight out of 27 cubic feet to the yard and the depth written as a fraction of a foot — and, in metric, out of the depth written in metres (100 mm is 0.1 m, so m² ÷ 10). They are the sums a foreman does on the tailgate; the calculators do the same thing with your waste factor on top.

Quick referencearea straight to yards

Depth as feet: 2 in = 0.167 · 3 in = 0.25 · 4 in = 0.333 · 6 in = 0.5 · 8 in = 0.667 · 12 in = 1
Yards at 2 in: ft² ÷ 162
Yards at 3 in: ft² ÷ 108
Yards at 4 in: ft² ÷ 81
Yards at 6 in: ft² ÷ 54
Yards at 12 in: ft² ÷ 27
Cubic metres: m² × depth in metres — 100 m² at 100 mm is 10 m³
m³ at 50 / 75 mm: m² ÷ 20 · m² ÷ 13.3
m³ at 100 / 150 mm: m² ÷ 10 · m² ÷ 6.67
Yards from m³: m³ × 1.308 · m³ from yards: yd³ × 0.7646
Sheets: ft² ÷ 32 for a 4 × 8 · m² ÷ 2.977 for a 1220 × 2440
Studs: wall in inches ÷ spacing, rounded up, + 1 — or wall in mm ÷ spacing in mm
Diagonal of a rectangle: √(L² + W²) — a 3-4-5 is square

Try it

Questions

How do I turn a decimal into a tape fraction?: Multiply the part after the decimal point by 16 and round to the nearest whole number — that is how many sixteenths you have. 0.6875 × 16 = 11, so 2.6875 in is 2 11/16. If the count is even, reduce it: 8 sixteenths is 1/2, 12 sixteenths is 3/4. The strip at the top of this page does it for you, and the chart lists all sixteen.
How many millimetres are in an inch?: Exactly 25.4 — that is the definition, not a rounding. So 1/16 in is 1.5875 mm, 1/8 in is 3.175 mm, and 3/4 in is 19.05 mm. Going the other way, divide millimetres by 25.4 to get decimal inches, then read the nearest sixteenth off the chart.
How do I add fractions on the tape?: Put everything into the smallest mark you are using — sixteenths, usually — add the counts, then reduce and carry. 2 11/16 + 3/8: 3/8 is 6/16, so 11 + 6 = 17 sixteenths, which is one whole and 1/16. The answer is 3 1/16. Subtraction works the same way, borrowing a whole inch (16 sixteenths) when the top fraction is smaller.
What is 0.5 feet in inches?: Six inches — decimal feet are tenths of a foot, not inches, and the two get mixed up constantly. Multiply the decimal part by 12: 14.142 ft is 14 ft and 0.142 × 12 = 1.7 in, so 14 ft 1 11/16 in on the tape. A 9.1 typed into a calculator as inches is nine and a tenth inches, not nine foot one.

Mistakes to catchbefore you trust the number

Adding fractions with different bottoms by adding the tops: ⅜ + ¼ is ⅝, not 4/12 — bring both to sixteenths first.
Reading a decimal-feet survey figure as feet and inches: 12.5 ft is 12 ft 6 in, not 12 ft 5 in.
Dividing by 25 instead of 25.4 for a quick mm-to-inch — the error is 1.6%, which is ¼ in on a 16 in run.

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