Feet and Inches Calculator

How to use this calculator

1. Type measurements as you would read them: 10' 6 3/8" — feet with ' and inches with ".
2. Bare numbers are treated as inches (6 1/2 = 6½ in).
3. Choose add, subtract, multiply or divide; for × and ÷ the second box is a plain number.
4. Read the result in feet-inches-fractions, decimal feet, inches or millimetres.

Method & assumptions

Inches = Feet × 12 + Inches + Numerator ÷ Denominator
Result fraction = round(inches × 16) ÷ 16, reduced
1 in = 25.4 mm (exact)

1/16″

≈ 1.59 mm — the usual tape-measure resolution

Assumptions & limits

• Rounding to 1/16 in happens only for display; decimal results are exact.

Rounding: purchase quantities are always rounded up to whole bags, boxes, sheets or bundles. See our methodology.

Worked examples

In depth: feet & inches planning notes

Working with tape-measure fractions

Imperial tape measures are marked in feet, inches and fractions of an inch — usually halves, quarters, eighths and sixteenths. Adding 5′ 7⅜″ to 3′ 9¾″ by hand means converting the fractions to a common denominator, carrying inches into feet and simplifying the result. The calculator does this for you: it converts every value to inches, does the arithmetic and converts back, rounding to the nearest 1/16 in.

Inputs are flexible. You can type 5' 7 3/8", 5ft 7-3/8in, 67.375 or 5.6146 ft. Results are shown in feet-and-inches, decimal feet, decimal inches and millimetres, so you can move between plans, tools and metric product specifications.

Dividing lengths and practical precision

Dividing a length is common on site — spacing balusters evenly, dividing a wall for shelves or finding the centre of a board. Divide the length by the number of spaces, not the number of objects: five equal shelves in a cabinet need six spaces if you include top and bottom. When the result does not land on a tape mark, the calculator rounds to the nearest sixteenth, which is about 1.6 mm.

Rounding errors accumulate. If you mark 12 balusters by repeatedly measuring a rounded spacing, the error multiplies; it is better to mark cumulative distances from a single starting point. A story stick — a strip of wood marked with the spacing — avoids the problem entirely.

For metric work there is no conversion problem to solve: 1,712 mm + 1,162 mm is simply 2,874 mm. One inch is exactly 25.4 mm by definition, so conversions in this calculator are exact before rounding.

Common fraction pitfalls

The most frequent tape-measure mistake is reading the wrong tick: on many tapes the 1/16 marks are shortest, 1/8 slightly longer, 1/4 longer again and 1/2 the longest below the inch. Count from the nearest whole inch rather than from the last foot mark. Another trap is decimal feet: 0.5 ft is 6 in, but 0.1 ft is 1.2 in, not 1 in. Surveyors and some plans use decimal feet; carpenters use inches and fractions. This calculator shows both so you can move between them without mistakes.

Subtracting is where errors creep in on site — for example deducting the blade kerf (often ⅛ in) from each cut. Enter each piece as a separate subtraction to see the remaining stock.

Tips & common mistakes

• Always include saw kerf when dividing a board into pieces.
• Write measurements as you measure — 10′ 6⅜″ — rather than converting in your head.
• Measure twice from the same reference edge.

Sources

1. NIST – the international inch (25.4 mm exactly) — checked 2026-10; verify before launch
2. NIST SP 811 – unit conversion — checked 2026-10; verify before launch
3. UK NPL – imperial units — checked 2026-10; verify before launch

Default values (bag yields, coverage rates, densities) are typical figures from manufacturer literature. Always check the label on the product you buy.

Cite or embed this calculator

DIYCalcLab. “Feet and Inches Calculator.” Oct 18, 2026. https://diycalclab.com/conversions/feet-and-inches-calculator/

An embeddable version for blogs and contractor sites is planned.