Fraction math

Fraction calculator for woodworkers

Tape measures read in fractions. Phone calculators read in decimals. Most of a woodworker’s day is spent translating between the two — and rounding errors at 1/64" stack up fast on a long part list.

The formula

a/b + c/d = (a·d + c·b) / (b·d)

Mixed numbers like 3-7/8 are converted to improper fractions (31/8) before adding, then converted back to a mixed number with the smallest power-of-two denominator that fits — halves, quarters, eighths, sixteenths, thirty-seconds, sixty-fourths. The same denominators your tape reads.

Example: 3-7/8 + 1-5/16

3-7/8 becomes 31/8, which is 62/16. 1-5/16 stays as 21/16. Sum: 83/16, which simplifies to 5-3/16". Try doing that in your head with a glove on. Dead On accepts mixed-number input straight from the keypad and renders the result in the same fraction system you measure with — never decimals.

Why it matters

Decimal calculators round at three places. 0.1875" reads as 0.19" on a cheap calculator — that’s nearly 1/64" of error per number, and over six joints it’s 1/8" of slop. Real fraction math never loses anything between the input and the output.

Step-by-step

  1. 1

    Type the first measurement

    Use mixed-number form like 3-7/8 or 1 5/16 — just the way it reads on the tape.

  2. 2

    Choose the operator

    Plus, minus, multiply, or divide. Multiplying by a count is how you add up a part list.

  3. 3

    Type the second measurement

    Same fractional form. Dead On accepts whole numbers, fractions, or mixed numbers interchangeably.

  4. 4

    Read the answer in fractions

    Output stays in the smallest denominator that’s a clean tape-readable fraction (sixteenths or smaller, depending on the inputs).

  5. 5

    Tap to convert if needed

    Decimals, millimeters, board feet — Dead On has the conversions built in next to the answer.

Skip the math. Build the diagram.

Dead On does the fraction calculator and 12 other woodworking calculators with visual diagrams, offline, on every Apple device signed into your Apple ID.

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Frequently asked questions

Why fractions instead of decimals?

Because that’s what your tape measure, ruler, and combination square read. Translating to decimals introduces rounding error every step. Stay in fractions and the answer is exact.

Can I mix fractions and decimals?

Yes. Dead On accepts 3.875 or 3-7/8 for the same value. Output mode (fraction or decimal) is a single tap.

How does Dead On round the answer?

Down to the smallest power-of-two denominator that exactly represents the result — usually sixteenths or thirty-seconds. If a result doesn’t land cleanly, Dead On shows it as the nearest 1/64" with the residual.

Does it handle metric?

Yes. Tap the unit toggle to switch between inches and millimeters. The same calculator handles both, and conversions stay exact.

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