Feet-Inch-Fraction Calculator for Carpentry

Woodshop Construction Math

Skip the phone calculator gymnastics when you need to add story poles, subtract door reveal, or turn a roof pitch into a common rafter length. This calculator is tuned for cabinet shops and site carpentry: feet-inch-fraction inputs, right-triangle solutions for stairs and rafters, regular-plan hip helpers, and metric conversion when a plan lands in millimeters.

Use the Math tab to chain lengths the way you call them on a tape — add stock lengths for a glue-up, subtract hinge overlay from a finished opening, or divide a run into equal stile spacings. Results stay in feet and inches to the nearest sixteenth so you can mark the board without converting to decimals first.

How to use each tab

1. Math — Enter two feet-inch values and add, subtract, multiply by a count, or divide. Ideal for cut lists, reveal math, and totaling random-width boards.
2. Triangle — Fill any two of rise, run, or diagonal to solve the third. Use it for stair stringers, brace lengths, and common rafters when pitch is known as rise and run.
3. Roof — Enter common run and pitch (rise per 12″ of run). The tool returns theoretical common and hip lengths for a regular square plan (hip plan run = common run × √2).
4. Convert — Move between imperial and metric when hardware specs or imported plans use millimeters.

Input formats that work on the bench

The parser accepts the notations carpenters actually type:

• 10' 6 1/2" — standard feet and inches with fraction
• 10-6 1/2 — dash notation from many cut lists
• 10 6 1/2 — space-separated lazy entry
• 126.5 — raw decimal inches from a digital caliper or CAD export

For a fraction of an inch alone, use 0 3/4 or 0.75. A bare 3/4 is also read as three-quarters of an inch.

Framing and shop tips

Match finished vs rough dimensions to the job: cut lists usually want rough stock; reveal and overlay math should use finished openings. Roof helpers assume a regular 45° plan hip — always confirm birdsmouth and seat cuts against your framing square or layout software for irregular roofs. For unequal metal-building valleys, switch to StructLogic’s Hip & Valley calculator instead of this roof tab.

FAQ