Construction math guides with worked examples
The formulas behind the job, from stair stringer length to bags of concrete for a slab, each with a worked example you can check on your own tape. Every number on these pages was computed again by a program.
Stairs, decks and fences
- Stair calculatorHow to work out stair rise and run, tread depth, the number of risers and the stringer length, with a worked example for deck stairs and stairs with a landing.
- Deck baluster spacingHow to work out how many balusters a deck railing needs and the gap between them, using the 4 inch rule, with a worked example.
- Deck stairs calculatorHow to work out deck stairs: the rise from the deck to the ground, the number of steps, the riser height, the tread, the stringer length and how many stringers, with a worked example.
- Deck board calculatorHow to work out how many deck boards, deck screws or hidden clips a deck needs, from the deck size, board width, gap, board length and joist spacing, with a worked example.
- Fence calculatorHow to work out the posts, rails and pickets for a run of fence, the real post spacing, and the concrete for each post hole, with a worked example for 100 ft of fence.
Roofs and rafters
- Roof pitch and raftersHow to calculate roof pitch, roof slope and angle, common rafter length, roof square footage and the number of shingle bundles, with a worked 6/12 example.
- Rafter length and birdsmouthHow to work out a common rafter for a gable roof from the span: run, ridge height, the take-off for the ridge board, the tail for the overhang and the birdsmouth heel and seat, with a worked 6/12 example.
- Roof pitch chartA roof pitch chart from 1/12 to 12/12 with the angle in degrees, the slope factor and the rafter length per foot of run, plus how to turn degrees back into a pitch.
Measuring and converting
- Feet and inches calculatorHow to add feet and inches with fractions, convert inches to feet and decimal feet, and find square feet from feet and inches, with worked examples.
- Inch fraction chartA chart of inch fractions in sixteenths with their decimal and millimeter values, how to read a tape measure, and how to turn decimals and millimeters back into the nearest fraction.
Concrete and materials
- Concrete calculatorHow to calculate concrete for a slab in cubic yards, how many 80 lb, 60 lb or 40 lb bags you need, and how much to add for waste, with a worked example.
- Board foot calculatorHow to calculate board feet for lumber and hardwood, nominal and actual sizes, and linear feet versus board feet, with worked examples.
- Drywall calculatorHow to work out how many sheets of drywall a wall, a ceiling or a whole room needs, in 4x8, 4x10 or 4x12 sheets, with waste and a worked example.
- Stud calculatorHow to work out how many studs a wall needs at 12, 16, 19.2 or 24 in on center, plus the plates and the extra studs for corners and openings, with worked examples.
Cutting, layout and conduit
- Cut list calculatorHow to build a cut list with the saw kerf, fit the cuts on as few boards as possible and cut sheets of plywood, with a worked example.
- Conduit offset bendsConduit offset multipliers and shrink for 10, 22.5, 30, 45 and 60 degree bends, three-point saddles and stub-ups, with worked examples for EMT.
How this page was made
Written with the help of an AI assistant (Claude, by Anthropic). Every worked example and every chart was computed again by a program and checked against its formula. Building codes differ by place: check yours before you build.