The best construction math reference is the one you stopped carrying around 2014. That dog-eared laminated card from the supply house. The one with coffee rings and concrete dust ground into the corners.
Everything since then has been worse. Government PDFs nobody can read on a phone. $7 quick-cards from Amazon that cover half of what you need. Gated downloads that want your email before they'll show you how to calculate board feet.
This is the replacement. Every formula a working tradesperson reaches for across 10 trades, organized the way work actually happens — by what you're building, not by what chapter you're in.
Framing
(Wall length in inches ÷ OC spacing) + 1
Example: 12-foot wall at 16" OC: (144 ÷ 16) + 1 = 10 studs
(Thickness × Width × Length in feet) ÷ 12
Example: A 2×8 that's 10 feet long: (2 × 8 × 10) ÷ 12 = 13.33 board feet
√(Rise² + Run²)
Example: 6/12 pitch, 12-foot run: rafter table gives 13.42" per foot → 12 × 13.42 = 161" = 13'-5"
Total rise ÷ number of risers = individual rise
Example: 108" total rise (9' ceiling): 108 ÷ 15 risers = 7.2" rise per step
Concrete
(L × W × Depth in feet) ÷ 27
Example: 20×20 slab at 4": (20 × 20 × 0.333) ÷ 27 = 4.94 cubic yards. Order 5.5.
Volume in cubic feet ÷ yield per bag
Example: 80-lb QUIKRETE yields 0.6 cu ft. A 2'×2' footer pad at 4": 1.33 ÷ 0.6 = 2.2 bags. Buy 3.
A 12"-diameter sono tube, 48" deep needs about 5 bags of 80-lb mix. Four footings = 20 bags. At $7 per bag, a ready-mix truck starts making financial sense around 8-10 sono tubes.
Roofing
Building footprint area × slope factor
Example: 30×40 building with 6/12 pitch: 1,200 × 1.118 = 1,342 sq ft of roof area
Roof area ÷ 100 = squares. Each square = 3 bundles.
Example: 1,342 sq ft = 13.42 squares = 41 bundles. Add 10-15% for waste.
Tile and Flooring
Floor area ÷ tile area × waste factor
Example: 60 sq ft bathroom with 12"×12" tiles: 60 × 1.10 = 66 tiles
(Length × Width in feet) ÷ 9
Example: 12×15 room: 180 ÷ 9 = 20 square yards. Add 10% for seams.
Electrical
15A → 14 AWG (lighting, general outlets) | 20A → 12 AWG (kitchen, bathroom) | 30A → 10 AWG (dryer) | 40A → 8 AWG (range) | 50A → 6 AWG (sub-panel) | 60A → 4 AWG (EV charger). Copper, standard runs under 100 feet. Longer runs upsize for voltage drop. Aluminum needs two sizes larger.
Total wire cross-section ≤ 40% of conduit internal area (3+ wires per NEC)
Example: 1/2" EMT: max 9 #12 THHN wires. 3/4" EMT: max 16. 1" EMT: max 26.
Painting
Perimeter × ceiling height − openings
Example: 12×14 room, 8' ceilings: 52 × 8 = 416 sq ft − 51 sq ft openings = 365 sq ft
Paintable area ÷ 375 sq ft per gallon
Example: 365 sq ft ÷ 375 = ~1 gallon per coat. Two coats = 2 gallons.
Drywall
Total area ÷ sheet area (32 sq ft for 4×8)
Example: 12×14 room: 584 sq ft total → 19 sheets of 4×8 + 10% waste
Plumbing
Run in feet × slope per foot = total drop
Example: 20' run of 2" pipe: 20 × 0.25 = 5" total drop needed
Masonry
Standard modular brick: 6.75 bricks per sq ft
Example: 120 sq ft accent wall: 120 × 6.75 = 810 bricks + 5-10% waste
The Pattern
Every formula on this page takes inputs you can measure or look up, runs them through multiplication and division, and gives you a number you can take to the supply house.
The math isn't hard. The problem is remembering which formula goes with which situation when you're standing in a gutted kitchen at 6 AM trying to figure out how much thinset to order before the supply house opens at 7.
That's why we built Journeyman Voice Calcs. Say "how many studs for a 14-foot wall at 16 on center" and get the answer in fractions, ready for the tape. 155 formulas across 18 trades. No buttons. No typing. No putting your phone down to wipe your hands first.
Every formula on this page — and 120+ more — works by voice. And the "Show the Math" button breaks down every calculation step by step, so you're not just getting answers. You're seeing the formula work.