Field math for trades — enter values, get answers
Builder's Formula Lab is a free set of field calculators for common construction math: roofing, concrete, framing, electrical, plumbing, HVAC, flooring, painting, excavation, driveways & base, solar & off-grid, and general site geometry. Enter measurements, get results instantly, and read a short explanation of the formula used.
Pick a trade below, then open the calculator that matches the problem on the job.
Formula:
cu yd = (L_ft × W_ft × thickness_in ÷ 12) ÷ 27.
Divide inches of thickness by 12 to get feet, multiply volume in cubic feet, then divide by 27
(3×3×3). Add waste (often 5–10%) before ordering ready-mix.
A 6:12 pitch rises 6 inches per 12 inches of run (~26.6°).
Roof surface area ≈ plan area × √(1 + (pitch/12)²).
One roofing square = 100 square feet of roof surface.
Formula: (thickness_in × width_in × length_ft) ÷ 12.
A nominal 2×4×8 is 5.33 board feet. Suppliers often price on nominal dimensions.
Formula: fall_inches = slope_in_per_ft × run_ft.
A common minimum is ¼ in/ft (~2% grade) for many smaller drains;
confirm pipe size and local plumbing code.
Multiply length (ft) × width (ft) × thickness (ft). Convert thickness from inches by dividing by 12.
Then divide cubic feet by 27: cu yd = (L × W × (t_in ÷ 12)) ÷ 27.
Add 5–10% waste for most pours. Use the Slab / Pad Volume calculator for bags and overage.
Degrees = arctan(rise ÷ run) × (180 ÷ π). For 6:12, rise/run = 0.5 → about 26.6°. Use Roof Pitch & Slope for pitch, percent grade, and the roof-area multiplier.
Estimate floor(length_ft × 12 ÷ O.C.) + 1, then add extras for openings and corners.
Open the Wall Stud Count tool for a planning total and plate stock.
BF = (T_in × W_in × L_ft) ÷ 12. See Board Feet.
Many designs target ≤3% on the branch circuit and ≤5% feeder+branch combined.
Approximate copper single-phase: VD = 2 × 12.9 × I × D ÷ CM.
Use Voltage Drop for a quick check — not a substitute for NEC tables.
Often ¼ in/ft minimum for smaller drains; some larger pipes allow ⅛ in/ft.
fall = slope × length. Calculator: Drain Pipe Slope / Fall.
Rule of thumb only: about 20–40 BTU/h per sq ft by climate; 1 ton = 12,000 BTU/h. Not a Manual J load calc. Try Rule-of-Thumb BTU Load.
Yes. They are free browser tools for planning and education. Always verify results against local codes, product data, and licensed professionals for permitted work.
Complete list of construction formula tools. Each link opens the interactive calculator (JavaScript required for live results).