How We Calculate
Every calculator uses a published formula or a standard material yield. This page documents the numbers, assumptions and rounding so you can check the result instead of trusting a black box.
Unit conversions
| From | To | Factor |
|---|---|---|
| 1 foot | inches / metres | 12 in / 0.3048 m |
| 1 square foot | square metres | 0.092903 m² |
| 1 cubic foot | cubic metres | 0.0283168 m³ |
| 1 cubic yard | cubic feet / cubic metres | 27 cu ft / 0.764555 m³ |
| 1 short ton | pounds / kilograms | 2,000 lb / 907.185 kg |
Material yields and coverage
| Material | Assumption |
|---|---|
| 60 lb concrete bag | 0.45 cu ft |
| 80 lb concrete bag | 0.60 cu ft |
| 80 lb mortar bag | 30 standard 8 × 8 × 16 blocks |
| Interior paint | 350 sq ft per gallon |
| Asphalt shingles | 3 bundles per 100 sq ft square |
| Roofing underlayment | 400 sq ft per roll |
| Gravel | 1.4 tons per cubic yard |
| Topsoil | 1.2 tons per cubic yard |
| 2 cu ft mulch bag | 13.5 bags per cubic yard |
Default waste allowances
| Calculator | Default waste |
|---|---|
| Concrete, gravel, mulch, topsoil, sod | 5% |
| Rebar, post holes | 10% |
| Paint, tile, drywall, wallpaper, pavers, deck boards, shingles | 10% |
Rounding
- Dimensions are converted to feet before calculating.
- Areas and volumes are shown to two decimal places.
- Bags, boxes, sheets, boards and pallets are rounded up to the next whole unit.
- Costs are shown in U.S. dollars and do not include tax, delivery, equipment or labor.
What we do not model
- Soil bearing capacity, compaction, drainage and frost depth.
- Structural design, reinforcement requirements and code compliance.
- Complex shapes, multiple rooms, openings and jobsite breakage beyond the waste allowance.
- Product-specific yields, coverage, density and packaging changes.
- Local labor rates, permits, disposal fees and delivery minimums.
Updates and corrections
Pages are reviewed when a formula, product standard or price assumption changes. The updated date appears at the top of every calculator. If a number looks wrong, email hello@winwinz.us.kg with the page, the inputs and the result you expected.
Why this matters
A calculator is only useful if you can audit it. Showing the formula, the worked example and the assumptions makes it possible to catch a bad input before you order a truckload of material — and makes the estimate easier to defend to a contractor or inspector.