Estimating formulas for common home projects, each with a worked example that matches the matching calculator. They give exact quantities; real jobs need extra for waste, cuts and uneven ground, so add 5 to 10% (more for tile with diagonal cuts) and round up to whole bags, sheets or boxes.
Formulas
| Quantity | Formula | Example |
|---|---|---|
| Concrete, gravel or soil volume | yd³ = length × width × depth(in) / 12 / 27 (feet) |
10 ft × 10 ft × 4 in slab: 1.23 yd³; with 10% extra: 1.36 yd³ |
| Mulch | same formula |
200 ft² at 2 in deep: 1.23 yd³, or 17 bags of 2 ft³ |
| Paint | gallons = (2(L + W) × H − openings) × coats / coverage |
12 × 15 ft room, 8 ft walls, 50 ft² of openings, 2 coats, 350 ft²/gal: 2.18 gal, so buy 3 |
| Drywall sheets | sheets = ceil(area / 32) for 4 × 8 ft sheets |
The same room's walls are 432 ft²: 14 sheets before openings and waste |
| Board feet | bf = thickness(in) × width(in) × length(ft) / 12 |
1 × 6 in, 10 ft board: 5 bf |
| Roof pitch to angle | angle = atan(rise / 12), area factor = √(1 + (rise/12)²) |
6/12 pitch: 26.57°, area factor 1.118 |
| Stair risers | risers = round(total rise / target rise) |
108 in rise at about 7.5 in: 14 risers of 7.714 in |
| Fence posts | posts = ceil(length / spacing) + 1 |
100 ft at 8 ft spacing: 14 posts |
| Pool volume | gallons = L × W × average depth × 7.48 |
20 × 10 ft, 5 ft average depth: 1,000 ft³, 7,480 gallons |
| Heat loss through a wall | BTU/h = area × ΔT / R; R-values in series add |
100 ft², R-13, 40 °F difference: 307.7 BTU/h |
Try them
Materials: Concrete Calculator, Gravel Calculator, Mulch Calculator, Paint Calculator, Tile Calculator, Flooring Calculator, Drywall Calculator, Roofing Calculator, Board Foot Calculator, Fence Calculator and Square Footage Calculator. Layout: Stair Calculator and Roof Pitch Calculator. Heating and water: Insulation R-Value Calculator, BTU Calculator and Pool Volume Calculator.
Rules of thumb
- Cubic yards. A cubic yard is 27 cubic feet; one yard covers
324 / depth in inchessquare feet, so about 108 ft² at 3 in deep. - Lumber sizes. A "2×4" is nominally 2 × 4 in but actually about 1.5 × 3.5 in. Board feet use the nominal size, which is how lumber is priced.
- Stairs. Common US residential limits are a rise of at most 7¾ in and a tread of at least 10 in; check your local code before building.
- Cooling. A rough room air-conditioner guide is about 20 BTU per square foot, adjusted for sun, ceilings and occupants; a "ton" of cooling is 12,000 BTU/h.