Why we built this
Estimating shouldn't mean a calculator app, a spreadsheet, and a notebook on the tailgate. GritCalc puts the everyday material math — yards, bags, squares, bundles, gallons, sheets — in one place, in the US units suppliers actually sell in.
How we calculate
Every tool uses the standard formulas tradespeople and estimators rely on, in US units. The table below documents the exact formula and the key assumptions behind each calculator, so you can see precisely what the result represents. Where a method is a rough rule of thumb (HVAC sizing) or safety-critical (wire size), we say so on the tool and point you to the proper method or a licensed professional.
| Calculator | Core formula | Key assumptions |
|---|---|---|
| Concrete | Length × Width × (Thickness ÷ 12) ÷ 27 = cubic yards | Bag yields: 80 lb ≈ 0.60, 60 lb ≈ 0.45, 40 lb ≈ 0.30 cu ft. Add 5–10% waste. |
| Paint | (Wall area − openings) × coats ÷ coverage = gallons | Coverage ≈ 350–400 sq ft per gallon per coat on sealed drywall. |
| Roofing | Footprint × pitch multiplier ÷ 100 = squares; × 3 = bundles | Pitch multiplier = √(1 + (rise/12)²). 3 bundles per square; +10% waste. |
| Gravel | Area × (Depth ÷ 12) ÷ 27 = cubic yards; × density = tons | Crushed stone ≈ 1.4 tons per cubic yard (dry). |
| Drywall | (Wall + ceiling area) ÷ sheet area × 1.10 = sheets | Standard 4×8 sheet = 32 sq ft. 10% waste allowance. |
| Flooring | Area × (1 + waste) ÷ box coverage = boxes | Waste 5% straight, 10% diagonal, 15% herringbone. |
| Tile | Area ÷ tile area × (1 + waste) = tiles | Thinset coverage varies with trowel notch size. |
| Mulch | Area × (Depth ÷ 12) ÷ 27 = cubic yards; ÷ 2 cu ft = bags | 1 cu yd covers 162 sq ft at 2 in deep. |
| Stairs | Risers = total rise ÷ target riser; stringer = √(rise² + run²) | IRC R311.7: max riser 7¾ in, min tread 10 in. |
| Board foot | (Thickness-in × Width-in × Length-ft) ÷ 12 × qty | Uses nominal lumber dimensions; 1 bd ft = 144 cu in. |
| HVAC BTU | Area × 20 × (Ceiling ÷ 8) + (Occupants − 2) × 600 + kitchen | Rule of thumb only; 12,000 BTU = 1 ton. Kitchen adds 4,000 BTU. True sizing uses ACCA Manual J. |
| Wire size | Circular mils = (2 × K × A × dist) ÷ (V × VD%); take larger of voltage-drop and ampacity gauge | K = 12.9 Cu / 21.2 Al. Ampacity per NEC Table 310.16 (75°C) with 240.4(D) small-conductor caps (#14→15 A, #12→20 A, #10→30 A). |
The calculators run entirely in your browser in plain JavaScript. Nothing you type is sent to a server or stored — your numbers stay on your device.
Sources & how we keep this accurate
Formulas and constants are drawn from published industry standards and manufacturer data: the International Residential Code (IRC) for stairs, the National Electrical Code (NFPA 70) Table 310.16 and 240.4(D) for wire, ACCA Manual J and Energy Star sizing guidance for HVAC, and standard manufacturer bag yields and coverage rates for concrete, mulch, paint, and tile. Each calculator carries its own cited source and a review date. We review the formulas and constants periodically and welcome corrections via the contact page.
Methodology last reviewed June 2026
What this is not
GritCalc gives educational estimates to help you scope and order materials. It is not engineering, code compliance, or professional advice. Always verify against manufacturer specifications, comparable measurements, and your local building code before ordering or building.
Get in touch
Want a calculator we don't have yet, or spot a number that looks off? Tell us on the contact page.