Methodology
How BuildCalculators works out its numbers — every constant, every formula and every rounding rule, published so you can check any result yourself.
Most calculator sites give you a number and no way to audit it. If it looks wrong, you cannot tell whether you mistyped a dimension, the tool assumed a different density, or it rounded in its own favour.
BuildCalculators publishes the lot. The table below is generated from the same module the calculators import, so it cannot drift out of date — if a value changes in the code, it changes here on the next build.
Every constant we use
| Constant | Value | Used in | Source |
|---|---|---|---|
| Cubic feet per cubic yard | 27 | Every volume calculator | Definition — 3 × 3 × 3 ft |
| Compacted hot-mix asphalt density | 145 lb/ft³ | Asphalt tonnage and cost | Asphalt Institute MS-2, typical compacted HMA |
| Asphalt tonnage factor | 0.006042 | tons = ft² × depth(in) × factor | Derived: 145 ÷ 12 ÷ 2,000 |
| Concrete yield, 80 lb bag | 0.6 ft³ (45/yd³) | Concrete bag counts | Manufacturer published yield (Quikrete, Sakrete) |
| Concrete yield, 60 lb bag | 0.45 ft³ (60/yd³) | Concrete bag counts | Manufacturer published yield |
| Concrete yield, 50 lb bag | 0.375 ft³ (72/yd³) | Concrete bag counts | Manufacturer published yield |
| Loose aggregate density | 1.4 tons/yd³ (2800 lb) | Gravel, crushed stone, road base | Typical loose bulk density, dry |
| Roofing square | 100 ft² = 3 bundles | Shingle counts | NRCA Roofing Manual; standard trade unit |
| Roof pitch multiplier, 6/12 | 1.118034 | Footprint → sloped roof area | Derived: √(rise² + 144) ÷ 12 |
| Crusher run density | 1.5 tons/yd³ (3000 lb) | Crusher run tonnage | Typical quarry conversion; fines raise it above clean gravel |
| Crushed concrete density | 1.3 tons/yd³ (2600 lb) | Recycled concrete tonnage | Typical RCA loose density — lighter than virgin stone |
| Rip rap placed density | 100 lb/ft³ placed | Rip rap tonnage and coverage | ~165 lb/ft³ solid rock at ~60% packing (DOT practice) |
| Decomposed granite density | 1.5 tons/yd³ (3000 lb) | DG tonnage | Typical loose DG — granite fines pack densely |
| Pea gravel density and bag | 1.4 tons/yd³; 0.5 ft³/bag | Pea gravel tons and bag counts | Clean rounded gravel; standard 0.5 ft³ retail bag |
| Stone dust density | 1.35 tons/yd³ (2700 lb) | Stone dust / screenings tonnage | Typical quarry screenings, loose |
| Fill dirt density | 1.25 tons/yd³ (2500 lb) | Fill dirt weight and grading orders | Typical unscreened subsoil, dry |
| Asphalt millings density | 1.4 tons/yd³ (2800 lb) | Recycled asphalt (RAP) tonnage | Crushed RAP, loose — matches clean gravel |
| Shed cost, kit installed | $25–$60/ft² | Shed cost calculator | 2026 US kit and prefab installed ranges |
| Shed cost, custom built | $60–$150/ft² | Shed cost calculator | 2026 US site-built carpentry ranges |
| Addition cost, room | $200–$400/ft² | House addition cost calculator | 2026 US single-storey addition, installed |
| Addition cost, second storey | $250–$500/ft² | House addition cost calculator | 2026 US second-storey addition, installed |
| Stucco base coat coverage | 25 ft² per 80 lb bag at 3/8 in | Stucco bag counts | Manufacturer published coverage (Quikrete, Sakrete) |
| Stucco finish coat coverage | 70 ft² per 80 lb bag at 1/8 in | Stucco bag counts | Manufacturer published coverage |
| Metal lath sheet coverage | 16 ft² usable per 27 in × 8 ft sheet | Three-coat stucco lath counts | 18 ft² sheet less standard edge and end overlap |
| Loose topsoil density | 2100 lb/yd³ (1.05 tons) | Topsoil weight and truckload estimates | Typical screened topsoil, moderate moisture |
| Topsoil bag volume | 0.75 ft³ (36/yd³) | Topsoil bag counts | Standard 40 lb retail bag, printed volume |
| Cold patch bag yield | 0.45 ft³ per 50 lb bag | Asphalt calculator, cold patch mode | Manufacturer published coverage (Quikrete, Sakrete) |
| Feet per metre | 3.280840 | Metric toggle on every calculator | Definition — 1 m = 3.280839895 ft |
Full reference list on the sources page.
How constants are guarded
Every value above is asserted in an automated test that runs on every build. The expected figure is written out independently of the source code — so changing a constant fails the build unless the test is updated in the same commit. That makes a change a deliberate act rather than something that slips through.
The tests also assert worked examples end to end. A 100 × 20 ft driveway at 3 inches must come out at 36.25 tons; a 24 × 24 ft slab at 4 inches must come out at 7.11 cubic yards. If a refactor breaks the arithmetic, the build stops.
Rounding
- Volumes and areas — two decimal places.
- Bag and unit counts — always rounded up. You cannot buy 0.4 of a bag.
- Costs — whole dollars. Cents on an estimate are false precision.
- Intermediate steps are never rounded. Rounding happens once, at display. Rounding mid-calculation compounds error.
Every displayed number passes through a single formatting module, so rounding behaves identically across every calculator.
Waste allowances
Waste is a field you control, not a hidden multiplier. You see the raw geometric volume and the order volume side by side, so you always know which number is which. Defaults are 10% for concrete and aggregate, which suits a flat prepared subgrade — hand-dug trenches and irregular forms need more.
Cost data
Cost figures are national US averages, stated as ranges with the assumption shown. They move with material markets, and regional labour rates, haul distance and site access routinely shift an installed price by 30% or more. They are a planning range, not a quote.
What these tools will not do
They will not size a structural element. Footings, retaining walls, lintels and anything carrying load need a qualified engineer and a check against your local building code — a volume calculator tells you how much concrete fills a form, not whether the form is the right size.
They also will not replace a site visit or a written quote. Access, drainage, existing subgrade and disposal are decided by conditions no calculator can see.
Corrections
If a number here is wrong, we fix it and log it publicly on the changelog. Getting it right beats being consistent with what we have already published. Report an error via the contact page — include the calculator and the inputs you used.
Methodology FAQ
Where do BuildCalculators constants come from?
Published industry sources — Asphalt Institute MS-2 for compacted asphalt density, manufacturer bag yields for concrete, the NRCA Roofing Manual for roofing squares, and ASTM specifications for aggregates. Every one is listed on this page with its origin.
How do you stop a constant from being changed by accident?
Each value is asserted in an automated test that runs on every build, with the expected figure written out independently of the source code. Changing a constant fails the build unless the test is updated at the same time, which makes it a deliberate act rather than a slip.
How do you round results?
Volumes and areas to two decimal places, bag and unit counts always rounded up (you cannot buy 0.4 of a bag), and costs to whole dollars. Every displayed number passes through one formatting module, so rounding is consistent across every calculator.
Are the cost figures accurate for my area?
They are national US averages presented as ranges. Regional labour rates, haul distance from the plant and site access routinely move an installed price by 30% or more, so treat them as a planning range rather than a quote.