How we calculate
Every calculator follows the same rules: show the formula, round the way materials are sold, and never publish a constant without a source.
Rules that apply to every calculator
- US units first. Feet, inches, cubic yards, pounds and tons, with metric as an option where it helps.
- Rounded up to what you can buy. Blocks and pieces are whole numbers; asphalt is rounded up to a tenth of a ton; sand to a quarter of a cubic yard.
- Waste is yours to set. Each calculator starts with a suggested allowance you can change.
- No guessed prices. Costs appear only when you enter your own prices.
- No constant without a source. If a value changes too much between products or suppliers (sand density, retaining wall block sizes), the calculator asks you for it instead of assuming.
- Unit conversions don't add phantom rows. Lengths converted from meters land a hair over whole inches; anything under 1/4 in (less than a mortar joint) is ignored when counting courses and blocks per course.
Constants and their sources
| Value | Used in | Source |
|---|---|---|
| Nominal block sizes include the mortar joint (16 × 8 in standard face) | Concrete block | Angelus Block, CMU Basics & Glossary |
| Hot mix asphalt: 145 lb/ft³ | Asphalt | Iowa DOT Design Manual 1B-4, Densities for use in estimating quantities (2026 update); FHWA Central Federal Lands, Application rates and weights for estimating purposes (2009) |
| Default extra for tile: 15% | Tile | Fireclay Tile, Ordering extras: everything you need to know |
| Premixed concrete yield per bag: 40 lb 0.30 ft³, 50 lb 0.375 ft³, 60 lb 0.45 ft³, 80 lb 0.60 ft³, 90 lb 0.675 ft³ | Concrete slab | QUIKRETE, Concrete Mix No. 1101 data sheet |
| Site mix 1 : 2 : 3 (cement : sand : gravel) by volume; 6 bags of 94 lb per cubic yard (top of the maker's 5–6 range) | Concrete slab | QUIKRETE, Portland Cement data sheet |
| A 94 lb bag of cement is about 1 ft³ | Concrete slab (sand and gravel for your own mix) | Houlsby, Construction and Design of Cement Grouting (1990), p. 69, via Wikipedia: Bag (unit) |
| Area of drawn shapes: shoelace formula, exact for any shape whose sides don't cross | Sand, asphalt, tile, concrete slab | Geometry |
| 1 cubic yard = 27 cubic feet; 1 board foot = 144 cubic inches | Sand, asphalt, board foot | Unit definitions |
Testing
Every formula is a separate function with automated tests built from hand-worked examples, including edge cases like openings larger than the wall and metric input. A calculator is not published until its tests pass.
Reviews
Pages show the month they were last reviewed. We recheck sources at least once a year and whenever a reader reports a problem.