Every number on this site comes from a formula you can check. Here they all are in one place.
Units and conversions
| 1 cubic yard | 27 cubic feet (0.7646 m³) |
| 1 cubic foot | 1,728 cubic inches |
| 1 square yard | 9 square feet |
| 1 acre | 43,560 square feet |
| 1 square meter | 10.764 square feet |
Rounding rules
Anything sold in whole units (bags, boxes, sheets, quarts) is rounded up, because being one bag short means another trip. Tiny floating-point errors are ignored, so exactly 27 cubic feet is 45 bags of 0.60 cu ft, not 46. Bulk mulch and soil are rounded up to the nearest ½ cubic yard, gravel to the nearest ½ ton and ready-mix concrete to the nearest ¼ cubic yard. Paint needing three or more extra quarts becomes one more gallon.
Constants and defaults
| Value | Used for | Why |
|---|---|---|
| 0.60 / 0.45 / 0.30 cu ft | 80 / 60 / 40 lb concrete bags | Yield printed on standard concrete-mix bags |
| 21 sq ft per door, 15 per window | Paint and drywall | A 3 × 7 ft door and 3 × 5 ft window |
| 350 sq ft per gallon (editable) | Paint | Middle of the 250–400 range on most wall-paint labels |
| 1.4 tons per cu yd (editable) | Gravel weight | Common middle value; real stone ranges about 1.3–1.7 |
| 1.1 tons per cu yd (editable) | Topsoil weight | Typical topsoil ranges about 1.0–1.3 depending on moisture |
| 10% (editable) | Concrete, flooring, drywall waste | Common starting allowance for spills, cuts and offcuts |
| 3.5 in, 5.5 in, 2.375 in | 4×4, 6×6 and round-post displacement | Actual dimensions of nominal lumber and standard fence post |
What these estimates don’t know
Calculators can’t see your site. They don’t account for uneven ground, settling and compaction, structural requirements, or brand-specific yields. For anything structural (footings, load-bearing slabs, retaining walls, driveways), follow local building code and a qualified professional. When a product label disagrees with a default here, trust the label and change the setting.
Testing
The calculation engine is covered by automated tests, including edge cases such as zero areas, exact multiples of a bag size, openings larger than walls and posts wider than their holes.
Spotted a mistake? Email hello@howmuchtobuy.com and it will be checked and fixed.