Reference
Calculation Methodology & Constants
Published by MyBuildCalcHub · Every figure verified against our audited estimating engine ·
Every number MyBuildCalcHub produces comes from the transparent constants and formulas on this page. We publish them in full so you can verify any result by hand. The values are the industry-standard figures printed on material bags and quoted by quarries; where a convention traces to a standards body such as the American Concrete Institute (ACI) or ASTM International, we follow it, and we note manufacturer-printed values as such rather than implying a certification we don't hold.
Core volume constant
All volume math resolves through a single identity: a cubic yard is a 3×3×3 ft cube, so 1 cubic yard = 27 cubic feet. Depths given in inches are divided by 12 to feet before multiplying, then the cubic-foot result is divided by 27 for cubic yards.
Concrete bag yields
Bag yields are the mixed-volume figures printed on major-brand bags, consistent across manufacturers:
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |
| 40 kg | 0.0187268 m³ | 54 per m³ |
| 25 kg | 0.0117043 m³ | 86 per m³ |
| 20 kg | 0.0093634 m³ | 107 per m³ |
Aggregate densities
Tonnage uses these planning densities. Real-world density varies with moisture and quarry source, so a supplier's stated figure always takes precedence, the calculator's custom-density field accepts it directly.
| Material | US tons per yd³ | Tonnes per m³ |
|---|---|---|
| Crushed gravel / pea gravel | 1.40 | 1.66 |
| Decomposed granite | 1.40 | 1.66 |
| Crushed limestone | 1.45 | 1.72 |
| Dry sand | 1.35 | 1.60 |
| Wet sand | 1.50 | 1.78 |
Coverage & mulch
Slab coverage per cubic yard is 324 ÷ thickness in inches square feet (metric: 100 ÷ depth in cm square metres per cubic metre). Mulch and topsoil bag sizes: 1.5, 2, and 3 cubic feet (imperial); 40, 50, and 70 litres (metric).
Waste allowances
Default buffers, added on top of exact geometry: concrete 10% (a short pour risks a cold joint), aggregates and mulch 5% (settling and edge loss). Both are user-adjustable on every tool.
Exact conversion constants
| Conversion | Exact factor |
|---|---|
| Foot to metre | 0.3048 (exact) |
| Cubic foot to cubic metre | 0.028316846592 |
| US short ton to tonne | 0.90718474 |
| Pound to kilogram | 0.45359237 |
Fence-post volume
Post-setting concrete subtracts the post from the hole: π × (hole diameter ÷ 24)² × depth − (post width ÷ 12)² × depth, in feet. Skipping the subtraction over-orders by roughly 15–20%.
How we verify
Every published figure, in these tables, in each calculator, and in every guide, is checked against the same audited math engine that powers the tools, so the numbers you read and the numbers the calculator returns can never disagree. Results remain planning estimates: confirm structural dimensions with your local building department and final quantities with your supplier.