How we calculate
Every result comes from one engine with tested formulas, exact unit conversions and cited data. Here's what it does, step by step.
Units and precision
All math happens in meters, cubic meters and kilograms, and converts to your units only for display. Conversions are exact by definition: 1 in = 0.0254 m, 1 ft = 0.3048 m, 1 yd³ = 27 ft³ = 0.764554857984 m³, and 1 lb = 0.45359237 kg.
Numbers are never rounded in the middle of a calculation. Rounding happens only for display and for the two things you buy: the ready-mix order and the bag count, which always round up.
You can type dimensions as 10' 6", 10 ft 6 in, 10.5, 3 1/2, 3½, 0.5 m, 50 cm or 120 mm. A unit you type overrides the field's default. Text that can't be read shows an error and is never treated as zero.
Formulas
Each shape's volume, as used by the calculator. Parts set to subtract are taken away from the total; if the subtractions are larger than the additions, the calculator shows an error rather than a negative volume.
| Shape | Volume |
|---|---|
| Rectangular slab | Length × width × thickness. |
| Slab with thickened edge | Slab (length × width × thickness) plus the perimeter strip below it (strip area × extra depth). The strip area is the full area minus the inner area, so corners are counted once. |
| Circular slab | π × (diameter ÷ 2)² × thickness. |
| Slab by area | Area × thickness. |
| Triangular slab | ½ × base × height × thickness. |
| Round column or tube | π × (diameter ÷ 2)² × height × quantity. |
| Square column | Width × depth × height × quantity. |
| Hollow ring | π ÷ 4 × (outer diameter² − inner diameter²) × height × quantity. |
| Strip footing | Length × width × depth. |
| Pad footing | Length × width × depth × quantity. |
| Bell footing | Shaft (π × shaft radius² × shaft height) plus bell (π × bell height ÷ 3 × (R² + R × r + r²), where R is the bell base radius and r the shaft radius), times quantity. |
| Wall | (Length × height − the area of every opening) × thickness. |
| Stairs | Width × run × rise × (1 + 2 + … + number of steps), because each step is a solid block from the ground up. A top landing adds landing depth × width × total height. |
| Post hole | (Hole area − post area) × (hole depth − gravel base) × number of posts. Hole area is π × (diameter ÷ 2)²; a square post's area is width², a round post's is π × (diameter ÷ 2)². |
| Curb and gutter | Cross-section (curb depth × (curb height + flag thickness) + gutter width × flag thickness) × length. |
Waste allowance
The National Ready Mixed Concrete Association suggests ordering 4% to 10% more than the volume calculated from plan dimensions. Our defaults sit inside that range. They are our design choice, not a published figure:
- 10% for shapes poured against the ground (slabs, footings, post holes, curbs), where the base is rarely perfectly level.
- 5% for formed shapes (columns, tubes, rings, walls, stairs), where the forms hold the size.
A project with several parts uses the largest default among the parts that add concrete. You can set any value from 0 to 20%.
| Shape | Default |
|---|---|
| Rectangular slab | 10% |
| Slab with thickened edge | 10% |
| Circular slab | 10% |
| Slab by area | 10% |
| Triangular slab | 10% |
| Round column or tube | 5% |
| Square column | 5% |
| Hollow ring | 5% |
| Strip footing | 10% |
| Pad footing | 10% |
| Bell footing | 10% |
| Wall | 5% |
| Stairs | 5% |
| Post hole | 10% |
| Curb and gutter | 10% |
Order quantity, bags and trucks
Order: volume × (1 + waste), rounded up to the supplier's increment. The default is 0.25 yd³ (US) or 0.1 m³ (metric), and you can change it. This is a design choice; ask your supplier what they sell in.
Bags: volume × (1 + waste) ÷ the bag's published yield, rounded up to a whole bag. Bag weight is the number of bags × the bag's labeled weight. Trips are that weight ÷ the payload you enter, rounded up.
Trucks: order ÷ truck capacity, rounded up. The default capacity is 10 yd³ (7 m³), the midpoint of NRMCA's 8 to 12 yd³ (5 to 9 m³) range.
Weight of poured concrete: order volume × 2,400 kg/m³ (150 lb/ft³), the upper end of NRMCA's 140 to 150lb/ft³ range for normal-weight concrete, so weights err high.
Cost: bags × your bag price, against order × your ready-mix price plus the fees you enter. We never show default prices.
Big-pour note: shown at 2 yd³ or 1.5 m³ and above. This threshold is our design choice.
Warnings
Warnings never stop the calculation. Each one comes from a cited source:
- Slabs thinner than 3.5 in: the IRC minimum for slab-on-ground floors.
- Slabs thinner than 2 in: the minimum thickness on the QUIKRETE data sheet.
- Post holes narrower than 3× the post, or shallower than 1/3 of the post's above-ground height plus 6 in: QUIKRETE's post-setting guidance.
Cross-checks against manufacturers
We compare the engine with figures manufacturers publish themselves. These checks also run as automated tests on every build.
| Scenario | Product | Published | Ours (exact) | Ours (to buy) | Result |
|---|---|---|---|---|---|
| 80-lb bags in 1 yd³ | SAKRETE High-Strength 80 lb | “It takes 45 bags to make 1 cubic yard” | 45 | 45 | Matches |
| 60-lb bags in 1 yd³ | QUIKRETE Concrete Mix 60 lb | 0.45 ft³ per bag (27 ÷ 0.45 = 60) | 60 | 60 | Matches |
| 40-lb bags in 1 yd³ | QUIKRETE Concrete Mix 40 lb | 0.30 ft³ per bag (27 ÷ 0.30 = 90) | 90 | 90 | Matches |
| 20 kg bags in 1 m³ | Australian Builders High Strength 20 kg | “Approximately 100 bags … produce 1m³” | 100 | 100 | Matches |
| 20 kg bags in 1 m³ | Easy Mix Super Strength 20 kg | “108 x 20kg Bags = 1.0m3 (approx)”, and also “9 litres” per bag | 111.11 | 112 | 4 more bags than the sheet's 108: we use its stated 9 L yield, the safer reading |
Limits
Results are estimates for buying concrete. They don't size footings, slabs or walls for loads, and they don't account for reinforcement taking up space. Confirm quantities with your supplier, and consult a professional for structural work.
Sources
- QUIKRETE Concrete Mix No. 1101 product data sheet, retrieved October 2, 2026
- QUIKRETE 5000 High Early Strength Concrete Mix No. 1007 product data sheet, retrieved October 2, 2026
- SAKRETE High-Strength Concrete Mix technical data sheet, retrieved October 2, 2026
- Easy Mix Super Strength Concrete technical data sheet, retrieved October 2, 2026
- Australian Builders High Strength Concrete Mix product data sheet (ICL), stated as about 100 bags per cubic metre, retrieved October 2, 2026
- NRMCA, Concrete in Practice 31: Ordering Ready Mixed Concrete, retrieved October 2, 2026
- NRMCA, Concrete in Practice 36: Structural Lightweight Concrete, retrieved October 2, 2026
- International Residential Code 2021, Section R506.1, retrieved October 2, 2026
- QUIKRETE, Setting Posts in Concrete, retrieved October 2, 2026
- Archtoolbox, Lumber Dimensions: Nominal vs Actual (based on the American Softwood Lumber Standard, PS 20), retrieved October 2, 2026
Last updated October 2, 2026. Read how we calculate, including formulas, waste defaults and rounding.