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Methodology

Last updated November 11, 2026.

Every calculator and table on FreeConcreteCalculator.org uses the same small, tested math library. This page lists the formulas, the defaults and where each figure comes from. Sources last checked November 11, 2026.

Formulas

Slab: Length (ft) × Width (ft) × Thickness (in) ÷ 12
Footing: Length (ft) × Width (in) ÷ 12 × Depth (in) ÷ 12 × count
Wall: Length (ft) × Height (ft) × Thickness (in) ÷ 12
Round column / Sonotube: π × (Diameter (in) ÷ 24)² × Height (ft) × count
Post hole: (π × (Hole diameter ÷ 2)² − post area) × (Hole depth − gravel) × count
Steps (solid, on grade): Width × Run × Rise × n × (n + 1) ÷ 2
With waste: volume × (1 + waste % ÷ 100)
Cubic yards = ft³ ÷ 27 · m³ = ft³ × 0.0283168
Bags = ft³ with waste ÷ yield per bag, rounded up

Metric inputs are converted exactly (1 m = 3.28084 ft, 1 in = 2.54 cm) before the same math runs. Square posts use actual dressed sizes: 3.5 in for a 4×4 and 5.5 in for a 6×6. The post is assumed to reach the bottom of the concrete; if you set it on a few inches of concrete instead, the difference is a small fraction of a bag.

In the post hole calculator the total is rounded up once for all holes. The "per hole" figure rounds up per hole, so it can add up to a little more.

Bag yields

Yields the calculator uses
BagYieldBags per yd³ / m³Source
40 lb0.30 ft³90 per yd³QUIKRETE Concrete Mix No. 1101
50 lb0.375 ft³72 per yd³QUIKRETE Concrete Mix No. 1101
60 lb0.45 ft³60 per yd³QUIKRETE Concrete Mix No. 1101
80 lb0.60 ft³45 per yd³QUIKRETE Concrete Mix No. 1101
20 kg0.008 m³125 per m³Hanson 40N Concrete (MaxiPack20)
25 kg0.010 m³100 per m³Hanson 40N Concrete (MaxiPack25)

The US yields are printed on QUIKRETE's data sheet for its Concrete Mix: "An 80 lb (36.2 kg) bag yields approximately 0.60 ft³", with 0.45 ft³ for 60 lb, 0.375 ft³ for 50 lb and 0.30 ft³ for 40 lb. Other brands of standard concrete mix publish similar figures.

Metric bag yields vary more by brand. We use Hanson's figures (0.008 m³ per 20 kg, 0.010 m³ per 25 kg) because one manufacturer lists both sizes and they are on the conservative side, so you are less likely to run short. For comparison, Easy Mix (Australia) lists 0.010 m³ per 20-kg bag and Firth's Dricon CivilCrete (New Zealand) 0.012 m³ per 25-kg bag. If your bag lists a different yield, use the Volume option and divide by your bag's figure.

Defaults

InputDefaultWhy
Waste10%Common estimating allowance for uneven bases, form bow and spillage; editable
Slab10 × 10 ft, 4 inA common small patio; 4 in is a usual patio thickness
Post hole10 in × 24 in, 4×4 postAbout 3 × the post width, per QUIKRETE's fence-post directions
Bag size80 lb (25 kg metric)Most concrete per bag; change it to see cost for another size
Concrete weight140 lb/ft³QUIKRETE Concrete Mix typical unit weight (ASTM C138); used only for the slab-weight notes

Prices

We don't publish concrete prices or averages; they vary too much by region, season and supplier. The optional cost fields multiply your own price per yard (or m³) and per bag by the quantity. Ready-mix minimums, short-load fees, delivery and waiting time are not included; ask your supplier.

Testing

The math library has automated unit tests for every shape, the unit conversions, waste, bag rounding, costs and shareable links. Checks against outside figures:

Limits

Results are estimates. Real bases are uneven, holes are rarely perfect cylinders, and some concrete stays in the mixer. Specialty mixes (fast-setting, high-strength, fiber) may yield differently from standard concrete mix. Spot a problem? Tell us.

Sources