Concrete calculator
Concrete for slabs, strip footings, post holes, pads and columns: the volume, the cement in bags, the sand and stone, and the largest stone that fits.
Concrete calculator
How it is worked out
Per m³ of concrete, a 1 : 2 : 4 mix takes about 285 kg cement, 0.62 t sand and 1.36 t stone; a 1 : 3 : 5 mix about 220 kg cement, 0.71 t sand and 1.31 t stone. Allow 5–10 % for waste.
A post hole is the hole less the post. The largest stone is at most a quarter of the thinnest part and a fifth of the narrowest form, and at most three quarters of the clear gap between bars and 5 mm less than that gap (see 4.1 and 4.4).
Sources
- FAO Farm Structures, ch. 3: Concrete: mixes and amounts per m³, stone ≤ ¼ of the thinnest part, sand silt test, bulk densities, layers ≤ 150 mm, keep wet at least 7 days, forms off after 3 days
- ACI 318-19 §26.4.2.1 (ACI FAQ): largest stone no more than ⅓ of slab depth, applied here to any layer
- USDA NRCS fence specification 382 (Texas): posts at least 3 ft (0.9 m) deep, deeper in sandy or wet soil; slope the concrete top so water runs off
Concrete calculator
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