Concrete and cement calculator
A 10 × 10 ft shed base at 4 inches deep is 33.3 cubic feet of hole, or 1.23 cubic yards. The panel applies the waste percentage before it reports anything, so at the 10% it opens on, the rows above already read 36.67 cubic feet and 1.36 yards, with the 0.944 m³ of actual hole on the “Before waste” line. In practice you then ask for 1.5 yards, because ready-mix is sold in quarter-yard steps and running short mid-pour is far worse than paying for a little extra.
Concrete volume is length × width × thickness, converted to cubic yards or cubic metres. A 10 × 10 ft slab at 4 inches thick is 1.23 cubic yards of hole, which the 10% waste allowance turns into the 1.36 the panel tells you to order; round that up to the quarter yard suppliers sell in and you are buying 1.5. The mix mode sizes a batch you are mixing yourself, cement included.
How to work out concrete
The arithmetic is the easy part; the judgement is in the waste allowance. A slab poured onto compacted hardcore with clean formwork might only lose 3%. The same slab onto a dug-out garden with soft spots can swallow 15% before you notice, because concrete finds every dip. Footings are worse again; trench sides slump inward and the trench you measured is not the trench you fill. If the pour is one continuous piece and running short means a cold joint, buy the extra quarter yard.
Batching it yourself is a different question, and it starts with the fact that cement is only the binder. A 1 : 2 : 4 mix is one part cement to two of sand and four of aggregate by volume, and seven parts of dry material yield roughly five parts of finished concrete, because the finer materials fill the voids in the coarser ones. That shrinkage is why a bucket count never matches the hole, and it is what the mix mode accounts for before it tells you how many cement bags to buy.
Which ratio depends on the job rather than on preference. A 1 : 2 : 4 is general-purpose concrete for slabs and footings, 1 : 3 : 6 a weaker fill for backfilling and bedding, 1 : 1.5 : 3 a stronger structural mix. Mortar drops the aggregate entirely: 1 : 4 cement to sharp sand for bricklaying, softer and richer for pointing. Water is the one ingredient never listed in the ratio and the one most likely to ruin the batch — more water makes it easier to work and weaker forever, so aim for a water-to-cement ratio near 0.5 by weight and mix smaller batches rather than loosening a stiffening one.
What people use it for
- Ordering ready-mix for a shed base or driveway
- Counting 25 kg bags for a small path you will mix by hand
- Sizing a strip footing before the building inspector arrives
- Working out concrete for a run of fence post holes
- Buying cement, sand and ballast separately and batching by the barrow
- Counting cement bags for a 1 : 2 : 4 batch before a merchant run
- Checking a builder quote that lists materials but not quantities
Questions
About 46 bags of 80 lb (36 kg) pre-mix, or 66 bags of 25 kg. Count bags by what a bag YIELDS, not by dividing the pour’s weight by the bag’s: a bag yields more than its dry weight suggests, because the water you add is part of the finished volume. The manufacturers put the figure on the bag — an 80 lb bag makes 0.60 cubic feet, a 25 kg bag about 11.6 litres.
Four inches (100 mm) is standard for a shed base, patio or footpath. Six inches (150 mm) if a vehicle will park on it. Thickness drives volume linearly, so going from four to six inches adds half again to the order.
Yes. Ten per cent is the default here. Concrete is unforgiving: a short pour means a cold joint, whereas a surplus is a wheelbarrow of spoil.
No. Cement is the grey powder that binds; concrete is cement, sand, aggregate and water together. Asking for "a load of cement" at a builders merchant gets you the powder.
About 2.4 tonnes. That matters for access: a single cubic metre is heavier than most small vans can legally carry.
A 1 ft diameter hole 3 ft deep takes 0.067 m³, so six of them is about 0.4 m³ before waste — 35 bags of 25 kg, and 38 once the default 10% allowance is added. The post-hole mode does the arithmetic for any diameter.
1 : 2 : 4 cement to sand to aggregate for general slabs and footings, 1 : 3 : 6 for weak backfill, 1 : 1.5 : 3 for a stronger structural pour.
For a 1 : 2 : 4 concrete, roughly 13 bags of 25 kg per cubic metre of finished concrete, plus about 0.5 m³ of sand and 1 m³ of aggregate.
As little as the mix will tolerate and still work. A water-to-cement ratio around 0.5 by weight is the usual target; every extra splash trades strength for ease.
No. Building sand is too fine and makes a weak, sticky concrete. Use sharp sand or all-in ballast for concrete, and keep the soft building sand for mortar.
About two hours at mild temperatures, much less in heat. Once it stiffens, adding water to loosen it destroys the strength; mix smaller batches instead.
Yes for anything load-bearing. Concrete poured on soft ground cracks along the soft spots, and an uncompacted base is also what turns a 5% waste allowance into 15%.