Mixing your own concrete? Pick a ratio - 1:2:3, 1:2:4 or your own - or a strength like 3000 or 4000 psi, enter your slab size and get exactly how much cement, sand and gravel to buy - by the bag, the weight and the cubic yard.
Wet concrete volume: 24.5 cu ft
Cement
565 lb
Sand
1256 lb
Aggregate
1708 lb
Water
33.9 gal
Batch breakdown (volume / weight)
Dry batched volume 37.7 cu ft (wet × 1.54)
A concrete mix ratio sets the proportions of cement, sand (fine aggregate) and coarse aggregate by volume - for example 1:2:3 means one part cement to two parts sand to three parts stone. To batch a mix, the calculator works backwards from the wet concrete you need:
dry volume = wet volume × 1.54
part volume = dry volume ÷ (cement + sand + aggregate parts)
cement = part volume × cement parts → weight → bags
sand = part volume × sand parts
aggregate = part volume × aggregate parts
water = cement weight × water-cement ratio
The 1.54 dry factor accounts for the air voids in loose materials that close up once the mix is wetted. Each material’s volume is turned into a weight using its bulk density (cement ~1440, sand ~1600, aggregate ~1450 kg/m³) so you can order by the bag or the tonne.
To get the wet volume, add one section per part of the pour - a rectangle for a slab or path, a circle for a round post hole, column or sonotube, a triangle for an angled infill - and the calculator pools them, adds your wastage allowance, then batches the whole lot in one mix.
This example follows the unit system you pick in the calculator above.
1 cubic yard of concrete at a 1 : 2 : 3 mix, water-cement ratio 0.5, 94 lb cement bags:
Strength grades go by different names around the world - C-grades in the UK and Europe, M-grades in India, psi in the US - but they map onto the same handful of nominal volume ratios. Use this to pick the ratio for the grade you have been given:
| Grade (UK / EU) | Grade (India) | US strength | Nominal mix (C : S : A) | Typical use |
|---|---|---|---|---|
| C10 / GEN1 | M10 | ~1450 psi | 1 : 3 : 6 | Blinding, trench fill, mass fill |
| C15 | M15 | ~2200 psi | 1 : 2 : 4 | Paths, shed bases, kerbing |
| C20 / GEN3 | M20 | ~3000 psi | 1 : 1.5 : 3 (or 1 : 2 : 3) | Domestic floors, foundations, drives |
| C25 | M25 | ~3600 psi | 1 : 1 : 2 | Heavy-duty slabs, structural work |
| C30+ | M30+ | 4000 psi+ | design mix* | Driveways, structural, exposed concrete |
*C30 and above (4000 psi / 30 MPa+) are normally design mixes - the proportions are worked out and tested for the job, not taken from a fixed ratio - and often use admixtures. For structural work at these strengths, follow an engineered mix design and your local building requirements.
The same concrete strength has a different name depending on where you are, so match yours before you pick a ratio:
Whatever the label, find the strength in the table above, read across to the nominal mix, and enter that ratio in the calculator.
For most home jobs, 1:2:3 (one part cement, two sand, three aggregate) gives a strong general-purpose concrete suitable for footings, structural slabs and posts - roughly a C20/25 strength. Use 1:2:4 for paths, shed bases and non-structural slabs, and a leaner 1:3:6 for mass fill and blinding under a slab. The ratio is measured by volume, so the same bucket or shovel is used for every part.
For a 1:2:3 mix, about 1 cubic metre of finished concrete needs roughly 370 kg of cement (around 7 to 15 bags depending on bag size), 0.5 m³ of sand and 0.77 m³ of aggregate. The dry materials add up to about 1.54 m³ because they lose volume to air voids once water binds them - that 1.54 dry factor is built into the calculator.
Loose, dry cement, sand and stone are full of air gaps. When you add water and mix, the fine material fills the voids between the coarse stone, so the mix shrinks to roughly 65% of its loose volume. To end up with the volume of wet concrete you actually need, you batch about 1.54 times that in dry materials. This is the standard site allowance and the calculator applies it for you.
Water is set by the water-cement ratio - the weight of water divided by the weight of cement. Around 0.5 is standard: half a litre of water per kilo of cement. Lower (0.45) gives a stiffer, stronger mix; higher (0.55 to 0.6) is easier to place but weaker. Add water gradually and stop when the mix is workable but not soupy - the calculator gives a starting figure, not a hard limit.
On site, concrete ratios are measured by volume - the same bucket, gauge box or shovel for each part - because it is fast and needs no scales. The calculator shows both: the volume of each material to batch and its approximate weight, so you can order sand and aggregate by the bag, the bulk bag or the tonne, whichever your supplier sells.
Mixing from cement, sand and aggregate is the cheapest per cubic metre and lets you control the ratio, but it is hard work and the mix quality depends on you. Pre-blended bags (like Quikrete) cost more but only need water. Ready-mix delivered by the truck is best once you pass roughly a cubic metre. Use this calculator for batching raw materials, and the concrete calculator to compare bags and ready-mix.
C20 (UK and Europe) and M20 (India) both describe concrete reaching about 20 MPa, roughly 3000 psi, compressive strength. The standard nominal mix is 1 part cement to 1.5 parts sand to 3 parts coarse aggregate (1:1.5:3) by volume. A 1:2:3 mix lands in a similar strength range and is common for general home work. Pick either in the ratio dropdown and the calculator batches the cement, sand, aggregate and water for your volume.
Around 3000 psi (20 MPa) comes from a 1:2:3 or 1:1.5:3 mix, which covers most home slabs, footings and driveways. 4000 psi (about 28 MPa) and 5000 psi (about 35 MPa) are stronger than a fixed nominal ratio reliably delivers, so they are normally specified as design mixes with tested proportions and often a water-reducing admixture. For structural 4000 psi and above, follow an engineered mix design rather than a volume ratio.
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