Concrete Mix Calculator

1. Enter the concrete volume you need and choose its unit (cubic feet, yards or meters). Size it with the concrete calculator first if you only have dimensions.
2. Pick a mix ratio by nominal grade, or choose Custom parts and enter your own cement, sand and gravel parts.
3. Adjust the dry-volume factor, water-cement ratio, bag size and waste if needed, then read the materials.
4. Buy whole bags: the cement result is rounded up, with the exact figure shown in the working line.

Here is what each result means:

A concrete mix calculator works out how much cement, sand, gravel and water go into a given volume of concrete at a chosen mix ratio. It answers the question behind every bagged or site-mixed pour: how much of each material do I buy?

It is built for small, on-site mixing: paths, footings, posts, small slabs and repairs, where you batch by the bag rather than order ready-mix. A mix ratio such as 1:2:4 means one part cement to two parts sand to four parts gravel by volume. The calculator is a planning aid for nominal mixes; it is not a mix design, and it does not set the strength your structure needs.

Freshly mixed concrete takes less space than its dry ingredients because water and cement paste fill the gaps between the sand and gravel. To get the dry material volume you multiply the wet volume by a dry-volume factor, commonly 1.54.

1. Multiply the wet volume (plus waste) by the dry-volume factor to get the dry material volume.
2. Add the ratio parts (for 1:2:4 the sum is 7) and give each material its share of the dry volume.
3. Convert the cement volume to weight at 1440 kilograms per cubic meter, then divide by the bag size for a bag count.
4. Find the mixing water from the water-cement ratio: water weight equals the ratio times the cement weight.

Suppose you are mixing 1 cubic meter of concrete at a 1:2:4 ratio, with a 1.54 dry factor and 50 kg bags.

Calculation: dry volume = 1 x 1.54 = 1.54 cu m. The parts add to 7, so cement is 1.54 x 1/7 = 0.22 cu m, sand is 1.54 x 2/7 = 0.44 cu m and gravel is 1.54 x 4/7 = 0.88 cu m. Cement weight = 0.22 x 1440 = 316.8 kg, which is 316.8 / 50 = 6.34 bags, so you buy 7.

At a 0.5 water-cement ratio, mixing water is 0.5 x 316.8 = 158 liters. These are loose, uncompacted volumes, so order a little extra sand and gravel.

The Mix Ratio

A richer mix (more cement, such as 1:1.5:3) needs more bags and gives more strength; a leaner mix (1:3:6) needs fewer. The ratio is the single biggest driver of the cement count.

The Dry-volume Factor

Most calculators use 1.54, but values from 1.52 to 1.57 are used. A higher factor raises every quantity, so the result is sensitive to it; leave it at 1.54 unless you have a reason to change it.

Water-cement Ratio

More water makes the mix easier to place but weaker. The water figure here follows the ratio you set, not the slump you feel on site, so treat it as a starting point.

Bulking and Waste

Damp sand bulks up and loose aggregate has voids, so real volumes vary. The waste allowance covers spillage and over-ordering; add more for a long barrow run or a rough site.

Nominal mixes are fixed ratios; designed mixes are proportioned by a lab for a target strength. The table shows the common nominal mixes and their rough strengths, but strength on site also depends on materials, water and curing.

Strengths are approximate conversions of nominal grades and are not code values. For anything structural, use a designed mix and quantities from an engineer rather than a nominal ratio.

Buying Bags for Site Mixing

Size how many bags of cement, and how much sand and gravel, a small pour needs before you load the mixer or the car.

Choosing a Ratio for the Job

Compare a richer and a leaner mix for the same volume to see the trade-off between cement cost and strength.

Checking a Supplier Quantity

Sanity-check a merchant list of materials against the ratio and volume before you pay for a delivery.

1. Forgetting the Dry-volume Factor

Splitting the wet volume straight by the ratio under-counts every material by about a third. Always multiply by the dry factor first.

2. Mixing Up Ratio Order

The ratio is cement : sand : gravel. Reading it the wrong way swaps the sand and gravel and changes the cement share.

3. Treating Nominal Strengths as Guaranteed

The psi figures are rough. They are not a substitute for a tested, designed mix on a structural job.

4. Ignoring Bulking and Voids

Sand and gravel are loose volumes. Order a little extra rather than the exact figure, and round cement up to whole bags.

5. Adding Water to the Ratio, Not the Job

The water figure follows the water-cement ratio. Too much water on site weakens the concrete even if the bag count is right.

The arithmetic is exact for the ratio, factor and densities you enter; real site quantities vary with the materials.

For a 1:2:4 nominal mix, about 6.34 bags of 50 kg cement, which you round up to 7. The exact figure comes from 1.54 x 1/7 x 1440 kg divided by the 50 kg bag size. A richer 1:1.5:3 mix needs more, a leaner 1:3:6 fewer.

It is one part cement, two parts sand and four parts gravel by volume, a nominal M15 mix of roughly 2,200 psi. It suits paths, small slabs and kerbs. For structural or reinforced work, use a richer mix or, better, a designed mix.

It is the ratio of dry material volume to finished wet concrete. Dry sand, gravel and cement take more space than the mixed, compacted result, so you multiply the wet volume by about 1.54 to find how much dry material to buy.

Of the common nominal mixes, 1:1:2 (about M25) is the richest and strongest, then 1:1.5:3, 1:2:4 and the lean 1:3:6. More cement means more strength and more cost. Above M20, a designed mix is preferred over a nominal ratio.

Each gets its share of the dry volume by the ratio. For a 1:2:4 mix the sand is two sevenths and the gravel four sevenths of the dry volume. The calculator shows both in your chosen unit; order a little extra because they are loose volumes.

Multiply the cement weight by the water-cement ratio. At a 0.5 ratio, 316 kg of cement needs about 158 liters of water. Lower ratios give stronger but stiffer concrete; more water is easier to place but weaker.

No. Ready-mix is ordered by the cubic yard or meter and proportioned at the plant. This tool is for site mixing by the bag. Use the concrete calculator to size a ready-mix order by volume instead.

Treat them as nominal mixes for small, non-structural work. Columns, beams, suspended slabs and anything reinforced should use a designed mix and quantities set by a qualified engineer.

Sand bulks up when damp and loose aggregate contains voids, so the delivered volume rarely matches the calculated figure exactly. A small surplus and a waste allowance keep you from running short mid-mix.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• American Cement Association (Portland Cement Association) (American Cement Association; cement and concrete basics).
• National Ready Mixed Concrete Association (NRMCA; concrete materials and proportioning).
• QUIKRETE Concrete Mix product data (QUIKRETE; bag sizes and coverage).
• QUIKRETE concrete and bag calculator (QUIKRETE; bag-count method).