How to use the concrete mix ratio calculator
- Enter the slab length, width and thickness. Thickness is in inches and the other two are in feet.
- Pick a mix ratio of cement to sand to gravel, or choose Custom and enter the parts.
- Set the waste allowance. Open the advanced options if you want to change the dry volume factor or the size of a bag of cement.
- Read the bags of cement and the volumes of sand and gravel in cubic feet and cubic yards.
Formula
Wet volume = length x width x thickness / 12 x (1 + waste)
Dry volume = wet volume x 1.54
Cement = dry volume x cement parts / total parts
Sand and gravel follow the same rule; bags = cement volume / 1 cubic foot per bag
Dry sand, gravel and cement pack together once water is added, so you need roughly one and a half times the finished volume in dry material. The 1.54 factor is common and some guides use 1.5. A 94 pound bag of portland cement is about one cubic foot.
Worked example
A 10 ft by 10 ft slab 4 inches thick is 100 x 4 / 12 = 33.33 cubic feet. With 10% waste the wet volume is 36.67 cubic feet, or 1.36 cubic yards.
Dry volume is 36.67 x 1.54 = 56.47 cubic feet. For a 1:2:3 mix there are 6 parts, so cement is 9.41 cubic feet, sand is 18.82 and gravel is 28.23 cubic feet (1.05 cubic yards).
At about one cubic foot per bag, that is 9.41 bags, rounded up to 10 bags of cement.
What the ratio means
A ratio such as 1:2:3 means one part cement, two parts sand and three parts gravel by volume. The parts can be measured with any container as long as you use the same one for each material, such as a bucket or a shovel count.
Leaner mixes with more sand and gravel are cheaper and less strong. Richer mixes with more cement are stronger and cost more. Common general purpose ratios include 1:2:3 and 1:2:4, but the right choice depends on the job and the strength needed.
Before you order materials
Estimating is only half the job. A few points help the pour go well.
- Add the water last and slowly. Too much water weakens the concrete, so use only what is needed to make it workable.
- Order a little extra, and make sure the subbase is compacted and the forms are set before mixing.
- Use clean, sharp sand and washed gravel of a suitable size for the thickness of the slab.
- For structural work, driveways or anything load bearing, get a design from your supplier or an engineer. For ready mix or bagged concrete, use the concrete calculator.
Assumptions and limits
- Ratios are by loose volume of cement, sand and gravel, not by weight.
- The waste allowance covers spillage and uneven ground. The default is 10%, and you can set your own.
- The default dry volume factor of 1.54 and a bag size of 1 cubic foot are common conventions, and both can be changed.
- This tool does not calculate water, admixtures, strength class or reinforcement.
Frequently asked questions
What is the best concrete mix ratio for a slab?
A 1:2:3 or 1:2:4 mix of cement, sand and gravel is common for general slabs. The right ratio depends on the load and the climate, so ask your supplier or an engineer for anything structural.
How many bags of cement do I need for a slab?
It depends on the volume and the ratio. A 10 by 10 foot slab 4 inches thick at 1:2:3 with 10% waste needs about 10 bags of 94 pound cement. Enter your dimensions for your own number.
Why is the dry volume bigger than the finished concrete?
Dry materials have air between the grains. When water is added the smaller particles fill the gaps and the mix compacts, so you need about 1.5 times the finished volume in dry materials.
What does a 1:2:3 ratio mean?
One part cement, two parts sand and three parts gravel by volume. Six parts in total, so cement is one sixth of the dry volume, sand one third and gravel one half.