Concrete Calculator
Find out exactly how much concrete your project needs — in cubic yards for ready-mix orders, or in bags for mix-it-yourself jobs. Runs 100% in your browser.
Concrete Calculator
The three custom parts are only used when the mix ratio above is set to Custom ratio. Common bag weights: 80 or 60 lb pre-mix, 94 lb cement (US), 50 or 25 kg cement (metric).
About this concrete calculator
Ordering too little concrete stalls a pour; ordering too much wastes money. This calculator converts your slab, footing, or post-hole dimensions into the numbers suppliers actually use: cubic yards for ready-mix trucks, bag counts for mix-it-yourself jobs (enter your bag weight in pounds or kilograms), and a full material quantity breakdown — cement, sand, aggregate and water — for site mixing at any standard mix ratio.
The math is simple volume: length × width × thickness, converted to cubic feet and then cubic yards (27 cubic feet per yard). Bag counts use the yield printed on the bag — an 80 lb bag of concrete mix yields about 0.6 cubic feet, a 60 lb bag about 0.45 cubic feet. The calculator rounds bag counts up, because a half bag short is a ruined afternoon.
Always add a waste allowance. Forms leak, subgrades are never perfectly level, and a slab that is half an inch thicker than planned across 100 square feet eats an extra 4 cubic feet of concrete. Ten percent is the standard recommendation for slabs; use 15% on uneven or hand-dug ground.
How to use
- Measure the length and width of your slab or footing in feet.
- Enter the thickness in inches — 4 inches is standard for patios and walkways, 6 inches for driveways.
- Choose pre-mix bags or ready-mix. For bags, enter the bag weight and pick lb or kg — 80 and 60 lb are the common US pre-mix sizes, 94 lb is a US cement sack, 50 kg is the standard metric cement bag.
- Pick a mix ratio (1 : 2 : 4 is the standard all-purpose mix) and a waste allowance.
- Press Calculate. You get the volume, the pre-mix bag count, and the full material list — cement, sand, aggregate and water — if you are mixing on site.
Mix ratios and material quantities
Concrete is cement, sand and coarse aggregate (gravel) in a set proportion, written as cement : sand : aggregate. A 1 : 2 : 4 mix (grade M15) is the standard all-purpose choice for slabs, floors and paths. 1 : 1.5 : 3 (M20) is stronger, for structural work and driveways. 1 : 3 : 6 (M10) suits light, non-structural work, and 1 : 4 : 8 (M7.5) is a lean mix for blinding and bedding. You can also enter a custom ratio.
The material breakdown uses the standard dry-volume method: dry materials take about 1.54× the finished (wet) volume, each material takes its ratio share of that dry volume, and weights use typical densities — cement 1,440 kg/m³, sand 1,600 kg/m³, aggregate 1,450 kg/m³. Water is estimated at a 0.5 water-cement ratio. Treat the weights as close buying estimates: sand and aggregate moisture and grading shift them a little in practice.
Good to know
- Bag yields vary slightly by brand and mix — check the yield printed on your specific bag.
- Footings, thickened edges, and post holes are separate volumes; calculate each and add them.
- This is a materials estimate, not structural advice. Slabs carrying vehicles or structures need proper thickness, rebar, and often a permit — check local code.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10×10 ft slab at 4 inches thick is about 1.24 cubic yards (33.3 cubic feet), which works out to 56 bags of 80 lb mix or 74 bags of 60 lb mix before waste. Add 10% and you are at 62 eighty-pound bags. Enter your exact dimensions above for your project.
How many cubic feet are in a yard of concrete?
One cubic yard = 27 cubic feet. A standard full-size mixer truck carries about 10 cubic yards.
How many 80 lb bags of concrete are in a yard?
About 45 eighty-pound bags make one cubic yard (27 ÷ 0.6 = 45). For 60 lb bags it is 60 bags per yard.
What is the formula for concrete volume?
Length (ft) × width (ft) × thickness (in ÷ 12) = cubic feet. Divide by 27 for cubic yards.
How much cement, sand and aggregate do I need for a 10x10 slab?
For a 10×10 ft slab at 4 inches thick with a standard 1 : 2 : 4 mix and 10% waste, the calculator gives: cement ≈ 329 kg (10 bags of 80 lb, or 7 bags of 50 kg), sand ≈ 16.1 cu ft (≈ 731 kg), aggregate ≈ 32.3 cu ft (≈ 1,325 kg), and roughly 164 liters of water. Pick a different ratio — or enter your slab size above — for your own numbers.
What does a 1:2:4 concrete mix ratio mean?
It is the proportion of cement : sand : aggregate by volume. One part cement, two parts sand, four parts aggregate. 1 : 2 : 4 (grade M15) is the standard all-purpose mix for slabs and floors; 1 : 1.5 : 3 (M20) is stronger for structural work, while 1 : 3 : 6 and 1 : 4 : 8 are leaner mixes for light or non-structural jobs.
Should I order ready-mix or bags?
As a rule of thumb, bags make sense under about half a cubic yard (roughly 20 eighty-pound bags). Above that, ready-mix is usually cheaper and far less labor.