How to Calculate Concrete
This guide shows how to work out how much concrete a project needs, from a single patio to a full foundation. You will get the core formula, the math for every common shape, bag counts, mix ratios, weight, a cost method and a cure time chart, each with a worked figure you can check by hand. When you want an instant answer, the calculators linked through the page do the arithmetic for you.
How concrete is measured and ordered
Ready mix concrete is bought by the cubic yard in the United States and by the cubic meter in most other countries. The conversion worth memorizing is that one cubic yard equals 27 cubic feet, because almost every residential dimension starts in feet and inches. One cubic meter is about 1.31 cubic yards. Small jobs are poured from bags instead of a truck, and a bag is rated by the volume of wet concrete it yields, not by its dry weight alone.
The core formula and the one rule that matters
For any rectangular pour, volume equals length times width times thickness. The rule that trips people up is unit discipline: every dimension has to be in the same unit before you multiply. Thickness is the usual problem, since slabs are quoted in inches while length and width are in feet.
Worked example. A 10 foot by 10 foot slab at 4 inches thick. Convert the thickness first: 4 divided by 12 is 0.333 feet. Multiply: 10 times 10 times 0.333 is 33.33 cubic feet. Divide by 27 to get 1.23 cubic yards.
Formulas for every shape
| Shape | Formula | Quick figure |
|---|---|---|
| Slab or footing | Length × Width × Thickness | 10 × 10 × 4 in = 1.23 cu yd |
| Wall | (Length × Height, minus openings) × Thickness | 20 × 8 ft, 8 in = 3.95 cu yd |
| Square column | Width × Depth × Height | 1 × 1 × 10 ft = 0.37 cu yd |
| Round column or tube | 3.1416 × radius² × Height | 12 in × 10 ft = 0.29 cu yd |
| Circular pad | 3.1416 × radius² × Thickness | 10 ft × 4 in = 0.97 cu yd |
Break an odd footprint into simple rectangles and circles, work out each piece, then add them before you order.
Bags or ready mix, and how many bags per yard
For small jobs, bagged concrete is cheaper and easier than meeting a truck minimum. Each bag yields a set volume, so the count is your cubic feet divided by the yield per bag. These yields come from the Quikrete Concrete Mix data sheet.
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
The crossover is around one cubic yard. A yard of 80 pound bags is 45 sacks, which a confident crew can handle. Two yards is 90 bags and a long day of mixing, so a truck is both cheaper and more consistent.
Concrete mix ratios
Concrete is cement, sand and coarse aggregate bound by water. The general purpose ratio is 1:2:3, one part cement, two parts sand, three parts stone, which works out to roughly 22 percent cement, 33 percent sand and 45 percent aggregate by volume. That mix suits most residential flatwork. Higher strength structural work tightens the ratio, and leaner footings can run 1:3:6. The water to cement ratio matters as much as the aggregate split: less water means more strength but a stiffer mix.
How much concrete weighs
Standard concrete weighs about 150 pounds per cubic foot, so one cubic yard weighs close to 4,050 pounds, just over two tons. Reinforced concrete is a little heavier near 156 pounds per cubic foot, and lightweight structural mixes drop to around 110. Weight sets form pressure, what the ground has to carry, and what a trailer can legally haul.
Always add a waste allowance
Never order your exact calculated volume. Order volume equals the calculated volume times one plus the waste percentage. Use 5 to 10 percent for clean, formed pours on stable ground, and 10 to 15 percent for rough terrain, hand dug footings or irregular shapes. A second partial truck costs far more than the small over order a waste allowance covers.
Concrete strength by project (PSI)
Concrete strength is measured as 28 day compressive strength in pounds per square inch. The Quikrete standard mix reaches about 4,000 psi at 28 days and 2,500 psi at 7 days. Match the strength to the job.
| Strength | Typical use |
|---|---|
| 2,500 to 3,000 psi | Sidewalks, some residential flatwork |
| 3,500 to 4,000 psi | Driveways, footings, slab foundations |
| 4,000 to 5,000 psi | Commercial floors, warehouses |
| 6,000 psi or more | Bridges, high rise, industrial |
How long concrete takes to cure
Fresh concrete sets within a few hours and is usually safe to walk on after 24 to 48 hours. It reaches enough strength for light vehicle traffic in about 7 days and full design strength in about 28 days. The Portland Cement Association notes that most strength gain happens in the first month, and that the longer concrete stays moist, the stronger it becomes. Curing by keeping the surface damp during the first week is what produces a hard, crack resistant finish.
| Milestone | Time |
|---|---|
| Safe to walk on | 24 to 48 hours |
| Light vehicle traffic | about 7 days |
| Full design strength | about 28 days |
Estimating cost
Ready mix is priced by the cubic yard and set regionally, so two local quotes beat any national figure. Price has three habits worth knowing. Most plants charge a short load fee under a full truck and set a minimum delivery, often one to one and a half yards. Round up rather than down, since a second partial truck costs far more than your waste allowance. Confirm the mix design, telling the plant the strength and any additives so the right batch arrives.
Tips for an accurate estimate
- Measure twice and verify every dimension on site before ordering.
- Keep units consistent, or let the calculator convert, to avoid the inches as feet error.
- Split odd shapes into rectangles and circles, then add the results.
- Match the strength grade to the application.
- Ask the plant about minimum order and short load fees before you finalize the pour size.
Use the right calculator for your project
- Concrete calculator for a quick all in one estimate
- Slab calculator for patios, driveways and floors
- Column calculator for piers, sonotubes and columns
- Footing calculator for strip and pier footings
- Bags calculator to turn a volume into a bag count
Frequently asked questions
How do I calculate concrete in cubic yards?
Multiply length by width by thickness in feet, then divide the cubic feet by 27. A 10 by 10 slab at 4 inches is 33.3 cubic feet, or 1.23 cubic yards.
How much concrete do I need?
It depends on the area, thickness and shape. Measure the dimensions, multiply for volume, convert to cubic yards, then add 5 to 10 percent for waste. The calculators above do this for you.
How many bags of concrete make a yard?
About 45 bags of 80 pound mix, 60 bags of 60 pound, or 90 bags of 40 pound.
What is the best mix ratio for concrete?
A 1:2:3 ratio of cement, sand and aggregate suits most general work. Structural work tightens the ratio, and the water to cement ratio controls strength.
What PSI should my concrete be?
Sidewalks and patios use 3,000 to 3,500 psi, driveways and footings 3,500 to 4,000, and commercial floors 4,000 or more.
How long does concrete take to cure?
You can walk on it in 24 to 48 hours and drive on it after about 7 days. Full strength takes about 28 days, and keeping it moist for the first week helps it get there.
How much does a yard of concrete weigh?
About 4,050 pounds at the standard density of 150 pounds per cubic foot.
How much extra concrete should I order?
Add 5 to 10 percent for clean pours and 10 to 15 percent for rough ground or hand dug footings.
References
- Quikrete, Concrete Mix 1101 data sheet (yields, 4,000 psi at 28 days, 2,500 at 7 days): https://www.quikrete.com/PDFs/DATA_SHEET-Concrete Mix 1101.pdf
- Portland Cement Association, Curing in Construction: https://cement.org/Learn/concrete-technology/concrete-construction/curing-in-construction
- American Concrete Institute, ACI 318 and ACI 332: https://www.concrete.org
- ASTM C94, Standard Specification for Ready Mixed Concrete: https://www.astm.org/c0094_c0094m-22.html
- National Ready Mixed Concrete Association: https://www.nrmca.org
Methodology: formulas convert all dimensions to a common unit, with 1 cubic yard equal to 27 cubic feet; bag counts, strength and cure figures come from the manufacturer data sheet and the Portland Cement Association. Estimates are for planning. Confirm mix design, thickness and reinforcement with a licensed engineer or local code. Recommended: attribute this guide to a named author with construction or engineering experience and add a review date.