How to use this calculator
- Choose what you’re pouring: a slab (patio, driveway, shed base), a wall or footing, a round column or post hole, or a set of steps.
- Choose US (feet and inches) or metric (meters and centimeters) units.
- Enter the dimensions. Lengths are in feet and thicknesses in inches, as you’d measure them on site.
- Set the quantity if you’re pouring several identical pieces, like eight post holes.
- Adjust the waste allowance (10% by default).
- Optionally open Cost and enter a ready-mix price per cubic yard or a price per bag.
The result shows the cubic yards to order, the same volume in cubic feet and cubic meters, and how many 40, 60 or 80 lb bags of premixed concrete it takes.
The formulas
Every shape comes down to a volume in cubic feet, which is then divided by 27 to get cubic yards.
- Slab: length × width × thickness (thickness in inches ÷ 12).
- Wall or footing: length × height (or depth) × thickness (or width).
- Round column or post hole: π × radius² × height, where the radius is half the diameter.
- Steps: each step is a solid block resting on the ground, so the bottom step is one riser tall, the next two, and so on. For n steps that’s width × rise × run × (1 + 2 + … + n), or width × rise × run × n(n + 1) ÷ 2.
Then the waste allowance is added, and the bag count is the volume divided by the bag’s yield, rounded up to a whole bag.
Worked example
A 10 × 10 foot patio, 4 inches thick:
- Thickness in feet: 4 ÷ 12 = 0.333 ft.
- Volume: 10 × 10 × 0.333 = 33.33 cubic feet.
- Add 10% for waste: 33.33 × 1.1 = 36.67 cubic feet.
- Cubic yards: 36.67 ÷ 27 = 1.36 cubic yards (1.04 m³).
- Bags: 36.67 ÷ 0.60 = 61.1, so 62 bags of 80 lb concrete, or 82 of 60 lb, or 123 of 40 lb.
Other defaults in the calculator: a 20-foot wall 4 feet high and 8 inches thick needs 2.17 cubic yards with waste. A 12-inch round column 4 feet tall needs 0.13 cubic yards, or 6 bags of 80 lb mix. Three steps 4 feet wide with a 7-inch rise and 11-inch run need 0.52 cubic yards, or 24 bags.
Concrete for common slab sizes
Including a 10% waste allowance:
| Slab size | Thickness | Cubic yards | Cubic feet | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 10 × 10 ft | 4 in | 1.36 | 36.67 | 62 | 82 |
| 10 × 10 ft | 6 in | 2.04 | 55 | 92 | 123 |
| 12 × 12 ft | 4 in | 1.96 | 52.8 | 88 | 118 |
| 12 × 12 ft | 6 in | 2.93 | 79.2 | 132 | 176 |
| 10 × 20 ft | 4 in | 2.72 | 73.33 | 123 | 163 |
| 10 × 20 ft | 6 in | 4.07 | 110 | 184 | 245 |
| 20 × 20 ft | 4 in | 5.43 | 146.67 | 245 | 326 |
| 24 × 24 ft | 4 in | 7.82 | 211.2 | 352 | 470 |
Going from 4 to 6 inches thick adds 50% to the concrete, so it pays to get the thickness right before you order. Follow your plans or local building code for the thickness and any reinforcement your project needs.
Bag yields
Premixed concrete is sold by weight, but what matters is the volume it fills once mixed. The figures here come from the QUIKRETE Concrete Mix product data sheet:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | about 0.30 ft³ | 90 |
| 60 lb | about 0.45 ft³ | 60 |
| 80 lb | about 0.60 ft³ | 45 |
Other brands publish their own yields, and lightweight and “high-yield” mixes cover more per bag, and fast-setting or specialty mixes may cover less. Check the yield printed on the bag you plan to buy and adjust if it differs.
Tips for ordering
- Measure in several places. Ground is rarely perfectly flat. Use the average depth, or the deepest point if it varies a lot.
- Round up. Order a little over your figure rather than under. Topping up a pour that ran short is costly and can leave a weak joint.
- Count your helpers. Mixing and placing dozens of 80 lb bags by hand is heavy work. For bigger pours, a mixer rental or a ready-mix truck can save a long day.
- Price it out. Enter your local ready-mix price per cubic yard and a bag price to compare both options side by side. If you’re financing a bigger project, our home equity loan calculator can help with the numbers.
- Plan the surface area too. For pavers, tile or a coating on top of the slab, use the square footage calculator.
Assumptions and limitations
- The volume of a post set in a hole isn’t subtracted, so post-hole counts err on the generous side.
- Shapes are treated as perfect rectangles, cylinders and stair blocks. Thickened slab edges, sloped surfaces and openings aren’t modeled; add them separately or raise the waste allowance.
- Steps are assumed to be solid concrete down to the ground, with no landing. If your steps sit on fill or have a top landing, adjust the result.
- Bag counts use QUIKRETE’s published yields for standard Concrete Mix. Other products differ.
- The calculator estimates material only. It doesn’t include gravel base, rebar or mesh, forms or delivery fees.
Sources
Frequently asked questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. QUIKRETE lists its Concrete Mix at about 0.60 cubic feet per 80 lb bag, 0.45 per 60 lb bag and 0.30 per 40 lb bag. That works out to 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb per cubic yard.
How much concrete do I need for a 10 x 10 slab?
A 10 by 10 foot slab 4 inches thick is 33.33 cubic feet, or 1.23 cubic yards. With 10% extra for waste that's 1.36 cubic yards, or 62 bags of 80 lb concrete. At 6 inches thick you'd need 2.04 cubic yards with the same allowance.
How do I calculate cubic yards of concrete?
Multiply length by width by thickness, all in feet, to get cubic feet, then divide by 27. Convert inches to feet by dividing by 12 first, so a 4-inch slab is 0.333 feet thick. Then add 5% to 10% for spillage, uneven ground and form flex.
How much extra concrete should I order?
An allowance of around 10% is a common starting point, and it's the default here. Use more if the ground is uneven or the forms are long and complicated, and less for small, precise jobs like post holes. Running short in the middle of a pour is far worse than having a little left over.
Should I use bags or ready-mix?
Bags make sense for small jobs like post holes, steps or a small pad, where you can mix and place each batch quickly. As the count climbs into dozens of bags, ready-mix delivered by truck is often easier and gives a more even pour, but suppliers may charge extra for small loads. Price both before deciding.
How many bags of concrete do I need for a fence post?
It depends on the hole. A 10-inch-wide hole 3 feet deep holds about 1.64 cubic feet before the post goes in, a little under three 80 lb bags. For 8 posts that size, with 10% extra, the calculator gives 24 bags of 80 lb concrete.
Updated November 2026. Results are estimates for educational purposes and depend on the inputs you provide.