Concrete calculator
Work out how much concrete a slab, footing, round column or set of steps needs, and how many premixed bags that is.
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Concrete needed
Enter the measurements to see how much concrete you need.
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How to use the concrete calculator
- Pick the shape. Slab for patios, pads and floors. Footing for rectangular footings or walls. Column for round columns and post holes. Steps for a solid set of stairs.
- Choose US or metric units. Every box has its own unit, so you can mix feet and inches. Switching systems converts what you typed.
- Enter the measurements. For footings and columns, add how many there are. For steps, add the number of steps.
- Set the waste allowance. 10% is filled in. The result shows the volume with and without it, and the bags to buy.
Concrete volume formulas
- Slab or footing: length × width × thickness.
- Round column: π × (diameter ÷ 2)² × height.
- Steps: width × tread depth × rise × n(n + 1) ÷ 2, where n is the number of steps. Each step is treated as a solid block from the ground up to its top.
- Units: 1 cubic yard = 27 cubic feet = 0.7646 cubic metres.
All measurements are turned into metres with exact factors (1 inch = 2.54 cm), so mixing units gives the same answer. Bags are worked out from typical yields: about 0.30, 0.45 and 0.60 cubic feet for 40, 60 and 80 lb bags, from the Quikrete Concrete Mix data sheet, and about 11.7 litres for a 25 kg bag at the same yield per kilogram. Bag counts are always rounded up.
Concrete for common 4-inch slabs
| Slab, 4 in thick | Cubic yards | 80 lb bags | With 10% extra |
|---|---|---|---|
| 10 × 10 ft | 1.23 | 56 | 1.36 yd³, 62 bags |
| 12 × 12 ft | 1.78 | 80 | 1.96 yd³, 88 bags |
| 10 × 20 ft | 2.47 | 112 | 2.72 yd³, 123 bags |
| 20 × 20 ft | 4.94 | 223 | 5.43 yd³, 245 bags |
| 4 × 3 m, 10 cm thick | 1.2 m³ | 103 (25 kg) | 1.32 m³, 113 bags of 25 kg |
Tips before you pour
Measure the real depth. Ground is rarely flat. Measure the depth of the form in a few places and use the average, or the deepest spot if you want to be safe.
Round up, not down. Running short in the middle of a pour leaves a weak joint. A few spare bags cost less than a second delivery.
Check the bag. Yields differ between brands and mixes, such as fast-setting or high-strength concrete. The bag lists its own yield.
Laying a gravel base first? Use the gravel calculator for the stone under the slab.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness in feet to get cubic feet, then divide by 27 for cubic yards. A 10 × 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. Add 5 to 10% for waste.
How many 80 lb bags of concrete make a cubic yard?
About 45. An 80 lb bag of Quikrete Concrete Mix makes about 0.6 cubic feet, and a cubic yard is 27 cubic feet: 27 ÷ 0.6 = 45. You need 60 bags of 60 lb mix or 90 bags of 40 lb mix for the same cubic yard.
How thick should a concrete slab be?
4 inches (about 10 cm) is a common thickness for patios, walkways and shed bases. Driveways and slabs that carry vehicles are often thicker. Local building codes and your project's plans decide the right thickness, so check them before you order.
Should I use bags or ready-mix concrete?
Bags suit small jobs such as post holes, a few steps or a small pad. For larger pours, ready-mix delivered by truck saves a lot of mixing: one cubic yard already takes about 45 bags of 80 lb mix. Many suppliers have a minimum order, so ask when you get a quote.
How do I measure for round columns or post holes?
Choose Column, then enter the diameter of the hole or tube form and its depth. The volume is π × (diameter ÷ 2)² × height. A 12-inch tube 4 feet deep holds about 3.14 cubic feet, so it takes 6 bags of 80 lb mix.