Order the right amount of concrete
Running short of concrete halfway through a pour is a headache; ordering a truckload too much is money down the drain. This concrete calculator works out the volume for a slab, a patio or a set of round columns or post holes, adds a sensible margin for waste, and tells you the answer in cubic yards, cubic feet, cubic metres and premix bags.
Ready-mix suppliers sell by the cubic yard, while bags suit smaller jobs. The calculator gives both, so you can decide which makes sense.
How to use it
- Choose Slab or patio, or Round columns or post holes.
- Enter the length and width in feet and the thickness in inches, or the diameter, depth and number of holes.
- Set the extra for waste. 10% is a safe default for most home jobs.
- Pick the bag size if you plan to mix your own.
- Read the cubic yards, then the cubic feet, cubic metres and number of bags.
The concrete formulas
slab volume = length × width × (thickness in inches ÷ 12)
column volume = π × (diameter in inches ÷ 24)² × depth × number of columns
cubic yards = cubic feet ÷ 27
bags = cubic feet ÷ yield per bag, rounded upThickness is usually measured in inches, so it is divided by 12 to turn it into feet before multiplying. A cubic yard is 3 feet by 3 feet by 3 feet, which is 27 cubic feet. Typical yields printed on premix bags are about 0.6 cubic feet for an 80 lb bag, 0.45 for 60 lb and 0.3 for 40 lb.
Worked example: a 10 by 10 foot patio
A patio slab will be 10 feet by 10 feet and 4 inches thick.
- Volume: 10 × 10 × (4 ÷ 12) = 33.33 cu ft
- Plus 10% for waste: 36.67 cu ft
- In cubic yards: 36.67 ÷ 27 = 1.36 cubic yards
- In 80 lb bags: 36.67 ÷ 0.6 = 61.1, so 62 bags
The patio needs about 1.36 cubic yards. That is 62 bags of 80 lb premix, which is a lot of mixing by hand; many people would order a small ready-mix load for a slab this size.
Common slab thicknesses
| Thickness | Typical use | Cubic yards per 100 sq ft |
|---|---|---|
| 4 in | Patios, paths, shed bases | 1.23 |
| 5 in | Heavier patios, hot tub pads | 1.54 |
| 6 in | Driveways and garage floors | 1.85 |
| 8 in | Heavy loads | 2.47 |
Your local building rules may require a certain thickness, a gravel base or steel reinforcement for some jobs. Check before you pour.
Mistakes that leave you short
- Using the thickness in inches as if it were feet. 4 inches is 0.33 feet.
- Measuring the top and not the bottom of uneven ground. Dips need extra concrete.
- Leaving out the waste margin. Spills, forms bowing out and over-digging all eat into it.
- Confusing cement with concrete. Cement is one ingredient; concrete is cement mixed with sand, stone and water.
- Forgetting the base. A slab usually sits on compacted gravel; the gravel calculator works out how much.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply the length and width in feet by the thickness in feet, which is inches divided by 12, to get cubic feet. Divide by 27 for cubic yards. A 12 by 12 foot slab 4 inches thick is 48 cubic feet, or 1.78 cubic yards. Add 5% to 10% for waste.
How many bags of concrete do I need?
Divide the cubic feet by the yield of one bag and round up. A typical 80 lb bag makes about 0.6 cubic feet, a 60 lb bag about 0.45 and a 40 lb bag about 0.3. Check the yield printed on the bag you buy, as brands vary a little.
How many cubic yards are in a 10 by 10 slab?
At 4 inches thick, a 10 by 10 foot slab is 33.33 cubic feet, which is 1.23 cubic yards before waste and about 1.36 cubic yards with 10% extra. At 6 inches thick it is 1.85 cubic yards before waste. The calculator shows both figures for any size.
Is it cheaper to use bags or ready-mix concrete?
For small jobs such as a few post holes or a little pad, bags are usually simpler and cheaper. For anything over about a cubic yard, ready-mix often wins once you count the time and labour of mixing dozens of bags. Get a quote from a local supplier to compare.
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