Key takeaways
- Volume = length × width × thickness, with thickness converted to feet.
- There are 27 cubic feet in a cubic yard.
- Bag counts use QUIKRETE's published yields, so check the bag if you buy another brand.
Enter the slab's size and thickness. The extra allowance covers spills and uneven ground. It's a planning figure, so raise it if your base isn't level.
How the calculator works
- Cubic feet: length × width × (thickness ÷ 12)
- With extra: cubic feet × (1 + allowance %)
- Cubic yards: cubic feet ÷ 27
- Bags: cubic feet ÷ yield per bag, rounded up
QUIKRETE's Concrete Mix data sheet gives these approximate yields:
| Bag size | Yield |
|---|---|
| 80 lb | 0.60 cu ft |
| 60 lb | 0.45 cu ft |
| 40 lb | 0.30 cu ft |
Worked example
A 10 × 12 foot slab, 4 inches thick, with a 5% allowance:
- 10 × 12 × (4 ÷ 12) = 40 cubic feet
- 40 × 1.05 = 42 cubic feet
- 42 ÷ 27 = about 1.56 cubic yards
- 42 ÷ 0.60 = 70 bags of 80 lb, about 5,600 lb to move and mix
For the same slab, you'd need 94 bags of 60 lb or 140 bags of 40 lb.
Bags or ready-mix?
Look at the bag count and weight first. Mixing dozens of bags by hand is heavy work, and you need to place the concrete before it starts to set. QUIKRETE says to allow at least 1 cubic foot of mixer capacity for each 80 lb bag mixed at one time.
For larger jobs, ask local ready-mix suppliers for a quote, including any short-load fees and whether a truck can reach your site.
Before you pour
- Check how thick your slab should be.
- Ask your building department whether you need a permit.
- Prepare the ground: QUIKRETE's directions call for staking forms in place and compacting the sub-base until firm.
How a small depth change adds up
Forms and ground are rarely perfect. For the same 10 × 12 foot slab, with no extra allowance:
| Actual depth | Cubic feet | 80 lb bags |
|---|---|---|
| 4 in | 40 | 67 |
| 4 1/2 in | 45 | 75 |
| 5 in | 50 | 84 |
Just half an inch of extra depth adds 12.5%, which is more than our default allowance. So check your base and forms at several points before you buy.
Frequently asked questions
How much concrete do I need for a 10 × 10 slab?
At 4 inches thick, that's 33.3 cubic feet, or about 1.23 cubic yards, before any extra allowance.
How do I work out cubic yards?
Multiply length and width in feet by thickness in feet, then divide by 27.
Why is there an extra allowance?
Slabs rarely turn out exactly as measured, and running short during a pour is a real problem. We default to 5% as a planning figure, and you can change it.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- QUIKRETE: Concrete Mix data sheet, product 1101 (yields, mixer capacity, surface preparation)