How to use the concrete calculator
- Choose a slab, patio or footing for rectangular pours, or a round column or post hole.
- Enter the dimensions. Slab thickness is in inches (or centimetres in metric).
- Enter how many identical pours you have, such as four footings of the same size.
- Keep the 10% extra unless your forms are perfectly level and you are ordering a little over anyway.
The results show ready-mix volume and the number of 60 lb and 80 lb premix bags. Compare both answers before you buy. Small pours are often cheaper in bags and larger ones by truck, which is why Decoratoradvice shows both.
Concrete volume formulas
Slab or footing: volume = length × width × thickness
Round column or hole: volume = π × (diameter ÷ 2)² × height
Cubic yards = cubic feet ÷ 27
Convert inches to feet before multiplying: a 4 in slab is 4 ÷ 12 = 0.333 ft thick.
Worked example: a 10 × 10 ft patio slab
10 ft × 10 ft × 0.333 ft = 33.3 cu ft
33.3 cu ft ÷ 27 = 1.23 cu yd
With 10% extra: 36.7 cu ft, or 1.36 cu yd
QUIKRETE lists a yield of about 0.60 cu ft for an 80 lb bag of its Concrete Mix and 0.45 cu ft for a 60 lb bag. So 36.7 ÷ 0.60 = 61.1, which rounds up to 62 bags of 80 lb, or 82 bags of 60 lb.
Bag yields
| Bag size | Approximate yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Yields are from the QUIKRETE Concrete Mix data sheet. Other brands are similar but check the bag.
Bags or ready-mix?
Bags make sense for post holes, steps and small pads, where you can mix as you go. Above about a cubic yard, mixing 45 or more 80 lb bags by hand is heavy, slow work, and the first batches start to set before the last are placed. For larger slabs, ask local suppliers about ready-mix delivery and their minimum order.
Tips for a better estimate
- Check slab depth in several places. A base that dips by half an inch over a large area adds noticeably to the volume.
- For a slab with a thickened edge, calculate the edge as a separate footing and add it.
- Plan reinforcement at the same time with the rebar calculator.
- Fence posts set in concrete: the fence calculator works out bags per hole after subtracting the post.
Common mistakes
- Forgetting to convert inches. 10 × 10 × 4 = 400 is not cubic feet; the 4 must be 0.333 ft.
- Dividing by 9 instead of 27. A cubic yard is 3 × 3 × 3 ft, so it holds 27 cu ft.
- No allowance for waste. Spillage, form bulges and an uneven base can use 5–10% more than the neat volume.
- Treating weight as volume. An 80 lb bag yields about 0.6 cu ft, not 0.8.
Frequently asked questions
How many bags of concrete are in a yard?
About 45 bags of 80 lb, or 60 bags of 60 lb, based on QUIKRETE's published yields of 0.60 and 0.45 cu ft per bag.
How thick should a concrete patio be?
Four inches is common for patios and walkways that only carry foot traffic. Driveways and slabs under heavy loads usually need more, so follow local code or your contractor's design.
How do I calculate concrete for a round column or post hole?
Choose the round option and enter the diameter and depth. The calculator uses π × radius² × height. For fence posts, use the fence calculator, which also subtracts the volume of the post.
How much concrete do I need for a 12 × 12 slab?
At 4 in thick, 12 × 12 × 0.333 = 48 cu ft, or 1.78 cu yd before any extra. With 10% added it is 1.96 cu yd.
Is concrete the same as cement?
No. Cement is the binding powder; concrete is cement mixed with sand, gravel and water. Premix bags already contain all the dry ingredients.
What is the difference between a cubic yard and a yard of concrete?
They are the same. Suppliers say "a yard of concrete" to mean one cubic yard. The cubic yard calculator converts any bulk volume.
Key takeaways
- Volume = length × width × thickness, with every measurement in feet; divide cubic feet by 27 for cubic yards.
- Add 5–10% for spillage and an uneven base.
- An 80 lb bag yields about 0.60 cu ft, so a cubic yard takes about 45 bags.
- Above about a cubic yard, ready-mix delivery is usually more practical than bags.
Sources
- QUIKRETE Concrete Mix (No. 1101) data sheet: bag sizes and yields.