Concrete Calculator
Find how many bags or yards of concrete you need, and what it will cost.
Measure your project
Your answer
That is 36.7 cubic feet. We added 10% extra.
If you buy bags
One 80 lb bag makes 0.6 cubic feet of concrete.
If you order a truck (ready-mix)
Ask for 1.50 cubic yards. We round up to the next quarter yard.
This answer is a guide. For footings, driveways or anything that holds weight, check your local building rules or ask a builder or engineer.
Which costs less?
But you must mix 62 bags by hand. That is 2.5 tons. For a big job, a truck saves a lot of hard work.
Prices are US middle prices from October 2026. Ask your store for their price.
Quick answers
- When does a truck cost less?
- From about 3.0 cubic yards, at our middle prices. For less, bags usually cost less.
- A 10 × 10 ft slab, 4 in thick
- 1.23 cubic yards. That is 56 bags of 80 lb, before extra.
- How thick?
- 4 in for a patio, path or car driveway. 5 in for heavier vehicles.
- How much extra?
- Add about 10%, so you don't run short. Sakrete suggests the same.
How to work it out
- Measure the length and width in feet.
- Measure how thick it will be in inches. Divide by 12 to get feet.
- Multiply length × width × thickness. This gives cubic feet.
- Add 10% extra. Then divide by 27 to get cubic yards.
- For 80 lb bags, divide the cubic feet by 0.6. Round up.
10 × 10 × 0.333 = 33.3 cubic feet
33.3 + 10% = 36.7 cubic feet
36.7 ÷ 27 = 1.36 cubic yards
36.7 ÷ 0.6 = 61.1 → 62 bags
Concrete for common slab sizes
This table shows how much concrete common slabs need. Each slab is 4 in thick, with 10% extra added.
| Slab size | Cubic yards | 80 lb bags |
|---|---|---|
| 4 × 4 ft | 0.22 | 10 |
| 8 × 8 ft | 0.87 | 40 |
| 10 × 10 ft | 1.36 | 62 |
| 10 × 12 ft | 1.63 | 74 |
| 12 × 12 ft | 1.96 | 88 |
| 10 × 20 ft | 2.72 | 123 |
| 12 × 20 ft | 3.26 | 147 |
| 20 × 20 ft | 5.43 | 245 |
| 24 × 24 ft | 7.82 | 352 |
For a 5 in slab, you need about a quarter more.
Not a slab? For footings, steps, tube forms or fence posts, use our footing, steps, Sonotube or fence post calculator.
Bags or a truck?
Bags are good for small jobs. You mix them yourself, in a wheelbarrow or a rented mixer.
A truck brings concrete that is already mixed. This is called ready-mix. You pay for each cubic yard. Our middle price is $179 a cubic yard, from 5 US sellers.
Small orders cost more. 4 cost guides say plants charge about $30–80 for each cubic yard short of a full truck. A full truck holds about 10 cubic yards. Some plants charge one fee per load instead. These are guides, not plant prices, so ask your plant.
At our middle prices, a truck starts to cost less from about 3.0 cubic yards. Below that, bags usually win. Step 3 in the calculator checks this for your job.
Think about the work too. A 10 × 10 ft slab, 4 in thick, takes 62 bags of 80 lb with extra. That is about 2.5 tons to lift and mix by hand.
How thick should the slab be?
- Patios and walks: at least 4 in.
- Driveways for cars: at least 4 in.
- Driveways for garbage trucks or other heavy vehicles: 5 in or more.
- House floors on the ground: at least 3.5 in.
Going from 4 in to 5 in adds about 20% to the cost. But the slab can carry almost 50% more weight, says a ready-mix maker.
A house floor below ground level needs a base, says the US home building code. It is 4 in of clean sand, gravel or crushed stone. Ground that drains well may not need it.
These are general rules. For driveways and anything that holds weight, check your local building rules. Or ask a builder or engineer.
Why bag counts don't agree
Ask how many bags a slab needs, and you may hear many answers. There are two main reasons.
- Bags make different amounts. A bigger bag makes more concrete. Check the size you will buy.
- Some counts add extra, and some don't. We add the extra you pick, and we show it.
You can't buy part of a bag, so we round up to whole bags.
This table shows what each bag size makes, from the makers' data sheets.
| Bag size | Makes | Bags per cubic yard |
|---|---|---|
| 90 lb | 0.675 cubic feet | 40 |
| 80 lb | 0.6 cubic feet | 45 |
| 60 lb | 0.45 cubic feet | 60 |
| 50 lb | 0.375 cubic feet | 72 |
| 40 lb | 0.3 cubic feet | 90 |
Before you pour
- Measure each part in feet. Measure the thickness in inches.
- Add about 10% extra.
- Pick bags or a truck. For a truck, ask the plant about its small-order fee.
- Have the wooden edges (forms) ready before the concrete comes.
- Save or print your shopping list, so you buy the right amount.
Questions people ask
How much concrete do I need?
Measure each part. Work out the cubic feet. Add 10% extra. Divide by 27 for cubic yards. The calculator above does it all for you.
How do I estimate concrete yards?
Change the thickness from inches to feet first. Then multiply it by the length and width. Divide by 27, and add about 10%.
How many yards of concrete for a 10 × 10 slab?
About 1.23 cubic yards at 4 inches thick. With 10% extra, about 1.36.
How much does a yard of concrete cost?
About $179 a cubic yard from a truck. That is the middle price of 5 US sellers. Small orders also pay a fee.
Is it cheaper to buy bags or order a truck?
For a small job, bags usually cost less. Plants charge a fee when you order less than a full truck. At our middle prices, a truck costs less from about 3.0 cubic yards.
How thick should a concrete slab be?
At least 4 in for a patio or a car driveway. Use 5 in or more if heavy trucks will drive on it. House floors need at least 3.5 in.
Is a 5 inch slab worth it?
It costs about 20% more than 4 in. But it can carry almost 50% more weight, says a ready-mix maker.
Where our numbers come from
- Bag sizes and weight: Quikrete Concrete Mix data sheet
- Extra for spills: Sakrete slab calculator (10%)
- How thick: a ready-mix maker's driveway guide
- How thick: a town building handout (Palatine, IL)
- House floors: US home building code (IRC R506.1)
- Base under a slab: US home building code (IRC R506.2.2)
- Bag price: the middle price of 9 US stores (Oct 2026)
- Truck price: the middle price of 5 US concrete sellers (Oct 2026)
- Small-order fee: the middle of 4 US cost guides (2026)
Made by BuildersCount. Last checked 3 Oct 2026.
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