Concrete Calculator
Enter your dimensions to get the volume in cubic yards, how many 40, 50, 60 or 80 lb bags to buy, and whether ready-mix makes more sense.
Enter your dimensions to see how much concrete you need.
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How to use the concrete calculator
- Pick the shape. Use Slab / footing / wall for anything rectangular — patios, sidewalks, shed pads, footings and walls are all length × width × thickness. Use Post hole for fence posts, deck piers and sonotube forms.
- Enter the dimensions in whatever units you measured. Each box has its own unit, so you can mix feet and inches.
- Set the number of pieces if you're pouring several identical ones, such as 12 post holes.
- Keep some extra. The calculator adds 10% by default for spillage, an uneven base and forms that bow slightly.
- Read the result: cubic yards (how ready-mix is sold), cubic feet, and the number of bags for each common bag size.
How to calculate concrete by hand
Concrete is ordered by volume. Work out the volume in cubic feet, then divide by 27 to get cubic yards, because one cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet. The only trap is units: convert everything to feet first, so a 4-inch slab is 4 ÷ 12 = 0.333 ft thick.
Worked example: a 10 × 10 ft patio, 4 inches thick
- Volume: 10 × 10 × 0.333 = 33.3 ft³
- In cubic yards: 33.3 ÷ 27 = 1.23 yd³
- Add 10% extra: 33.3 × 1.10 = 36.7 ft³ (1.36 yd³)
- 80 lb bags: 36.7 ÷ 0.60 = 61.1, so buy 62 bags — or 56 bags with no allowance at all.
Worked example: fence posts in 12-inch holes, 4 ft deep
- Radius 0.5 ft: π × 0.5² × 4 = 3.14 ft³ per hole
- 80 lb bags per hole: 3.14 ÷ 0.60 = 5.2, so about 6 bags per hole. Subtract a little for the post itself if you want to be exact: a 4×4 post (3.5 in square) takes up about 0.34 ft³ over 4 ft.
How many bags of concrete are in a yard?
Bag yields below come from the QUIKRETE Concrete Mix data sheet. Other brands of standard concrete mix are very close, but check the yield printed on your bag.
| Bag size | Yield per bag | Bags per cubic yard | Bags for a 10 × 10 ft × 4 in slab* |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | 123 |
| 50 lb | 0.375 ft³ | 72 | 98 |
| 60 lb | 0.45 ft³ | 60 | 82 |
| 80 lb | 0.60 ft³ | 45 | 62 |
*Including 10% extra.
How far does one bag or one yard go?
| Slab thickness | One 80 lb bag covers | One cubic yard covers |
|---|---|---|
| 2 in | 3.6 sq ft | 162 sq ft |
| 3 in | 2.4 sq ft | 108 sq ft |
| 4 in | 1.8 sq ft | 81 sq ft |
| 5 in | 1.44 sq ft | 65 sq ft |
| 6 in | 1.2 sq ft | 54 sq ft |
How thick should the concrete be?
Thickness drives the quantity more than anything else — going from 4 to 5 inches adds 25% more concrete. As a starting point:
- Walkways, patios and shed floors: 4 inches is the common standard.
- Driveways: at least 4 inches for passenger cars. Going to 5 inches uses 25% more concrete — the Tennessee Concrete Association puts the extra cost at about 20% — but raises load capacity by nearly half, which is worth it for trucks, RVs or soft soil.
- Thin toppings: standard concrete mix is designed for pours of 2 inches or more. For thinner layers, use a topping or resurfacing product instead.
- Footings and structural work: follow your plans and local building code — frost depth sets how deep footings go, and it varies a lot across the country.
Bags or ready-mix?
A cubic yard of concrete takes 45 bags of 80 lb mix — 3,600 lb that you lift, haul, mix and place, ideally in one continuous pour so you don't get cold joints between batches. That's why most people switch to ready-mix somewhere around 1 to 2 cubic yards.
- Bags make sense for post holes, footings for a small deck, steps, repairs and pads up to roughly a cubic yard, especially if the site is hard to reach with a truck.
- Ready-mix makes sense for driveways, large patios and anything over a couple of yards. Plants usually sell in quarter- or half-yard steps and many charge a short-load fee for small orders, so ask for the total delivered price before you decide.
- In between, some suppliers rent tow-behind mixer trailers loaded with a yard or less — a good middle ground for 1 to 2 yard jobs.
How much extra should you order?
Running short halfway through a pour is far worse than having a few bags left over. Plan on 5–10% extra: subgrades are never perfectly flat, forms bulge a little under the weight, and some concrete stays in the mixer and wheelbarrow. Measure the depth at several points after you prepare the base — if some spots are an inch deeper, your volume can jump by 25%.
Frequently asked questions
How many bags of concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 ft slab is 1.23 cubic yards. That's 56 bags of 80 lb mix with no allowance, or about 62 bags with 10% extra. With 60 lb bags you'd need about 82.
How many 80 lb bags make a yard of concrete?
45. Each 80 lb bag yields about 0.60 cubic feet, and a cubic yard is 27 cubic feet.
How much does a cubic yard of concrete weigh?
Roughly 3,800 to 4,000 lb — close to two tons. It's why a full yard of bagged mix is a serious workload for a small crew.
Should I subtract the post from the hole volume?
For wooden 4×4 posts the post takes up only about 10% of a 12-inch hole, so most people don't bother — it becomes part of your safety margin. For large holes and thin posts the difference is negligible.
Does rebar or wire mesh change how much concrete I need?
Not in any meaningful way. Steel reinforcement takes up well under 1% of a typical slab's volume.
Can I use this for metric measurements?
Yes. Choose m or cm next to each measurement and read the result in cubic meters as well as cubic yards.
Sources
- QUIKRETE, Concrete Mix (No. 1101) product data sheet — bag yields and density.
- Concrete Network, How thick should a concrete driveway be? — driveway thickness guidance, citing the Tennessee Concrete Association.