Outdoor & Landscape · Calculator
Concrete Calculator
Use this concrete calculator to get cubic yards and the number of 40, 60 or 80 lb bags for a slab, footing, post holes or steps. Waste is built in, and it tells you when a ready-mix truck makes more sense than bags.
Your pour
How to calculate concrete for a slab, posts or steps
Slabs and footings are boxes: length times width times thickness. Convert the thickness from inches to feet (divide by 12), multiply, and divide cubic feet by 27 for cubic yards.
Post holes are cylinders. Find the hole volume, then subtract the post, because it takes up space the concrete does not have to fill. A 4x4 is really 3.5 x 3.5 in and a 6x6 is 5.5 x 5.5 in. If you put gravel in the bottom for drainage, the calculator removes that depth too and shows bags per hole.
Solid steps are a stack of blocks: the bottom step is one rise tall, the next is two, and so on. Planning wooden stairs off a new pad instead? The stair stringer calculator lays out the rise and run.
Bag yields: how many bags make a yard
Standard concrete mix yields about 0.30 cubic feet per 40 lb bag, 0.45 per 60 lb and 0.60 per 80 lb, based on Quikrete and Sakrete published data. That works out to 90, 60 and 45 bags per cubic yard.
The 80 lb bag is the best value per cubic foot, but it is heavy to lift and hard to mix in a wheelbarrow. For a handful of post holes, 50 or 60 lb bags are easier on your back and cost only a little more.
Mortar, sand mix and fast-setting post mixes yield less per pound than regular concrete mix. Check the yield printed on the bag.
| Bag | Yield | Bags per cu yd | Bags per 10 cu ft |
|---|---|---|---|
| 40 lb | 0.3 cu ft | 90 | 34 |
| 60 lb | 0.45 cu ft | 60 | 23 |
| 80 lb | 0.6 cu ft | 45 | 17 |
Slab thickness and concrete strength
Thickness matters more than strength for most home projects. Going from 4 to 5 inches adds 25 percent more concrete and a lot more load capacity. Pour on 4 inches of compacted gravel base; the gravel calculator sizes that layer.
| Project | Thickness |
|---|---|
| Walkway or sidewalk | 4 in |
| Patio | 4 in |
| Shed floor | 4 in |
| Hot tub pad | 4 to 6 in (check the maker) |
| Driveway, cars | 5 in |
| Driveway, trucks or RVs | 6 in |
| Garage floor | 4 to 6 in |
| Mix | Strength | Use it for |
|---|---|---|
| Standard concrete mix | 4,000 psi at 28 days | Slabs, walks, steps, footings |
| High-strength mix | 5,000 psi | Driveways, heavy loads, thin sections |
| Fast-setting mix | Sets in 20 to 40 min | Fence and mailbox posts, no mixing |
| Ready-mix (order spec) | 3,000 to 4,000 psi | Larger slabs; ask for air entrainment in freeze areas |
Bags vs ready-mix
Bagged concrete costs more per yard than ready-mix, but a truck has delivery charges and short-load fees on small orders. Under about 1 cubic yard, bags win. The calculator prices both so you can compare with your local numbers.
Labor is the hidden cost. Mixing 45 bags means lifting 3,600 lb of dry mix plus water, and a slab has to be placed and finished before the first batch starts to set. On anything bigger than a small patio, line up helpers or order a truck.
Measure the forms, not the plan
| Volume | 80 lb bags | Best option |
|---|---|---|
| Under 0.5 cu yd | Up to 23 | Bags, mixed by hand |
| 0.5 to 1 cu yd | 23 to 45 | Bags with a rented drum mixer |
| 1 to 2 cu yd | 45 to 90 | Price a short load or mixer trailer |
| Over 2 cu yd | 90+ | Ready-mix truck |
Tips for a smooth pour
- Order 10 percent extra. Running short mid-pour leaves a cold joint. Leftover bags keep for the next project if you store them dry.
- Plan control joints. Cut or tool joints every 8 to 12 ft on a 4 in slab (spacing in feet of 2 to 3 times the thickness in inches), about a quarter of the slab thickness deep.
- Use the right tools. A circular saw cuts form boards fast, and an angle grinder with a diamond blade cuts joints in cured concrete (see how to use an angle grinder).
- Odd shapes? Break them into boxes and add them up with the cubic yard calculator, then run the total through here for bags.
Measure carefully, add waste, and use the concrete calculator to decide between bags and a truck before you build the forms.
Frequently asked questions
How many 80 lb bags of concrete are in a yard?
It takes 45 bags of 80 lb concrete mix to make one cubic yard. Each 80 lb bag yields about 0.6 cubic feet, and a yard is 27 cubic feet. With 60 lb bags it takes 60, and with 40 lb bags it takes 90.
How many bags of concrete do I need for a 10x10 slab?
A 10 x 10 ft slab at 4 inches thick needs 33.3 cubic feet of concrete, or 56 bags of 80 lb mix. With 10 percent extra for waste and an uneven base, buy 62 bags. At that size, compare the price of a ready-mix short load.
How many bags of concrete do I need per fence post?
A 10 inch hole 30 inches deep around a 4x4 post takes about 1.15 cubic feet of concrete, which is 2 bags of 80 lb mix. A 12 inch hole 36 inches deep around a 6x6 takes about 1.7 cubic feet, or 3 bags (4 with a little extra for waste).
When should I order ready-mix instead of bags?
Bags make sense under about 1 cubic yard (45 bags of 80 lb). Above 2 yards, order a ready-mix truck. Between 1 and 2 yards, compare a short-load delivery or a rental mixer trailer against a long day of hand mixing.
How thick should a concrete slab be?
Pour 4 inches for patios, walkways and shed floors. Use 5 to 6 inches for driveways and anywhere a vehicle will park, and thicken the edges where a slab carries a wall or heavy load. Local code may require more.
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