Concrete Calculator
Multiply length × width × thickness in feet and divide by 27 to get cubic yards: a 10 × 10 ft slab 4 in thick needs 1.23 cubic yards, or 62 bags of 80 lb mix with 10% extra.
Concrete estimator
How to calculate concrete for a slab
Concrete is ordered by volume. Work in feet, then convert:
cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12 cubic yards = cubic feet ÷ 27 bags = cubic feet ÷ bag yield (cu ft)
Sakrete prints the same method on its data sheets: multiply length by width by thickness in inches, divide by 12, then divide by the yield of the bag. For the 10 × 10 ft, 4 in example: 10 × 10 × 4 ÷ 12 = 33.33 cu ft, and 33.33 ÷ 27 = 1.235 cu yd.
How many bags of concrete do I need?
Divide the cubic feet by what one bag makes. The bag yields below are copied from each manufacturer's data sheet. Bags per cubic yard is our arithmetic (27 ÷ yield), not a number the manufacturers print.
| Product | Bag | Yield | Bags per cu yd |
|---|---|---|---|
| QUIKRETE Concrete Mix (4,000 psi) | 40 lb | 0.3 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 | |
| 60 lb | 0.45 cu ft | 60 | |
| 80 lb | 0.6 cu ft | 45 | |
| 90 lb | 0.675 cu ft | 40 | |
| QUIKRETE 5000 | 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 | |
| 80 lb | 0.6 cu ft | 45 | |
| QUIKRETE Fast-Setting | 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 | |
| Sakrete High-Strength | 40 lb | 0.3 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 | |
| 80 lb | 0.6 cu ft | 45 | |
| 90 lb | 0.66 cu ft | 40.9 | |
| Sakrete 5000 Plus | 50 lb | 0.38 cu ft | 71.1 |
| 60 lb | 0.47 cu ft | 57.4 | |
| 80 lb | 0.6 cu ft | 45 | |
| Sakrete Fast Setting | 50 lb | 0.38 cu ft | 71.1 |
Sakrete's 5000 Plus 60 lb bag yields 0.47 cu ft, a little more than the 0.45 cu ft of QUIKRETE's 60 lb bags, so check the yield printed on the bag you buy and type your own number if it differs.
How thick should a concrete slab be?
| Project | Thickness | Source |
|---|---|---|
| Patio | 4 in | QUIKRETE |
| Sidewalk (minimum) | 4 in | QUIKRETE |
| Driveway, passenger cars | 4 in | QUIKRETE |
| Driveway, trucks and heavy vehicles | 5 to 6 in | QUIKRETE |
| Small slabs and bases | 4 to 6 in | QUIKRETE |
Sakrete adds that you should leave room for both the gravel base and a minimum of 4 in of concrete. Local codes and any engineered plans take priority over these general figures.
Concrete for fence posts and round columns
A round hole or tube is a cylinder: π × (diameter ÷ 2)² × depth. QUIKRETE's post-setting guide says the hole should be about 3 times the post's width, its depth one third of the post's length plus 6 in for a gravel base, and at least 6 in below the frost line for structural posts such as deck supports. Subtract the post itself if you want a tighter number: the calculator gives the volume of the full hole, which builds in a little extra.
For scale, QUIKRETE's Fast-Setting data sheet says two 50 lb bags set a 3 in square post in a 9 in diameter hole 30 in deep.
Concrete for steps
Poured steps are a stack of blocks. Step 1 (bottom) is one rise tall, step 2 is two rises tall, and so on, each one tread deep. The calculator adds width × run × (step number × rise) for every step. It does not include a landing at the top: add that as a slab.
How long before you can walk or drive on it?
- Sakrete High-Strength Concrete Mix: foot traffic after 24 hours, vehicles after 72 hours.
- QUIKRETE 5000: walk-on time of 10 to 12 hours.
- QUIKRETE Fast-Setting: wait at least 4 hours before walking on it.
- Keep it moist: QUIKRETE says 5 to 7 days depending on temperature; NRMCA says at least 3 to 7 days. Protect it from freezing for the first 48 hours.
- No de-icers for the first 28 days (QUIKRETE).
Strength ratings such as 4,000 psi are measured at 28 days, which the American Cement Association calls standard practice. A cubic yard weighs about 3,780 lb at QUIKRETE's 140 lb/cu ft (140 to 155 lb/cu ft per the FHWA), which matters for wheelbarrow trips and trailer loads.
Frequently asked questions
How many bags of concrete are in a yard?
A cubic yard is 27 cubic feet. Using QUIKRETE Concrete Mix yields, that is 45 bags of 80 lb (0.60 cu ft each), 60 bags of 60 lb (0.45 cu ft) or 90 bags of 40 lb (0.30 cu ft), before any extra for waste.
How much concrete do I need for a 10x10 slab?
A 10 × 10 ft slab 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. With 10% extra that is 36.7 cu ft: 62 bags of 80 lb or 82 bags of 60 lb.
How thick should a concrete slab be?
QUIKRETE's project guides give 4 inches for patios, sidewalks and driveways that serve only passenger cars, and 5 to 6 inches for driveways used by trucks and other heavy vehicles.
How much extra concrete should I order?
Sakrete's concrete calculator page recommends adding 10% to your estimate. Uneven subgrade, form bulges and spillage all use up concrete, and running short mid-pour is worse than a few leftover bags.
How much does a yard of concrete weigh?
QUIKRETE lists the unit weight of its Concrete Mix at about 140 lb per cubic foot, so one cubic yard (27 cu ft) weighs roughly 3,780 lb. The FHWA gives a normal-weight concrete range of about 140 to 155 lb per cubic foot.
When can I walk or drive on new concrete?
It depends on the mix, so check your bag. Sakrete's High-Strength Concrete Mix data sheet says foot traffic after 24 hours and vehicles after 72 hours. QUIKRETE Fast-Setting allows walking after 4 hours. QUIKRETE says to protect new concrete from freezing for the first 48 hours.
Sources
- QUIKRETE Concrete Mix Data Sheet (Product No. 1101), QUIKRETE. Used for: bag yields (40, 50, 60, 80, 90 lb), unit weight 140 lb/cu ft, curing 5 to 7 days, freeze protection.
- Concrete Calculators, Sakrete. Used for: the 10% extra allowance.
- Sakrete High-Strength Concrete Mix Technical Data Sheet, Sakrete. Used for: Sakrete yields, foot traffic 24 h and vehicles 72 h, 4 in minimum for flatwork.
- Sakrete 5000 Plus Concrete Mix Technical Data Sheet, Sakrete. Used for: Sakrete 5000 Plus yields and the bag-count formula.
- Sakrete Fast Setting Concrete Mix Technical Data Sheet, Sakrete. Used for: Sakrete Fast Setting yield.
- QUIKRETE 5000 High Early Strength Concrete Mix Data Sheet (Product No. 1007), QUIKRETE. Used for: QUIKRETE 5000 yields and walk-on time.
- QUIKRETE Fast-Setting Concrete Mix Data Sheet (Product No. 1004-50), QUIKRETE. Used for: Fast-Setting yields, walk-on time, post example, de-icer wait.
- Project Guide: Concrete Driveways and Curbs, QUIKRETE. Used for: driveway thickness.
- Project Guide: Concrete Patios, QUIKRETE. Used for: patio thickness.
- Project Guide: Concrete Sidewalks and Small Slabs, QUIKRETE. Used for: sidewalk and small slab thickness.
- Project Guide: Setting Posts, QUIKRETE. Used for: post hole diameter, depth and gravel base.
- FHWA-HRT-05-057, Appendix C: Proposed Revisions to Specifications, U.S. Federal Highway Administration. Used for: normal-weight concrete range 140 to 155 lb/cu ft.
- Concrete in Practice CIP 11: Curing In-Place Concrete, National Ready Mixed Concrete Association (NRMCA). Used for: curing for at least 3 to 7 days.
- Cement & Concrete FAQ, American Cement Association (formerly PCA). Used for: strength measured at 28 days.
Estimates only. Check local codes and your product's label. Found a number that does not match its source? Send a correction.