Concrete Calculator
Work out how much concrete a slab, footing, post hole, tube or set of stairs needs, in cubic yards, cubic meters and bags.
Cost (optional)
Includes 10% extra for spillage and an uneven base.
Slab 10 ft × 10 ft × 4 in, quantity 1.
Premixed bags
Or 123 bags of 40 lb or 82 bags of 60 lb. Each 80 lb bag yields about 0.60 ft³ of concrete.
Your calculation
The same steps the calculator runs, with your numbers.
- Convert the thickness to feet: 4 in ÷ 12 = 0.3333 ft.
- Volume: 10 × 10 × 0.3333 = 33.33 ft³.
- Add 10% waste: 33.33 × 1.1 = 36.67 ft³.
- Convert to cubic yards: 36.67 ÷ 27 = 1.36 yd³.
- Bags: 36.67 ÷ 0.6 = 61.11, rounded up to 62 bags of 80 lb.
Other slab thicknesses
Same shape, quantity and waste allowance, with only the thickness changed.
| Thickness | Total yd³ | 80 lb bags |
|---|---|---|
| 3 in | 1.02 | 46 |
| 5 in | 1.70 | 77 |
| 6 in | 2.04 | 92 |
| 8 in | 2.72 | 123 |
An estimate from the dimensions you enter. Check slab thickness, footing size and reinforcement against your plans and local building code.
How the Concrete Calculator Works
This concrete calculator finds the volume of a pour from its dimensions, adds a waste allowance, then converts the total into cubic yards, cubic meters and bags of premixed concrete. Ready mix concrete is sold by the cubic yard or cubic meter, and bagged mix is sold by weight, so the calculator gives both answers for the same job.
Every shape follows the same pattern used in the American Concrete Institute's guide to slabs on ground: convert all measurements to one unit, multiply to get the volume, then divide cubic feet by 27 to get cubic yards. One cubic yard is a cube 3 feet on each side, so it holds 3 × 3 × 3 = 27 cubic feet.
Slab, footing or wall
A wall or a continuous footing is the same rectangular box. A footing 20 feet long, 18 inches wide and 12 inches deep is 20 × 1.5 × 1 = 30 cubic feet, or 1.11 cubic yards before waste.
Column, pier or post hole
Tube (hollow cylinder)
Stairs
The stairs formula treats each step as a solid block resting on the ground: the first step is one rise tall, the second is two rises tall, and so on. It does not include a top landing. Add a landing as a separate slab with the same thickness as the top step.
How Much Concrete Do I Need? Slab Chart
Cubic yards of concrete for a slab by area and thickness, before any waste allowance. Multiply length by width to find the area in square feet, then read across to the thickness.
| Slab area | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|
| 100 ft² (10 × 10) | 1.23 yd³ | 1.54 yd³ | 1.85 yd³ |
| 200 ft² (10 × 20) | 2.47 yd³ | 3.09 yd³ | 3.70 yd³ |
| 400 ft² (20 × 20) | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ |
| 600 ft² (20 × 30) | 7.41 yd³ | 9.26 yd³ | 11.11 yd³ |
| 1,000 ft² (25 × 40) | 12.35 yd³ | 15.43 yd³ | 18.52 yd³ |
Each extra inch of thickness adds a quarter to a 4 inch slab's volume. A two car driveway pad of 20 × 24 feet at 6 inches is 240 cubic feet, or 8.89 cubic yards, and 9.78 cubic yards with 10 percent extra.
Bags of Concrete per Cubic Yard
Quikrete's data sheet for its Concrete Mix lists the approximate yield of mixed concrete from each bag size. Dividing 27 cubic feet by the yield gives the number of bags in one cubic yard.
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb (18.1 kg) | 0.30 ft³ (8.5 L) | 90 |
| 50 lb (22.6 kg) | 0.375 ft³ (10.6 L) | 72 |
| 60 lb (27.2 kg) | 0.45 ft³ (12.7 L) | 60 |
| 80 lb (36.2 kg) | 0.60 ft³ (17 L) | 45 |
| 90 lb (40.8 kg) | 0.675 ft³ (19.1 L) | 40 |
The calculator always rounds bags up to the next whole bag, because a partial bag still has to be bought. Bags suit small pours such as post holes, steps and repairs, where mixing by hand or in a small mixer is practical. Larger slabs are usually ordered from a ready mix supplier by volume.
Concrete for Post Holes and Columns
Concrete per foot of depth for round holes and column forms such as cardboard tube forms. Multiply by the depth in feet and by the number of holes, then add your waste allowance.
| Diameter | ft³ per foot of depth | 80 lb bags per foot | 80 lb bags, 4 ft deep, 10% extra |
|---|---|---|---|
| 6 in | 0.196 | 0.33 | 2 |
| 8 in | 0.349 | 0.58 | 3 |
| 10 in | 0.545 | 0.91 | 4 |
| 12 in | 0.785 | 1.31 | 6 |
| 16 in | 1.396 | 2.33 | 11 |
| 18 in | 1.767 | 2.95 | 13 |
| 24 in | 3.142 | 5.24 | 24 |
Doubling the diameter multiplies the volume by four, because the area of a circle grows with the square of the radius. A 24 inch pier needs four times the concrete of a 12 inch pier of the same depth.
How Much Extra Concrete to Order
A pour almost always takes more concrete than the plan dimensions suggest. The base settles or dips, forms bulge slightly, and some concrete is spilled or left in the mixer. The American Concrete Institute's guide to slabs on ground says it is common to add 10 or 15 percent to small loads and 5 to 10 percent to larger orders. The National Ready Mixed Concrete Association recommends ordering at least 4 to 10 percent more than required.
The calculator defaults to 10 percent. Enter 0 to see the exact geometric volume, or raise the allowance for an uneven base, a small pour, or a shape with many edges and corners. The ACI guide also notes that ready mix orders are usually rounded up to the next whole cubic yard or cubic meter.
How to Use the Calculator
- Pick units. Feet and inches, or meters and centimeters. Switching converts the sizes already entered.
- Choose the shape. Slab for patios, driveways, footings and walls. Column for post holes and piers. Tube for hollow round forms. Stairs for solid steps.
- Enter the size and quantity. Use the inside dimensions of the forms.
- Set the waste allowance and bag size. Add prices to see the cost.
The large number is the volume to order, with the other unit beside it. The card below converts it to bags, and the table shows how the total changes with a different thickness, depth, height or number of steps.
Common Mistakes
- Leaving thickness in inches. 10 × 10 × 4 is 400, not 33.33. Divide inches by 12 first.
- Dividing by 9 instead of 27. 9 converts square feet to square yards. Cubic feet need 27.
- Using the radius as the diameter. A 12 inch hole has a 6 inch radius.
- Forgetting the post. A post 3.5 inches square set 3 feet deep takes up about 0.26 ft³ of the hole.
- Ordering the exact volume. Add a waste allowance.
Concrete Units, Conversions and Weight
NIST lists the exact conversion factors used here: 1 cubic foot is 0.0283168 cubic meters and 1 cubic yard is 0.764555 cubic meters. Going the other way, 1 cubic meter is 1.3080 cubic yards or 35.315 cubic feet. In metric mode the calculator multiplies meters directly and converts centimeters by dividing by 100, so a 3 × 3 meter slab 10 cm thick is 3 × 3 × 0.10 = 0.90 cubic meters, or 0.99 with 10 percent extra.
| Quantity | Value |
|---|---|
| 1 cubic yard | 27 ft³ = 0.7646 m³ |
| 1 cubic meter | 1.3080 yd³ = 35.315 ft³ |
| Weight of 1 yd³ at 145 lb/ft³ | 3,915 lb |
| Weight of 1 m³ at 145 lb/ft³ | 2,323 kg |
| Reinforced concrete, 150 lb/ft³ | 4,050 lb per yd³ |
The weight tile uses 145 pounds per cubic foot, the value an FHWA research report lists for plain normal weight concrete up to 5,000 psi. The same report notes that reinforced concrete is generally taken as 5 pounds per cubic foot heavier, and that measured unit weights range from about 140 to 155 pounds per cubic foot. Lightweight concrete weighs much less, so use the weight as a guide for hauling and handling, not for structural design.
Frequently Asked Questions
How much concrete do I need for a 10x10 slab?
A 10 by 10 foot slab that is 4 inches thick holds 33.33 cubic feet of concrete, which is 1.23 cubic yards. Multiply 10 × 10 × 0.3333 feet, then divide by 27 to convert cubic feet to cubic yards. With a 10 percent allowance for spillage and an uneven base, plan on 36.67 cubic feet, or 1.36 cubic yards. At 6 inches thick the same slab needs 1.85 cubic yards before waste. In 80 lb bags, a 4 inch slab takes 56 bags with no allowance or 62 bags with 10 percent extra.
How many 80 lb bags of concrete make a yard?
Quikrete rates an 80 lb bag of its Concrete Mix at about 0.60 cubic feet of mixed concrete. A cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45 bags of 80 lb mix make one cubic yard. The same yard takes 60 bags of 60 lb mix, which yields 0.45 cubic feet each, or 90 bags of 40 lb mix at 0.30 cubic feet each. Buy a few more than the exact count, because yields are approximate and the base is rarely perfectly flat.
How do I calculate concrete for a post hole or column?
Use the cylinder formula: volume = π × radius² × depth, with every measurement in feet. A hole 12 inches wide has a radius of 0.5 feet, so a 4 foot deep hole holds π × 0.5² × 4 = 3.14 cubic feet. Adding 10 percent gives 3.46 cubic feet, or 6 bags of 80 lb mix. Multiply by the number of holes for a fence or deck. If a post stands in the hole, subtract the post's volume, since the post takes up space the concrete would otherwise fill.
How much extra concrete should I order?
The American Concrete Institute's guide to slabs on ground says it is common to add 10 to 15 percent to small loads and 5 to 10 percent to larger orders, to cover subgrade settlement, an uneven subgrade, spillage and waste. The National Ready Mixed Concrete Association suggests ordering at least 4 to 10 percent more than the plan volume. This calculator defaults to 10 percent. Use the higher end for small pours, hand mixing, or a base you could not grade accurately.
How much does a yard of concrete weigh?
A Federal Highway Administration research report lists 145 pounds per cubic foot for plain normal weight concrete up to 5,000 psi, and notes that reinforced concrete is generally taken as 5 pounds per cubic foot heavier. At 145 pounds per cubic foot, one cubic yard of 27 cubic feet weighs about 3,915 pounds, close to 2 US tons. The same report gives measured unit weights of about 140 to 155 pounds per cubic foot, or roughly 3,780 to 4,185 pounds per cubic yard. In metric units, 145 pounds per cubic foot is about 2,323 kilograms per cubic meter.
How do you calculate cubic yards of concrete?
Multiply length × width × thickness with all three in feet to get cubic feet, then divide by 27, because one cubic yard is 3 × 3 × 3 = 27 cubic feet. Convert inches to feet by dividing by 12 first, so a 4 inch slab is 0.3333 feet thick. For example, 20 × 24 feet at 6 inches is 20 × 24 × 0.5 = 240 cubic feet, and 240 ÷ 27 = 8.89 cubic yards. Add your waste allowance last: 10 percent extra brings the order to 9.78 cubic yards.
Sources & References
- QUIKRETE. Concrete Mix No. 1101 data sheet (yield per 40, 50, 60, 80 and 90 lb bag). quikrete.com
- American Concrete Institute, Committee E703. Concrete Craftsman Series: Slabs on Ground, CCS 1(10), 2010 (volume method, 27 ft³ per yd³, waste allowance, rounding up orders). concrete.org
- National Ready Mixed Concrete Association. Concrete in Practice CIP 8: Discrepancies in Yield (order 4 to 10 percent extra). concreteanswers.org
- Federal Highway Administration. FHWA HRT 05 057, October 2006: Appendix D, Proposed Revisions to the AASHTO LRFD Bridge Design Specifications (145 lb/ft³ plain concrete, reinforced 5 lb/ft³ heavier) fhwa.dot.gov and Appendix C (unit weights of about 140 to 155 lb/ft³) fhwa.dot.gov
- NIST. Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically (cubic foot and cubic yard to cubic meter). nist.gov
Results are estimates based on the dimensions entered and the bag yields listed above. They are not an engineering design. Every number on this page was recomputed with the same code the calculator uses. Methodology reviewed: 2026.