Concrete Calculator
To calculate concrete for a driveway or slab, multiply length × width in feet by the thickness in inches and divide by 324 for cubic yards. For example, a 20 × 20 ft driveway 4 inches thick needs 4.94 cubic yards, or 223 bags of 80 lb.
How to calculate concrete
- Measure the area. Measure the length and width in feet and multiply them for square feet. For a round slab, measure the diameter.
- Choose the thickness. A driveway needs at least 4 inches of concrete, and 5 inches where delivery trucks will drive on it.
- Find the cubic yards. Multiply the square feet by the thickness in inches and divide by 324, which is 12 inches × 27 cubic feet.
- Count bags. Divide the cubic feet by the bag's yield: an 80 lb bag makes 0.60 cubic feet, so a cubic yard takes 45 bags.
- Order extra. Order 4 to 10% more than the estimate to cover waste, uneven ground and forms that bulge.
Worked example: a 20 × 20 ft driveway 4 inches thick: 400 sq ft × 4 ÷ 324 = 4.94 cubic yards, or 133.33 cubic feet. That's 223 bags of 80 lb; with 4–10% extra, order 5.14–5.43 cubic yards.
How thick should a concrete driveway be?
At least 4 inches. The Michigan Concrete Association and the Illinois Ready Mixed Concrete Association both recommend a minimum of 4 inches of concrete for a residential driveway, and the Michigan guidelines call for 5 inches when delivery vehicles will use it. Their other recommendations:
- Base: a compacted base of crushed stone, gravel or sand at least 4 inches thick (Michigan). The gravel calculator works out the stone.
- Concrete: 4,000 psi at 28 days (both); Michigan also specifies air-entrained concrete.
- Slope: at least ⅛ inch per foot, for drainage (both).
- Joints: control joints cut a quarter of the slab's depth, no more than 8 feet apart in a 4-inch slab (Michigan) or 10 feet apart (Illinois).
- Traffic: wait 7 days before driving on it, unless tests show it has reached 3,000 psi (Michigan).
A cubic yard of concrete covers 81 sq ft at 4 inches thick, 65 sq ft at 5 inches and 54 sq ft at 6 inches.
| Driveway | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|
| 10 × 20 ft | 2.47 | 3.09 | 3.70 |
| 12 × 24 ft | 3.56 | 4.44 | 5.33 |
| 20 × 20 ft | 4.94 | 6.17 | 7.41 |
| 24 × 24 ft | 7.11 | 8.89 | 10.67 |
| 20 × 40 ft | 9.88 | 12.35 | 14.81 |
How to calculate concrete for stairs
For solid concrete steps, multiply the width by the tread depth and the rise, then by 1 + 2 + … up to the number of steps, because each step is a block that runs down to the ground. For example, 3 steps 4 ft wide with 7-inch risers and 11-inch treads: 7 × 11 × (1 + 2 + 3) × 48 = 22,176 cubic inches, which is 12.83 cubic feet (0.48 cubic yards), or 22 bags of 80 lb.
If the top step is a landing, use the landing's depth for the top step instead of the tread. The same 3 steps with a 36-inch landing on top take 27.42 cubic feet (1.02 cubic yards), or 46 bags.
Avoid the shortcut rise × tread × width × number of steps. It leaves out the concrete beneath the upper steps: for the 3 steps above it gives 6.42 cubic feet, half the true amount, and for 5 steps it gives a third.
| Steps | Height | Cubic feet | Cubic yards | 80 lb bags |
|---|---|---|---|---|
| 1 | 7 in | 2.1 | 0.08 | 4 |
| 2 | 14 in | 6.4 | 0.24 | 11 |
| 3 | 21 in | 12.8 | 0.48 | 22 |
| 4 | 28 in | 21.4 | 0.79 | 36 |
| 5 | 35 in | 32.1 | 1.19 | 54 |
| 6 | 42 in | 44.9 | 1.66 | 75 |
The 2021 International Residential Code sets these limits for stairs at a house; check your local code, which may differ:
- Risers: 7¾ inches high at most, varying by no more than ⅜ inch within a flight.
- Treads: at least 10 inches deep.
- Width: at least 36 inches.
- Landings: at the top and bottom, at least 36 inches deep on a straight flight.
- Concrete: outdoor steps need at least 2,500 psi where weathering is negligible, 3,000 psi where it's moderate and 3,500 psi where it's severe, air-entrained in the last two. The bagged mixes cited here are rated 4,000 psi at 28 days.
A flight that spans between supports on a sloped slab, rather than resting on the ground, is a structural element: its concrete and reinforcing come from an engineer's drawings, not this calculator.
How many bags of concrete are in a yard?
A cubic yard of concrete takes 45 bags of 80 lb, 60 bags of 60 lb, 72 bags of 50 lb or 90 bags of 40 lb, from the yields on Quikrete's and Sakrete's data sheets. An 80 lb bag makes 0.60 cubic feet, enough for 1.8 sq ft of slab 4 inches thick.
| Bag | Makes | Bags per cubic yard | Covers 4 in thick |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 0.9 sq ft |
| 50 lb | 0.375 cu ft | 72 | 1.1 sq ft |
| 60 lb | 0.45 cu ft | 60 | 1.4 sq ft |
| 80 lb | 0.60 cu ft | 45 | 1.8 sq ft |
Other materials
For the compacted base under a slab or driveway, use the gravel calculator. For a gravel or asphalt driveway instead, see the gravel driveway calculator and the asphalt calculator.
To convert any volume between cubic yards, cubic feet and cubic meters, use the cubic yard calculator.
Frequently asked questions
How do I figure out how much concrete I need for my driveway?
Multiply the length by the width in feet and the thickness in inches, then divide by 324 for cubic yards. A 20 × 20 ft driveway 4 inches thick needs 20 × 20 × 4 ÷ 324 = 4.94 cubic yards. The National Ready Mixed Concrete Association advises ordering at least 4 to 10 percent more, so order 5.14–5.43 cubic yards.
Is 4 inches of concrete good for a driveway?
Four inches is the minimum that the Michigan Concrete Association and the Illinois Ready Mixed Concrete Association recommend for a residential driveway, poured on a compacted base with 4,000 psi concrete. The Michigan guidelines call for at least 5 inches when delivery vehicles will use the driveway.
How much concrete do I need for a staircase?
For solid concrete steps, multiply the width by the tread depth and the rise, then by 1 + 2 + … up to the number of steps, because each step is a block that runs down to the ground. A flight of 3 steps 4 ft wide with 7-inch risers and 11-inch treads takes 12.83 cubic feet (0.48 cubic yards), or 22 bags of 80 lb. A stair flight that spans on a sloped slab is designed by an engineer instead.
What is the formula for calculating stairs?
Divide the total height by 7¾ inches and round up for the number of risers, then divide the height by that number for each riser: a 30-inch rise needs 4 risers of 7.5 inches. The International Residential Code limits risers to 7¾ inches and requires treads at least 10 inches deep and stairs at least 36 inches wide.
Related calculators
Sources and methodology
Volumes use (length × width × thickness) ÷ 27 with all measurements in feet; with the thickness in inches, that's square feet × inches ÷ 324. Solid steps add one block per step from the ground up: rise × tread × (1 + 2 + … up to the number of steps) × width, with the landing's depth used for the top step when there is one. Bags use the yields on Quikrete's and Sakrete's data sheets (an 80 lb bag makes 0.60 cubic feet) and are rounded up. The order line adds the 4–10% that the National Ready Mixed Concrete Association advises. Driveway figures are from the Michigan Concrete Association and the Illinois Ready Mixed Concrete Association; stair limits and step concrete strength are from the 2021 International Residential Code. Concrete is sold by volume, so the calculator doesn't give its weight.
- QUIKRETE Concrete Mix (No. 1101) product data sheet, revised October 2022: bag yields
- Sakrete High-Strength Concrete Mix technical data sheet (2023): bag yields
- Michigan Concrete Association: Driveway & Sidewalk Design and Placement Guidelines (August 2025)
- Illinois Ready Mixed Concrete Association: Driveways (technical resources for homeowners)
- National Ready Mixed Concrete Association: Concrete in Practice 8, Discrepancies in Yield (2016)
- International Code Council: 2021 International Residential Code, Section R311.7 (stairways)
- International Code Council: 2021 International Residential Code, Table R402.2 (minimum concrete strength)
- NIST Special Publication 811 (Guide for the Use of the SI), Appendix B.8: conversion factors