Concrete Calculator
Calculate how much concrete you need for slabs, footings, columns, and steps — in cubic yards, cubic feet, and exact bag counts.
Whether you're pouring a patio, setting fence posts, or forming up a driveway, this calculator handles the math for you. Pick your project shape below, enter the dimensions, and you'll instantly see cubic yards needed, exact bag counts at three common sizes, and an estimated delivered cost. A 10% waste allowance is included by default — industry standard for most residential pours.
40 lb bag = 0.30 ft³ · 60 lb bag = 0.45 ft³ · 80 lb bag = 0.60 ft³ of mixed concrete
How to calculate concrete yourself
At its core, figuring out how much concrete you need is a volume problem. You're filling a three-dimensional space, and concrete is sold by volume — specifically in cubic yards (for ready-mix delivery) or cubic feet (for bagged mix). Once you have the volume, the rest is unit conversion.
For a rectangular slab, the formula is straightforward:
The catch is that thickness is almost always given in inches, not feet. A 4-inch slab is 4 ÷ 12 = 0.333 feet thick. So a 10 ft × 12 ft × 4 in slab works out to 10 × 12 × 0.333 = 40 cubic feet. Divide that by 27 (the number of cubic feet in a cubic yard) and you get 1.48 cubic yards. Add 10% waste and you’ll want to order 1.63 yards — which in practice means 1.75 or 2 yards, since most suppliers round to the quarter-yard.
Footings use the same rectangular formula. Columns and round post holes use the cylinder formula: π × radius² × height. Stairs are trickier — each step is a rectangular prism, but the bottom step needs enough concrete for one rise, the second step needs two rises of height, and so on. The calculator above handles all of this automatically.
Cubic yards vs. cubic feet: which do I need?
Both units measure the same thing — volume — but you’ll use them differently depending on how you’re buying the concrete.
- Cubic yards are the currency of ready-mix delivery. Concrete trucks are sized in yards, and pricing, minimum orders, and short-load fees are all calculated per yard. One cubic yard equals 27 cubic feet.
- Cubic feet are how bagged concrete is measured. Each bag tells you how many cubic feet of wet mixed concrete it produces, so if you know your project volume in cubic feet, dividing by the bag’s yield gives you the exact bag count.
The calculator above shows both, so you can decide bagged or ready-mix without doing the conversion by hand.
How much concrete do I need for common projects?
Here’s a quick reference for typical residential projects, all at standard thicknesses, including 10% waste:
| Project | Dimensions | Cubic yards | 60 lb bags |
|---|---|---|---|
| Small patio | 10 × 10 × 4 in | 1.23 yd³ | ~74 bags |
| Two-car driveway | 20 × 20 × 6 in | 7.41 yd³ | ready-mix |
| Walkway | 3 × 20 × 4 in | 0.74 yd³ | ~44 bags |
| Fence post hole | 10 in dia × 24 in deep | ~0.04 yd³ | ~2 bags |
| Shed foundation | 12 × 12 × 4 in | 1.78 yd³ | ~106 bags |
| 3-step porch | 4 ft wide, 7.5 in rise, 10 in run | 0.24 yd³ | ~14 bags |
As a rough rule of thumb: if your project is under 1 cubic yard, bagging it yourself is practical. Between 1 and 3 yards, you’re in a gray zone — bags work but you’ll spend several hours mixing, and a half-yard ready-mix delivery is often competitive. Over 3 yards, always go ready-mix.
Bagged concrete vs. ready-mix: when to use each
The choice comes down to three things: total volume, access, and how much time you have before the concrete starts setting.
When bagged concrete makes sense
Bagged mix is ideal for small, discrete pours: a few fence posts, a mailbox base, a stepping-stone walkway, or repairs. You can work at your own pace, mix only what you need, and stop if it rains. A 60 lb bag of Quikrete or Sakrete costs around $5–6 at most home centers and yields 0.45 cubic feet of wet concrete.
When ready-mix is the right call
For anything over about a cubic yard — a patio, driveway, or slab foundation — ready-mix wins on speed, cost, and quality. A single truck delivers 8–10 yards of uniformly mixed concrete in minutes. Mixing the same volume from bags would take a crew several hours, and the first batches would start setting before the last ones were poured, creating weak cold joints between sections.
Ready-mix also gives you better quality control. The plant uses tested aggregates and precise water ratios, and you can order specific strengths (3,000 PSI, 4,000 PSI, etc.) that match your project’s load requirements. Expect to pay $120–$150 per cubic yard in most U.S. markets, with a short-load fee of $50–$100 for orders under 3–4 yards.
Concrete mix ratios and strength (PSI) explained
Concrete strength is rated in PSI (pounds per square inch) and measured at 28 days of curing. The number tells you how much compressive pressure a square inch of cured concrete can take before it fails.
- 2,500 PSI — Adequate for walkways, patios, and non-load-bearing slabs in mild climates.
- 3,000 PSI — Standard residential grade. Used for most slabs, driveways, and footings.
- 4,000 PSI — Recommended for driveways in cold climates (better freeze-thaw resistance), garage floors, and exterior footings.
- 5,000+ PSI — Structural applications, commercial work, and heavy-duty industrial floors.
For DIY bagging, the classic residential mix ratio is 1 : 2 : 3 — one part Portland cement, two parts sand, three parts gravel — with a water-to-cement ratio around 0.50. Pre-mixed bags already contain these ingredients in the correct proportions; you just add water per the label.
Why waste allowance matters
Every experienced contractor orders extra concrete. The 10% figure built into this calculator isn’t a fudge factor — it’s there because the subgrade is never perfectly level, forms always have small gaps, and you’ll always lose a bit to spillage during the pour.
Running short on a pour is much more expensive than ordering a little extra. If the truck leaves and you’re a quarter-yard short, you’re looking at a return trip, a second short-load fee, and a guaranteed cold joint where the pours meet. The cost of 10% extra concrete is trivial compared to the cost of a failed pour. For complex pours — round forms, heavily reinforced footings, or pours on rough terrain — bump the waste allowance to 15% or 20%.
Curing times and temperature considerations
Concrete doesn’t “dry” — it cures through a chemical reaction called hydration, and that reaction is sensitive to temperature and moisture. A few benchmarks to remember:
- 24–48 hours: Safe to walk on. Remove forms only if the concrete is firm to the touch.
- 7 days: About 70% of final strength. Light vehicles can drive on it.
- 28 days: Effectively full strength. Safe for heavy loads and sustained traffic.
Keep freshly poured concrete damp for at least the first 7 days. Cover with plastic sheeting, burlap, or use a spray-on curing compound. This single step can boost final strength by 30% or more. Never let fresh concrete freeze in the first 24 hours — ice crystals disrupt the curing reaction and can ruin the pour. In hot weather, the opposite problem applies: concrete can set too fast and crack. Pour early in the morning or in the evening when temperatures are below 85°F.
Common mistakes to avoid
A few things that experienced pourers watch for, and DIYers often learn the hard way:
- Adding too much water. Wet concrete is easier to place but weaker once cured. Stick to the water ratio on the bag.
- Skipping the subgrade prep. A proper slab sits on 4–6 inches of compacted gravel, not bare dirt. Skipping this step leads to settling and cracks.
- Forgetting reinforcement. Driveways and garage floors need rebar or wire mesh — plain concrete cracks under vehicle loads.
- Pouring in bad weather. Don’t pour in the rain, don’t pour if temperatures will dip below 40°F in the next 48 hours, and don’t pour at midday in 90°F+ heat.
- Finishing too early. Wait until bleed water evaporates from the surface before troweling. Trowel too soon and you’ll seal water inside, causing spalling later.