Concrete Calculator
Estimate concrete for slabs, footings, columns and stairs.
Result
Enter your dimensions to see how much concrete you need.
A 10 × 12 ft slab, 4 in thick needs about 1.75 cu yd of ready-mix concrete.
You need about1.75cu yd ready-mix
That's ~74 × 80 lb bags to hand-mix — ready-mix is the practical order above ~1 cu yd.
- 10% waste
- 80 lb bag yield
- ready-mix above ~1 cu yd
What you’ll need
- Ready-mix concrete1.75 cu yddeliveredShop ready mix concrete delivery on Amazon (opens in a new tab)
- Compactable gravel basesub-base under the pourShop paver base gravel on Amazon (opens in a new tab)
- Rebar or wire meshreinforcementShop concrete rebar and wire mesh on Amazon (opens in a new tab)
- Form lumber & stakesShop concrete form lumber and stakes on Amazon (opens in a new tab)
- Float, edger & trowelShop concrete float edger and trowel set on Amazon (opens in a new tab)
- Concrete sealerShop concrete sealer on Amazon (opens in a new tab)
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Estimated cost
See the full breakdown
- Volume
- 40.00 cu ft (1.48 cu yd)
- With 10% waste
- 44.00 cu ft (1.63 cu yd)
- 80 lb bags
- 74
- 60 lb bags
- 98
- 40 lb bags
- 147
- Ready-mix order (recommended)
- 1.75 cu yd
How this is calculated
- At 1.63 cu yd this is a ready-mix pour — bagged mix means 74 × 80 lb bags to hand-mix. Ready-mix delivery is the practical, usually cheaper order above ~1 cubic yard.
- Volume = length × width × thickness (all in feet), ÷ 27 for cubic yards.
- Waste: +10% added, then purchasable quantities rounded up.
- Bag yields: 80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft, 40 lb = 0.30 cu ft. Ready-mix rounded up to the next ¼ cu yd.
How much concrete do I need?
To figure out how much concrete you need, multiply length × width × thickness (converted to feet) to get cubic feet, divide by 27 for cubic yards, then add ~10% for waste. For example, a 10 ft × 10 ft × 4 in slab needs about 1.36 cubic yards. That works out to roughly 62 × 80 lb bags — but nobody hand-mixes that many: above about 1 cubic yard, ready-mix delivery is almost always cheaper and easier than bagged mix. Bagged mix makes sense for small pours (a few dozen bags at most). Enter your dimensions above for an instant, waste-adjusted estimate — in bags and ready-mix yards — that you can save or share.
How to use this calculator
- Pick a shape. Choose slab, footing, column or stairs.
- Enter dimensions. Type length, width and thickness in feet or inches — the tool converts and does the math.
- Set waste. Keep the default 10% waste, or adjust it for your pour.
- Read your estimate. See cubic yards, 40/60/80 lb bag counts and the ready-mix order, then save or share it.
Tips & real-world notes
- Above ~1 cubic yard, order ready-mix — hand-mixing dozens of bags isn’t realistic. Bagged mix is for small pours only.
- Over-order for uneven or soft subgrade — a slab on a rutted base uses more than its nominal thickness.
- Ready-mix has minimum loads and short-load fees; below ~1 cu yd, bagged mix is often cheaper.
- Bags are heavy: 45 × 80 lb bags ≈ 3,600 lb per cubic yard — check DIY feasibility.
- Add rebar or wire mesh and plan control joints — not part of the volume, but part of the job.
Frequently asked questions
How many bags of concrete are in a cubic yard?
One cubic yard equals about 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags of pre-mixed concrete. That many bags is also why anything over about a yard is usually a ready-mix order.
How much concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab at 4 inches thick is about 1.23 cubic yards, or 1.36 cubic yards with 10% waste. That’s roughly 62 × 80 lb bags — enough that ready-mix delivery (order 1.5 cu yd) is the practical choice.
How much concrete for a 24×24 garage slab?
A 24 ft × 24 ft slab at 4 inches is about 7.1 cubic yards, or 7.8 cubic yards with 10% waste — order about 8 cubic yards of ready-mix. At that size bagged mix (350+ bags) isn’t realistic; this is a ready-mix pour, often with a thicker 5–6 in edge.
How much concrete for a 20×20 slab?
A 20 ft × 20 ft slab at 4 inches is about 4.94 cubic yards, or 5.43 with 10% waste — order roughly 5.5 cubic yards of ready-mix. Go up to 5–6 inches for driveways or heavy loads.
How much concrete per fence or deck post?
Use the Column / round shape with your hole diameter and depth — that also covers sonotube and post-hole (tube form) pours. A typical 8 in diameter hole 2 ft deep is about 0.7 cu ft, so one 80 lb bag nearly fills it; buy one to two bags per post.
What waste factor should I use for concrete?
Use 10% for small residential jobs and uneven subgrade, and 5% only for large, form-controlled pours. Concrete can’t be re-ordered mid-pour, so it’s safer to slightly over-order.
Should I use bags or ready-mix?
Below about 1 cubic yard, bagged mix is usually cheaper and easier. Above that, ready-mix delivery saves a lot of mixing — watch for minimum-load and short-load fees.
How thick should a concrete slab be?
Patios and walkways are typically 4 inches; driveways and heavier loads often 5–6 inches. Thicker slabs use proportionally more concrete.
How much does a bag of concrete cover?
An 80 lb bag yields about 0.60 cu ft, a 60 lb bag about 0.45 cu ft, and a 40 lb bag about 0.30 cu ft of mixed concrete.
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