Concrete Calculator
Estimate the volume of concrete and number of bags needed for any slab, footing, column or wall.
Concrete Calculator — input your values
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Did you know?
The concrete calculator tells you how much concrete you need for common pours — slabs, footings, columns and walls — in cubic feet, cubic yards and cubic meters.
Recommendation
Buy 5–10% extra — running out mid-pour is costly because the surface must be poured in one go.
Also worth knowing
For slabs thicker than 4 inches, add rebar or wire mesh for strength.
What does this construction calculator do?
The concrete calculator tells you how much concrete you need for common pours — slabs, footings, columns and walls — in cubic feet, cubic yards and cubic meters.
It also converts the volume into the number of pre-mixed bags (40, 50, 60 or 80 lb) and tells you how many ready-mix trucks to order, including a waste allowance so you never run short on the job.
Formula and variables
Bag yields are approximate — check the label on the brand you buy.
| Symbol | Meaning |
|---|---|
| Slab | length × width × thickness |
| Column | Ï€ × r² × height |
| 1 cu yd | 27 cubic feet = 202 US gallons |
| 80 lb bag | ≈ 0.6 cubic feet of concrete |
Worked example
A 10 ft × 10 ft patio slab, 4 inches thick, using 60 lb bags with 10% waste.
- Area = 10 × 10 = 100 sq ft.
- Thickness = 4 in ÷ 12 = 0.333 ft.
- Volume = 100 × 0.333 = 33.3 ft³.
- In yards: 33.3 ÷ 27 = 1.23 cu yd.
- Bags = 33.3 ÷ 0.45 ≈ 74 bags, plus 10% = 82 bags.
Answer: Order ≈ 82 bags (or 1.25 cu yd ready-mix).
Tips and common mistakes
Tips
- Buy 5–10% extra — running out mid-pour is costly because the surface must be poured in one go.
- For slabs thicker than 4 inches, add rebar or wire mesh for strength.
- Pour in cool weather if possible; hot, dry conditions cause rapid drying and cracking.
- Check your local mixer-truck minimums — some ready-mix companies charge for a minimum of 3–5 yards.
Common mistakes to avoid
- Forgetting to convert inches to feet before multiplying.
- Using a 27 cu ft / yard conversion for wet concrete incorrectly — wet mix compacts to slightly less volume.
- Ordering exact volume with zero waste allowance.
Concrete Calculator — frequently asked questions
How do I calculate concrete for a slab?
Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 for cubic yards. For a 10×10 ft slab at 4 in: 100 × 0.333 = 33.3 ft³ = 1.23 yd³.
How many 80 lb bags are in a yard of concrete?
An 80 lb bag yields about 0.60 cubic feet, and there are 27 ft³ per yard — so roughly 45 bags of 80 lb mix make one cubic yard.
How many 60 lb bags are in a cubic yard?
A 60 lb bag yields about 0.45 ft³, so you need approximately 60 bags (27 ÷ 0.45) for one cubic yard.
Should I order ready-mix or bags?
For pours under about 2 cubic yards, bags are usually cheaper and more practical. For larger pours, ready-mix is faster and more consistent.
How much does a yard of concrete weigh?
Standard concrete weighs about 4,000–4,050 lb per cubic yard (≈ 2,400 kg/m³).
Can I pour concrete in the rain?
Light rain after finishing is okay with covers, but pouring during heavy rain washes out the surface cement. Check the forecast and cover fresh concrete.
How this calculator works
Concrete Calculator uses the formula shown above (Volume = area × depth; bags = volume ÷ bag yield) and computes the result instantly in your browser — no data leaves your device.