Concrete Calculator
Estimate the volume of concrete and number of bags needed for any slab, footing, column or wall.
Concrete Calculator — input your values
What does this construction calculator do?
The concrete calculator estimates the concrete volume 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 | π × 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.