Concrete Calculator
Calculate the concrete for a slab, footing, or pad in cubic yards and cubic feet, plus how many 40, 60, or 80 lb bags it takes, with a waste allowance. Calculates in your browser.
Concrete needed
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Show the math
Volume is length × width × thickness, converted to cubic yards:
cubic feet = length × width × (thickness ÷ 12) × (1 + waste%)
cubic yards = cubic feet ÷ 27
An 80 lb bag yields about 0.6 cu ft, a 60 lb bag 0.45, and a 40 lb bag 0.30. Add 5–10% waste for spillage and uneven subgrade. All math runs locally in your browser.
What this does
A concrete calculator finds the volume for a slab, footing, or pad in cubic yards and cubic feet, plus how many 40, 60, or 80 lb bags it takes.
How to use it
- Enter the length, width, and thickness.
- Choose a bag size.
- Add a waste allowance.
- Read the cubic yards and bag count.
How it works
Multiply the dimensions in feet for cubic feet, then divide by 27. An 80 lb bag yields about 0.6 cu ft, so roughly 45 make a yard.
cubic yards = L × W × thickness (ft) ÷ 27
Understanding your result
Order slightly more than the calculated volume, because you cannot top up a partial pour once it starts to set and coming up short ruins a slab. Bagged mix is convenient for small jobs but ready-mix is cheaper and faster beyond about a cubic yard, and remember to account for any over-dig or uneven sub-base that quietly increases the real volume.
Example
A 10×10 slab 4 inches thick is about 1.23 cubic yards.
Sources & methodology
Last updated .
Frequently asked questions
How do I find cubic yards?
Multiply length × width × thickness (all in feet) for cubic feet, then divide by 27. A 4-inch slab uses a thickness of 0.33 ft.
How many bags of concrete per yard?
An 80 lb bag yields about 0.6 cu ft, so roughly 45 bags make a cubic yard. The tool shows counts for 40, 60, and 80 lb bags.
How much waste should I add?
Add 5–10% for spillage, uneven subgrade, and over-excavation. For ready-mix, ordering a little extra avoids a short pour.
Do you store my numbers?
No. Everything is calculated locally in your browser.
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