How to use this concrete calculator
Estimate concrete volume, 80-pound bags and material price. Measure the longest and widest sides in feet and the material depth in inches. The tool estimates cubic yards and converts to short tons using the density you enter. Apply an allowance for uneven ground, compaction, or spillage as appropriate to the material.
Important assumptions
For a simple rectangular slab, multiply length by width by slab thickness. A nominal 80-pound bag yields around 0.6 cubic feet; verify the manufacturer's yield and building code requirements.
Formula used
Volume (cubic yards) = length (ft) × width (ft) × depth (in) ÷ 324. Then multiply by (1 + your additional allowance as a decimal). Estimated US short tons = cubic yards × your density in US short tons per cubic yard.
For bulk material, enter price per yard or ton as labelled. The calculator adds any delivery fee entered, but does not automatically include tax, labor, permits or disposal.
Common project planning mistakes
- Confusing cubic yards of volume with tons of weight.
- Ordering by loose volume when your specifications refer to compacted depth.
- Assuming one dump truck can legally transport every material at the same payload.
- Using a national average price instead of requesting a local supplier quote.
Disclaimer: These estimates are for planning only and do not replace engineering specifications or quotes.
Example concrete quantities
These examples use rectangular measurements with no extra allowance. Bag counts round up to whole bags.
| Project size | Depth | Cubic yards | 80 lb bags at 0.6 cu ft yield |
|---|---|---|---|
| 10 × 10 ft | 2 in | 0.62 | 28 bags |
| 20 × 10 ft | 3 in | 1.85 | 84 bags |
| 40 × 12 ft | 4 in | 5.93 | 267 bags |
Common questions
How much concrete do I need for a slab?
For a rectangular slab, enter length and width in feet and thickness in inches. Divide length × width × thickness by 324 for cubic yards, then add an appropriate ordering allowance. Footings, thickened edges and steps require separate calculations.
How many 80 lb bags make a cubic yard?
At an illustrative yield of 0.6 cubic feet per 80-pound bag, 27 ÷ 0.6 equals 45 bags per cubic yard before allowance. Read your exact product’s yield; bag weight alone does not guarantee that volume.
Does this calculate concrete strength or slab design?
No. It estimates volume, illustrative bag count and material cost. Mix strength, reinforcement, thickness, subgrade, joints and other design requirements must come from the project specifications or a qualified professional.
Understand cubic yards, US tons and supplier conversion factors →