How to calculate your concrete project
Concrete orders should include enough material to avoid a cold joint or an unfinished pour. This calculator estimates volume for a simple slab or circular pad and includes an adjustable overage.
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- Use finished inside dimensions for the form.
- Measure thickness at several points if the base is uneven.
- Confirm bag yield or ready-mix minimums before ordering.
Formula
The calculation
Area × slab thickness = cubic feet. Divide by 27 for cubic yards. Bag count uses an approximate 0.60-cubic-foot yield per 80 lb bag.
Worked example
A 10 × 10 ft slab at 4 inches thick with 10% extra requires about 1.36 cubic yards, or roughly 62 eighty-pound bags.
The result is a planning estimate. Round purchasing quantities up to available package sizes and confirm the exact product specifications.
Material assumptions
- Bag yield is estimated at 0.60 cubic feet per 80 lb bag.
- No volume is deducted for reinforcement.
- Footings, thickened edges, slopes, and voids must be calculated separately.
Read the complete calculator methodology for unit conversions, rounding rules, and limitations.
Why the waste factor matters
A waste or extra-material percentage allows for real-world variation: irregular edges, cuts, compaction, settling, spillage, and measurement error. Ten percent is a useful starting point, not a universal rule. Increase or reduce it according to the material, layout, and supplier guidance.
Unit conversions used
Questions
Concrete Calculator FAQ
How much extra concrete should I order?
Ten percent is a common planning allowance for uneven excavation, form variation, and spillage. Discuss large pours with the ready-mix supplier.
How many 80 lb bags make a cubic yard?
At an approximate yield of 0.60 cubic feet per bag, one cubic yard requires about 45 bags. Always use the yield printed on the product.
Is this calculator suitable for structural work?
It is a quantity-planning tool, not an engineering design. Structural dimensions, reinforcement, mix, and code compliance require qualified guidance.