Volume + bags + ready-mix
Concrete Calculator
Find the concrete you need for a slab, continuous footing, or round post holes. Compare bags with ready-mix and add your own prices if you have them.
Example only
- Measured volume
- 40 cu ft
- With 10% extra
- 44 cu ft
- At 0.6 cu ft / 80 lb bag
- 74 bags
Your result uses your own measurements, not this example.
Measuring guide
Choose the shape you are pouring
Measure the space the concrete will actually fill. Keep the same dimensions throughout each estimate.
Slab or pad
Measure length and width inside the forms, then the planned thickness.
Footing or trench
Measure the total run length, width, and depth of the continuous pour.
Round holes or piers
Measure diameter and depth, then enter the number of identical holes.
The math
From measurements to an order quantity
Both buying options use the same volume and your chosen allowance for extra concrete.
Example: a 10 × 12 ft slab, 4 in thick
- Convert thickness: 4 in ÷ 12 = ⅓ ft.
- Measured volume: 10 × 12 × ⅓ = 40 cu ft, or about 1.48 yd³.
- With 10% extra: 40 × 1.10 = 44 cu ft.
- Ready-mix: 44 ÷ 27 = 1.6296 yd³, shown as 1.63 yd³.
- 80 lb standard-mix bags: 44 ÷ 0.6, rounded up, is 74 bags.
A bag’s yield depends on the actual product. Confirm it on the package, and ask your ready-mix supplier about order increments, minimums, and delivery.
FAQ
Concrete calculator questions
How do I calculate concrete for a slab?
Multiply the slab length, width, and thickness after converting every dimension to feet. Divide cubic feet by 27 for cubic yards, then add an allowance for uneven ground and spillage.
How many 80 lb bags make a cubic yard?
For QUIKRETE standard Concrete Mix at about 0.6 cubic feet per 80 lb bag, 27 ÷ 0.6 = 45 bags before waste. Other products can have different yields; use the number printed on your bag.
How much extra concrete should I order?
This calculator starts with 10% extra. Your site, formwork, subgrade, and supplier may call for a different amount, so you can edit the allowance.
Does the post-hole estimate subtract the post?
No. It uses the full round hole volume, so an embedded post can make the estimate conservative. Measure actual hole dimensions and check your chosen product’s directions.
Are bagged concrete and ready-mix amounts different?
Both start from the same concrete volume. Bags are divided by your selected product yield and rounded up to whole bags. Ready-mix is shown in cubic yards, rounded up to 0.01 yd³ for planning.
Does the cost include installation?
No. Optional prices calculate bagged concrete or ready-mix material only. Delivery, short-load fees, tax, labor, excavation, reinforcement, and finishing are excluded.
References
Sources behind this calculator
Published measuring and product guidance informs the defaults. Check the instructions for the mix you buy.
Bag yields
QUIKRETE lists approximate yields for its standard 40, 50, 60, and 80 lb Concrete Mix bags.
QUIKRETE product data sheet ↗Measuring and allowance
The American Concrete Institute explains volume conversion and additional concrete for site variation.
ACI concrete placing guide ↗Common project types
Sakrete provides separate estimating guidance for slabs, post setting, and round deck footings.
Sakrete concrete calculators ↗More tools