Rectangular slabLive volume and bag estimate
Free construction calculator
Free concrete calculator
Calculate in your browser
Enter slab length and width in feet, thickness in inches, a waste allowance and an optional price per cubic yard. The calculator estimates cubic feet, cubic yards, cubic meters, 60 lb bags, 80 lb bags and concrete cost in your browser.
Free, no sign-up, and no measurements leave this page.
yd³Ready-mix volume
ft³US volume
m³Metric volume
60/80Pound bag estimates
Worked example
20 ft × 12 ft × 4 in slab with 10% waste
The example starts with 80 cubic feet and adds the selected waste allowance before converting the result.
80 ft³Base volume
88 ft³With 10% waste
3.26 yd³Ready-mix volume
2.49 m³Metric volume
Formula
How the concrete calculation works
The calculator treats the project as a rectangular prism. Measure irregular areas separately and add their volumes.
| Result | Formula | Important note |
|---|---|---|
| Base cubic feet | Length (ft) × width (ft) × thickness (in) ÷ 12 | Use finished dimensions for the rectangular slab. |
| Adjusted cubic feet | Base cubic feet × (1 + waste % ÷ 100) | Set the allowance for the actual job and supplier. |
| Cubic yards | Adjusted cubic feet ÷ 27 | NIST lists 27 cubic feet per cubic yard. |
| Bag count | Adjusted cubic feet ÷ stated bag yield, rounded up | Confirm yield on the product being purchased. |
| Material cost | Cubic yards × entered price per cubic yard | Does not include delivery, labor, pump, tax or minimum-load fees. |
From measurement to estimate
Use the result in four steps.
Measure the slab
Measure the rectangular slab length and width in feet and the planned thickness in inches.
Choose a waste allowance
Add a project-specific allowance for spillage, uneven excavation and ordering constraints.
Review volume and bags
Use the cubic-yard result for ready-mix pricing and the bag estimates only with products that state the listed yield.
Prepare the estimate
Confirm field measurements, mix requirements, delivery minimums, labor and local conditions before quoting the job.
Primary references
Calculation sources and limits
The unit conversion is based on NIST tables. Bag counts use the approximate yields published for the named QUIKRETE Concrete Mix; other products can differ.
- NIST Handbook 133, Appendix E — Volume conversions, including 27 cubic feet per cubic yard and cubic-foot-to-cubic-meter conversion.
- QUIKRETE Concrete Mix data sheet — Approximate yields of 0.45 cubic foot for a 60 lb bag and 0.60 cubic foot for an 80 lb bag of the named product.
Frequently asked questions
Questions about concrete volume and bags
How do you calculate yards of concrete for a slab?
Multiply length in feet by width in feet by thickness in feet to get cubic feet, then divide by 27 to get cubic yards. This calculator converts thickness from inches and adds the waste percentage you enter.
How many 80 lb bags of concrete are in a cubic yard?
At an approximate yield of 0.60 cubic foot per 80 lb bag, one cubic yard requires about 45 bags before any waste allowance. Always use the yield printed for the specific product.
How many 60 lb bags of concrete are in a cubic yard?
At an approximate yield of 0.45 cubic foot per 60 lb bag, one cubic yard requires about 60 bags before waste. Product yield can vary, so confirm the bag label.
How much extra concrete should I order?
The correct allowance depends on the form, excavation, placement method, supplier rules and site conditions. Enter the percentage appropriate to the job rather than relying on one universal rate.
Does the concrete calculator store my measurements?
No. The calculation runs in the browser and Pocket Invoice does not send or store the measurements or price you enter.
Next step
Turn the material quantity into a customer document.
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