Concrete calculator.
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Every estimate shows its assumptions. Adjust waste, material density, and delivery rules to match your job. Use your supplier’s actual prices for a more useful comparison.See how we calculate
How to calculate concrete quantities
For a rectangular slab, multiply length by width by thickness in the same units. Convert cubic feet to cubic yards, then add your chosen waste allowance. Measure thickened edges and footings separately.
- Measure the pour. Measure the formed length, width and thickness, or the diameter and depth of a footing. Split thickened edges and steps into separate sections.
- Convert to cubic yards. Use length × width × (thickness ÷ 12) ÷ 27 for a rectangle in feet and inches. Concrete in this calculator does not use a compaction allowance.
- Add waste and choose a product yield. Multiply by (1 + waste fraction). Convert to bags with the mixed yield of the bag you will buy, such as the 0.60 ft³ approximate yield of an 80 lb QUIKRETE Concrete Mix #1101 bag.
- Compare bags and ready-mix. Enter actual quotes. Include delivery, order increments and tax only when you have those figures.
With length and width in feet and depth in inches, use length × width × (depth ÷ 12) ÷ 27 to get cubic yards. In metric, length in metres × width in metres × (depth in centimetres ÷ 100) gives cubic metres. One cubic yard is 0.764554857984 m³.
Worked example: 20 ft × 20 ft × 4 in
20 × 20 × (4 ÷ 12) ÷ 27 = 4.9383 yd³ before allowances.
This example adds 10% waste to the measured slab volume. Concrete does not use the calculator’s compaction allowance. The example thickness is a calculation input, not a structural specification.
- Volume with allowances
- 5.4321 yd³
- Whole 0.6 ft³ bags
- 245 bags
- Bulk at 0.25 yd³ increments
- 5.5 yd³
Fixed teaching example, independent of your inputs above. Dimensions, allowances and order increments are illustrative. Calculations retain precision until final bag and bulk rounding; displayed volumes are rounded.
Turn volume into an order
Bag counts use mixed yield, not dry bag weight. This example uses the approximate 0.6 ft³ yield of a QUIKRETE Concrete Mix #1101 80 lb bag. Confirm the yield of your actual product and the ready-mix supplier’s order increment.
Bag-yield source: QUIKRETE Concrete Mix #1101 — 80 lb bag · checked 2026-09-15. Yield does not establish structural suitability.
Compare bags and bulk using your quote
Multiply whole bags by the price per bag. For bulk, multiply the rounded order quantity by the supplier’s price in matching units; convert to weight only with a product-appropriate density. Compare both options with their delivery or collection costs, minimum charges and taxes. Include handling, equipment and labor separately when relevant.
Enter actual quotes in the calculator above. A blank price is unknown, not zero, and a material subtotal is not a complete delivered cost.
Read the calculation methodology for shape formulas and assumptions, or choose a project guide for measurement instructions.
Concrete calculator FAQ
How much concrete do I need?
For a rectangular slab, multiply length by width by thickness in the same units and convert to cubic yards, then add waste. With feet and inches, use length × width × (thickness ÷ 12) ÷ 27. Measure thickened edges and footings as separate sections. The example thickness is a calculation input, not a structural specification.
How many 80 lb bags are in a cubic yard?
Bags equal purchase cubic feet divided by bag yield, rounded up. QUIKRETE Concrete Mix #1101 lists an approximate 0.60 ft³ mixed yield for an 80 lb bag, so one cubic yard is 45 of those bags before project waste. Confirm the yield printed on the bag you will buy; 40 lb and 60 lb bags have different yields.
When should I use bags instead of ready-mix?
Bags are practical for small pads, post holes and repairs you can mix on site. Ready-mix is usually easier once the pour is several cubic yards, access allows a truck, and you need a consistent mix. Compare bag handling and ready-mix delivery using your actual quotes; a blank price is unknown, not zero.
What thickness should I enter for a slab?
Enter the thickness from your design or local requirement. CubicTally does not recommend a structural depth. Typical planning examples use 4 inches only to show the arithmetic. Thickened edges, steps and footings belong in their own sections.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27. One cubic yard is 0.764554857984 m³. Keep waste as a separate percentage after the neat volume so you can change it without remeasuring.