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Concrete Calculator — Cubic Yards & Bags

Enter slab length, width, and thickness to calculate how much concrete you need in cubic yards, cubic meters, and complete 40, 60, or 80 lb bags.

  • Instant conversion
  • Referenced units
  • Adjustable waste

Slab dimensions

Enter your measurements

Measurement system

This device

Saved estimates

Save an estimate to keep it here. Data stays in this browser.

Transparent by design

Concrete slab formula

A rectangular slab is a rectangular prism. Its net volume is calculated by multiplying length, width, and thickness after all three measurements have been converted to the same unit.

Net volumeV = length × width × thicknessOrder volumeVorder = V × (1 + waste ÷ 100)

For imperial results, 27 cubic feet equals 1 cubic yard. Metric calculations use cubic meters, where 1 cubic meter equals 1,000 liters.

Worked example

A slab that equals exactly one cubic yard

Use a 9 ft × 9 ft slab at 4 in thick, 0% waste, and the published 0.60 ft³ yield for an 80 lb bag. This is the independent reference vector used in the test suite.

  1. 01
    Convert the thickness4 in ÷ 12 = 0.333333 ft
  2. 02
    Find cubic feet9 × 9 × 0.333333 = 27 ft³
  3. 03
    Convert to cubic yards27 ft³ ÷ 27 = 1 yd³
  4. 04
    Find complete bags27 ft³ ÷ 0.60 ft³ = 45 bags
Verified result1 yd³ and 45 × 80 lb bags

No waste is included in this controlled example. Add the project-specific allowance only after verifying the formed dimensions.

Common questions

Concrete calculator FAQ

Practical context for using the estimate responsibly.

How does the concrete calculator work?

It multiplies length by width by thickness after converting every dimension to a consistent unit. It then adds your chosen waste allowance and converts the result to cubic yards, cubic meters, cubic feet, and liters.

How much concrete do I need for a 10 × 10 slab at 4 inches thick?

A 10 ft × 10 ft slab at 4 inches thick is about 1.235 cubic yards before waste. With a 10% allowance it is about 1.358 cubic yards, or 62 complete 80 lb bags when the selected mix yields 0.60 cubic feet per bag.

How much extra concrete should I order?

The calculator starts at 10%, but the correct allowance depends on subgrade accuracy, formwork, spillage, order increments, and site conditions. Adjust the percentage and confirm the final quantity with your supplier or project professional.

Why is the bag count approximate?

Bag yield varies by product and the manufacturer's published yield is approximate. This calculator uses published Sakrete High-Strength Concrete Mix yields of 0.30, 0.45, and 0.60 cubic feet for 40, 60, and 80 lb bags respectively.

Can I use metric measurements?

Yes. Select Metric to enter length and width in meters and thickness in centimeters. Switching systems converts the values already entered.

Continue planning

Calculate, then verify the purchase plan.

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