BuildProjectCalc
Concrete & Masonry guide

How Much Concrete Do I Need?

Calculate concrete volume from project dimensions, convert cubic feet to cubic yards, add waste, and estimate whole premix bags.

The short answer

Multiply length by width by thickness to find concrete volume. Keep every dimension in the same unit, convert cubic feet to cubic yards by dividing by 27, and apply a stated waste percentage only after calculating the base volume.

Use the Concrete Calculator

Measurements to collect

Measure each rectangular section separately when the project changes size or thickness.

Length and width
Measure the finished footprint of each section. Do not combine unlike shapes until each area has been calculated.
Thickness or depth
Record thickness in inches for US customary calculations or centimeters for metric calculations, then convert it before multiplying.
Number of matching sections
Repeated pads or sections can be calculated once and multiplied by their quantity.

Concrete volume formula

Volume = length × width × thickness
Cubic yards = cubic feet ÷ 27
Cubic meters = meters × meters × meters

For feet-and-inches measurements, divide thickness in inches by 12 before multiplying. For metric measurements, divide centimeters by 100 to convert them to meters.

From dimensions to an order quantity

  1. Calculate every section

    Find the cubic volume of each rectangular section using consistent units.

  2. Combine the base volumes

    Add section volumes before applying waste so the allowance is calculated consistently.

  3. Convert to the purchasing unit

    Use cubic yards for many US bulk-concrete quantities, cubic meters for metric quantities, or cubic feet for bag-yield comparisons.

  4. Apply the chosen allowance

    Multiply base volume by 1 plus the waste rate expressed as a decimal. The appropriate allowance depends on excavation, form accuracy, placement, and project conditions.

Worked example: a small rectangular pad

A pad measures 12 feet long by 10 feet wide by 4 inches thick. This example uses a 10% allowance.

  1. Thickness: 4 in ÷ 12 = 0.3333 ft
  2. Base volume: 12 ft × 10 ft × 0.3333 ft = 40 ft³
  3. Base cubic yards: 40 ÷ 27 = 1.4815 yd³
  4. With 10% allowance: 1.4815 × 1.10 = 1.6296 yd³

Result: The planning quantity is about 1.63 cubic yards under the stated 10% assumption. Keep more precision until the final step so intermediate rounding does not reduce the estimate.

How whole concrete bags are counted

Premix bag estimates compare the required cubic feet with the nominal yield printed for the selected bag. Divide required volume by that yield, then round upward because a fraction of a bag cannot cover the remaining volume.

Bag yield varies by product and bag size. Use the package label or an editable calculator assumption rather than treating one yield as universal. The bag calculation is a volume conversion, not a recommendation about mix design or structural suitability.

Mistakes that change the result

Multiplying by inches as though they were feet
Four inches is one-third of a foot, not four feet.
Adding waste twice
Do not increase dimensions and then apply a second percentage unless that is an intentional, documented choice.
Rounding every intermediate value
Retain decimals through the cubic-yard conversion and round only the final planning quantity.

Concrete quantity questions

Should separate pours be combined?

They can be totaled for planning when they use the same purchasing basis, but keeping section volumes visible makes it easier to check dimensions and sequencing.

Does this method determine slab thickness?

No. Thickness is an input to the volume calculation. Project design, site conditions, and applicable requirements must determine it.