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Simple Calculators

Concrete Calculator

Cubic yards and bags of concrete for a slab, footing, or column

Pour Dimensions

4 in is common for patios and walkways

Number of identical pours

5–10% covers spillage and uneven ground

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Quick summary

Concrete volume is length × width × thickness, with thickness converted from inches to feet. A 10 × 10 ft slab 4 inches thick holds 100 sq ft × 4/12 ft = 33.33 cu ft, or 1.23 cubic yards (27 cu ft per yard). Add a 10% waste allowance and order 1.36 cubic yards — or about 62 bags of 80 lb premix at the typical 0.60 cu ft yield per bag.

Reviewed by Simple Calculators TeamLast reviewed

How to use this calculator

  1. Choose the shape: Slab, Footing or wall, or Round column.
  2. Enter the dimensions — Length (ft), Width (ft), and Thickness (in) for a slab; Length (ft), Width (in), and Depth (in) for a footing; or Diameter (in) and Height (ft) for a column.
  3. Set Quantity to the number of identical pours, such as several post holes.
  4. Set Waste (%) — 10% is a common planning margin.
  5. Optionally enter Price per Cubic Yard ($) to estimate the ready-mix cost.
  6. Read the cubic yards to order, plus cubic feet, cubic meters, and the number of 80, 60, or 40 lb bags.

Concrete Formulas

Slab Volume

Vyd3=L×W×T1227V_{\text{yd}^3} = \frac{L \times W \times \frac{T}{12}}{27}
L
Slab length in feet
W
Slab width in feet
T
Thickness in inches (divided by 12 to convert to feet)
27
Cubic feet in one cubic yard

Worked example

A 10 × 10 ft slab, 4 inches thick

Given
L = 10 ft
W = 10 ft
T = 4 in
Calculation
10×10×41227=33.3327\frac{10 \times 10 \times \frac{4}{12}}{27} = \frac{33.33}{27}
Result
V ≈ 1.23 cubic yards

Waste Allowance

Vorder=V×(1+w100)V_{\text{order}} = V \times \left(1 + \frac{w}{100}\right)
V
Calculated volume
w
Waste allowance in percent

Worked example

The same slab with a 10% allowance

Given
V = 1.2346 yd³
w = 10%
Calculation
1.2346×1.101.2346 \times 1.10
Result
Order ≈ 1.36 cubic yards

Premix Bags

Bags=⌈Vft3yield per bag⌉\text{Bags} = \left\lceil \frac{V_{\text{ft}^3}}{\text{yield per bag}} \right\rceil
V (cu ft)
Volume including waste, in cubic feet
Yield per bag
Typically 0.60 cu ft (80 lb), 0.45 (60 lb), or 0.30 (40 lb); check the bag

Worked example

36.67 cu ft using 80 lb bags

Given
V (cu ft) = 36.67
Yield per bag = 0.60 cu ft
Calculation
⌈36.670.60⌉=⌈61.1⌉\left\lceil \frac{36.67}{0.60} \right\rceil = \left\lceil 61.1 \right\rceil
Result
62 bags of 80 lb premix

Round Column Volume

Vft3=π(d24)2×HV_{\text{ft}^3} = \pi \left(\frac{d}{24}\right)^2 \times H
d
Diameter in inches (d ÷ 24 is the radius in feet)
H
Height in feet

Worked example

A 12 in diameter column, 4 ft tall

Given
d = 12 in
H = 4 ft
Calculation
π×0.52×4\pi \times 0.5^2 \times 4
Result
V ≈ 3.14 cubic feet

Frequently asked questions

How many bags of concrete do I need?

Divide the volume in cubic feet by the yield printed on the bag, then round up. Premix bags typically yield about 0.60 cu ft (80 lb), 0.45 cu ft (60 lb), or 0.30 cu ft (40 lb). A 10 × 10 ft slab at 4 inches with 10% waste is 36.67 cu ft — about 62 bags of 80 lb mix. Yields vary by product, so check the bag.

Should I use bags or order ready-mix?

Bags suit small jobs such as post holes, steps, and small pads where hand mixing is practical. For larger pours, ready-mix delivered by truck is usually easier and gives a more consistent pour. Many suppliers have a minimum order or a short-load fee, so ask before ordering a small amount.

How much extra concrete should I order?

A waste allowance of roughly 5–10% is a common planning margin. It covers spillage, forms that bow slightly, and a subgrade that is a little lower in places. Running short partway through a pour is far more costly than a small leftover.

How do I calculate concrete for a footing or wall?

Choose the footing shape and enter the length in feet and the width and depth in inches. A 20 ft footing that is 12 in wide and 12 in deep holds 20 × 1 × 1 = 20 cu ft, or about 0.74 cubic yards before waste.

How do I calculate concrete for round columns or post holes?

Use the round column shape: volume is π × radius² × height. A 12 in diameter column 4 ft tall is about 3.14 cu ft. Set the quantity to the number of identical columns or holes and the calculator multiplies for you.

Key terms

Cubic Yard
27 cubic feet (3 ft × 3 ft × 3 ft) — the unit ready-mix concrete is ordered in.
Bag Yield
The volume of placed concrete one premix bag makes, printed on the bag in cubic feet.
Ready-Mix
Concrete batched at a plant and delivered in a mixer truck, priced per cubic yard.
Waste Allowance
Extra volume ordered to cover spillage, form movement, and uneven ground.

Methodology, sources & limitations

This page shows the formula, assumptions, and limitations used for this estimate. Rates, thresholds, and other time-sensitive inputs can change, so verify them against the relevant official source before relying on a result.

For our review approach and the primary sources we consult, see the methodology and source policy. If you spot an outdated figure or formula, please contact us with a correction.