NOVORY

Concrete Calculator

Calculate how much concrete you need for a slab, pad, footing, or other rectangular project.

Quick answer

56 eighty-pound bags (61 with a 10% waste buffer). For ready-mix, order 1.5 cubic yards.

Calculator

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How it works

Length × Width × Thickness = Volume

For imperial inputs, length and width are in feet and thickness is in inches. The calculator converts inches to feet automatically, then multiplies the three dimensions to get cubic feet.

Cubic feet ÷ 27 = Cubic yards

Since one cubic yard equals 27 cubic feet (3 ft × 3 ft × 3 ft), dividing by 27 converts the volume to cubic yards — the standard unit for ordering ready-mix concrete.

The recommended order amount includes your selected waste factor so you have a buffer for spillage and uneven surfaces.

Worked example

A 20 ft × 10 ft slab at 4 inches thick:

20 × 10 × (4/12) = 66.67 ft³

66.67 ÷ 27 = 2.47 yd³

2.47 × 1.10 = 2.72 yd³ (with 10% waste)

Order approximately 2.72 cubic yards of concrete.

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Ready-mix concrete is sold by the cubic yard, but most failed pours fail long before the truck arrives — they fail on the tape measure. An inch of missing thickness across a 20-by-10 slab is a quarter yard of concrete you didn't order, and a short load means a cold joint right through the middle of your project.

This calculator turns your length, width, and thickness into cubic yards, cubic feet, and cubic meters, then adds a waste allowance so the number you order includes a buffer for spillage, over-excavation, and uneven subgrade. The 10% default is the industry standard for formed slabs on prepared ground.

How to measure for the concrete calculator

  1. Measure the inside of your forms — length and width in feet, not the outside of the lumber. Forms eat 1½ inches per side, and that phantom area adds up.
  2. Check thickness in at least four spots with a tape, not by eye. Subgrade that looks flat is rarely flat; the thinnest spot decides whether you run short.
  3. Enter thickness in inches (imperial) or centimeters (metric) — the calculator converts automatically. Four inches is the minimum for walkways and patios; driveways want six.
  4. Leave the waste factor at 10% for formed work on compacted base. Bump it to 15% if you're pouring against rough excavation with no forms.

Bagged concrete: bags per cubic yard

Bag sizeYield per bagBags per cubic yard
80 lb0.60 ft³45
60 lb0.45 ft³60
40 lb0.30 ft³90

Nominal yields — always check the yield printed on your specific product; it varies by manufacturer and mix.

Worked example: bags for a 10×10 slab

A 10 ft × 10 ft patio slab, 4 inches thick, using 80-lb bags:

  1. Convert thickness to feet: 4 ÷ 12 = 0.333 ft.
  2. Volume: 10 × 10 × 0.333 = 33.33 ft³.
  3. To cubic yards: 33.33 ÷ 27 = 1.23 yd³.
  4. Bags: 1.23 × 45 = 55.4 → 56 eighty-pound bags.
  5. With 10% waste: 1.23 × 1.10 = 1.35 yd³ → about 61 bags, or order 1.5 yd³ of ready-mix.

56 eighty-pound bags (61 with a 10% waste buffer). For ready-mix, order 1.5 cubic yards.

Pro tips for ordering concrete

  • A short load costs more than the extra yard — the redelivery fee alone dwarfs 10% waste on a small pour.
  • Call the batch plant, not just the calculator: tell them your yardage and ask what their minimum load is (often 1 yd³ with a short-load fee below 3–4 yd³).
  • Keep concrete off the tops of your forms — screed, don't pour proud. Every high spot is yardage you paid for and then shaved off.
  • Wet concrete is caustic: gloves and eye protection on every pour, and keep it damp for 7 days while it cures — strength comes from slow curing, not fast drying.

How to use the concrete calculator

1. Choose your unit system — Imperial (feet and inches) or Metric (meters and centimeters).

2. Enter the length and width of your project area.

3. Enter the thickness (depth) of the concrete pour.

4. Select a waste factor — 10% is recommended for most projects.

5. Click Calculate to see your results in cubic yards, cubic feet, and cubic meters.

How concrete volume is calculated

Concrete volume is calculated by multiplying the three dimensions of your project: length, width, and thickness. This gives you the total volume of space the concrete will fill.

In the imperial system, the result is in cubic feet. Since ready-mix concrete is sold by the cubic yard, the calculator divides by 27 to convert. In the metric system, the result is directly in cubic meters.

The waste factor is applied as a percentage increase to the base volume, giving you a recommended order amount that accounts for real-world conditions.

How much concrete do I need?

The amount of concrete you need depends on the size and thickness of your project. A small patio might need 1–2 cubic yards, while a driveway could require 5–10 cubic yards or more.

Always measure carefully and add a waste factor. Ordering slightly more than you need is far better than running short mid-pour, which can cause cold joints and structural weaknesses.

Why add a waste factor?

A waste factor accounts for concrete that is lost to spillage, over-excavation, uneven subgrade, or form variations. Without a buffer, you risk running out of concrete before the pour is complete.

The standard recommendation is 10%. For projects with complex forms or uneven ground, 15% is safer. For simple, well-prepared areas with precise measurements, 5% may be enough.

Concrete calculator example

Let's say you're pouring a concrete slab that is 20 feet long, 10 feet wide, and 4 inches thick.

First, convert the thickness to feet: 4 inches ÷ 12 = 0.333 feet.

Then calculate the volume: 20 × 10 × 0.333 = 66.67 cubic feet.

Convert to cubic yards: 66.67 ÷ 27 = 2.47 cubic yards.

With a 10% waste factor: 2.47 × 1.10 = 2.72 cubic yards.

You should order approximately 2.72 cubic yards of concrete for this project.

Frequently Asked Questions

How do I calculate how much concrete I need?+
Multiply the length by the width by the thickness of your project area. Make sure the dimensions use compatible units. In imperial units, this produces cubic feet, which can be divided by 27 to convert to cubic yards. In metric units, the result is cubic meters. Add a waste factor to allow for spillage, uneven ground, and minor measurement differences.
How many cubic yards of concrete do I need?+
Calculate the volume in cubic feet by multiplying length × width × thickness, with all dimensions converted to feet. Then divide the result by 27. For example, a 20 ft × 10 ft slab that is 4 inches thick has a base volume of approximately 2.47 cubic yards before waste.
How much extra concrete should I order?+
A 5% to 10% allowance is commonly used depending on measurement accuracy, site conditions, waste, and the complexity of the forms. Projects with uneven ground or more complex shapes may require a larger allowance. The appropriate amount depends on the specific project.
What is the difference between cubic feet and cubic yards?+
A cubic yard is equal to 27 cubic feet. One cubic yard measures 3 feet × 3 feet × 3 feet. Ready-mix concrete is commonly ordered in cubic yards, while cubic feet are useful for calculating the volume of smaller projects.
How many bags of concrete do I need?+
Bag requirements depend on the bag size and the manufacturer's stated yield. Do not assume a universal yield for every product. To estimate the number of bags accurately, divide the required concrete volume by the yield stated on the specific product's packaging.

Related Guides

How Much Concrete Do I Need?How Many Bags of Concrete Do I Need for a 10x10 Slab?How Many Bags of Concrete Are in a Cubic Yard?How Much Does a Yard of Concrete Cover?How Many Bags of Concrete Do I Need for a 4x4 Post Hole?How Much Concrete for a Round Footing (Sonotube)?How Many Bags of Concrete Do I Need for a 12x12 Slab?

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