CONCRETE CALCULATOR

Concrete Calculator — How Much Concrete Do I Need?

Estimate how much concrete you need for your project. Enter the length, width and thickness of a rectangular slab, or the diameter and thickness of a circular pad, and the calculator gives you the volume to order plus an equivalent bag count for on-site mixing.

Shape

Dimensions

Common thicknesses

Waste allowance

Total concrete needed
—m³

Fill in dimensions to see your estimate.

Plan Your Pour with a Fast, Accurate Concrete Calculator

Whether you are pouring a patio, a garage floor, a footing for a deck or a round pad for a hot tub, the Interior Tailor Concrete Calculator turns your measurements into the volume of concrete to order. Enter the dimensions of a rectangular slab or a circular pad, add a waste allowance, and get your total in cubic yards or cubic metres, plus an equivalent bag count if you plan to mix on site. It takes the guesswork out of ordering, so you can avoid a short pour or paying for concrete you do not need.

What the Concrete Calculator Does

  • Two shapes in one tool: rectangular or square slabs, and circular pads.
  • Imperial and metric units: feet and cubic yards, or metres and cubic metres.
  • Common thickness presets: one-click values for slabs and patios, footings and beams, 100 mm walls and driveways.
  • Adjustable waste allowance: set a percentage for uneven sub-grades, spillage and minor measurement differences.
  • Equivalent bag count: see how many pre-mix bags the pour equals, so you can compare bags with ready-mix delivery.

How Concrete Volume Is Calculated

For a rectangular slab the formula is:

Volume = Length × Width × Thickness

For a circular pad:

Volume = π × (Diameter ÷ 2)² × Thickness

Convert the result from cubic feet to cubic yards by dividing by 27. Always add 5–15% waste to account for uneven sub-grades, spillage and minor measurement differences — most contractors add 10% as a standard allowance.

Standard Concrete Thicknesses

  • 3–4 in (75–100 mm): Residential patios, walkways, light-traffic slabs
  • 4–5 in (100–125 mm): Driveways and garage floors
  • 6 in (150 mm): Heavy equipment pads and light commercial floors
  • 8 in (200 mm): Foundation walls and structural slabs
  • 10–12 in (250–300 mm): Heavy-duty industrial slabs

For driveways, a minimum of 4 inches with #4 rebar at 12 in o.c. is recommended.

Bags vs Ready-Mix Concrete

For pours under 1 cubic yard (27 cubic feet), mixing from bags is practical. For anything larger, a ready-mix truck is almost always more economical and produces better results:

  • 80 lb bag: ~0.60 cu ft yield — need ~45 bags per cubic yard
  • 60 lb bag: ~0.45 cu ft yield — need ~60 bags per cubic yard
  • Ready-mix minimum load: typically 1 yard — suits most residential pours

Ready-mix quality is more consistent and avoids the variability that comes from hand-mixing bags at different water ratios.

Concrete Projects You Can Estimate

Use the calculator as a starting point for most flatwork and small structural pours:

  • Patios, walkways and garden paths
  • Driveways, garage floors and shed bases
  • Footings and beams
  • Circular pads for hot tubs, fire pits and equipment
  • Thin concrete walls at 100 mm (4 in) thickness

Example Concrete Estimates

These examples use a 10% waste allowance. Swap in your own dimensions to see how quickly the numbers change.

Example Concrete Estimates
ProjectInputsVolumeWith 10% waste80 lb bags
Rectangular patio12 ft x 12 ft x 4 in1.78 cu yd1.96 cu yd88 bags
Circular pad8 ft diameter x 4 in0.62 cu yd0.68 cu yd31 bags
Strip footing (illustrative)20 ft x 1.5 ft x 10 in0.93 cu yd1.02 cu yd46 bags

Bag counts assume an 80 lb bag yields about 0.6 cubic feet. Check the yield printed on the bag you buy.

Tips for a More Accurate Concrete Estimate

  • Measure inside the forms, not outside them, and check thickness at several points.
  • Add 5 to 15% waste. Around 10% is a common allowance, and uneven sub-grades call for more.
  • For irregular shapes, split the area into rectangles and circles, calculate each and add the volumes.
  • Above roughly one cubic yard, price a ready-mix delivery against mixing bags by hand.
  • Confirm slab thickness and reinforcement for your load with a local contractor or building code.

Frequently Asked Questions

How much concrete do I need for a 10×10 slab?

A 10×10 ft slab at 4 inches (0.333 ft) thick requires 10 × 10 × 0.333 = 33.3 cu ft ÷ 27 = 1.23 cubic yards. Order 1.5 yards to allow for waste and sub-grade variations.

How long does concrete take to cure?

Concrete reaches about 70% of its strength at 7 days and full design strength at 28 days. You can typically walk on it after 24–48 hours and drive on it after 7 days for residential applications.

Do I need rebar in a concrete slab?

For structural slabs, driveways and any slab that may experience load or ground movement, yes. A 4-inch residential slab typically uses #4 rebar at 12–18 inches on center in both directions.

How many square feet does one cubic yard of concrete cover?

It depends on thickness. One cubic yard covers about 81 square feet at 4 inches thick, about 65 square feet at 5 inches and about 54 square feet at 6 inches, before any waste allowance.

Can I use this calculator for a round patio or footing?

Yes. Choose the circular shape and enter the diameter and thickness. For a trench footing or a thin wall, use the rectangular shape with the length, width and depth of the pour.

Should I order ready-mix or use bags?

Bags are practical for pours under about one cubic yard, such as posts and small pads. Larger pours are usually easier, more consistent and more economical with ready-mix delivery. The bag count in the results helps you compare both.