Concrete volume calculator

Slabs, footings, and columns. Cubic yards or cubic metres. With the waste factor done right and the bag-count comparison every supply yard wishes more people did before ordering.

01 How the math works

Volume is dimensions multiplied together. For a slab, that's length × width × thickness with everything in consistent units. For a column it's π × radius² × depth. For a footing, length × width × depth.

For ordering, the raw volume gets a waste factor applied (typically 10%) and then rounded up to the supplier's increment. Ready-mix concrete is sold in half-yard steps in the US (and half-cubic-metre steps in metric markets); bagged premix is sold by the bag. Most calculators give you the raw number and leave the rounding to you, which is where DIY orders go wrong.

02 Worked example

Take a typical backyard patio slab: 10 ft × 12 ft × 4 in (or 3.05 m × 3.66 m × 100 mm in metric).

Volume: 40 ft³ = 1.48 yd³ = 1.13 m³. With a 10% waste factor: 1.63 yd³ = 1.24 m³. Rounded up to the supplier's half-yard step: 2.0 yd³ — or in metric, 1.5 m³.

If you're going bagged premix instead of ready-mix, the same pour at the standard 0.60 ft³ yield per 80 lb bag works out to 74 bags with the waste included. That's 5,920 lb to lift, mix, and pour by hand. This is the threshold where ready-mix starts making more sense than bagged — see the section below.

03 When this calculator is wrong

The result above uses a flat 10% waste factor, which works for production pours on simple geometry. For small residential slabs under 2 yd³, real waste typically runs closer to 15%, because small batches have more form loss, more spillage, and less margin for the truck to scrape the chute clean. Add a couple of extra bags or bump the waste setting to 15%.

• It assumes uniform thickness. Real slabs have a slight slope for drainage. On a 10×12 patio with a 1/8 in/ft slope, the actual volume is about 2% higher than the calculation. Negligible on small pours; matters on driveways.
• It treats forms as exact. Wood forms bow outward under pour pressure. For a 4 in slab in a single-board form, expect about 3% more concrete than calculated.
• Bag yield is a manufacturer figure under ideal conditions. Real-world yield from an 80 lb bag runs 0.55–0.60 ft³ depending on water content. The calculator uses the spec figure of 0.60; subtract 5% if you're being honest.
• It ignores rebar displacement. Negligible for residential pours (rebar takes up less than 0.5% of slab volume), but worth knowing if you're doing a heavily reinforced footing.

04 What to do with the result

If the rounded-up order is under 1 yd³ (0.75 m³), bagged premix is fine and the bag count above is what to buy. Above that, ready-mix beats bags on cost per yard, on uniformity, and on time. Most US suppliers charge a short-load fee of $50–150 for orders under 2 yd³ — typically recovered before you've finished mixing the second bag.

Above 2 yd³, ready-mix is the only sensible call. Order the rounded-up number, not the raw — concrete can't be made up mid-pour, and finishing a slab short is the kind of mistake you only make once.

05 Common questions

How many 80 lb bags of concrete are in a yard?

At the standard 0.60 ft³ yield per 80 lb bag, one cubic yard takes 45 bags (27 ft³ ÷ 0.60). With a 10% waste factor: 50 bags. That's 4,000 lb to lift and mix, which is the limit of what most weekend pours can handle.

What's the difference between cubic yards and cubic feet?

27 cubic feet make one cubic yard. Ready-mix is sold by the yard in the US; bagged premix yields are quoted in cubic feet because the per-bag amount is so small. In metric markets ready-mix is sold by the cubic metre — about 1.3 yd³.

How much does a yard of concrete weigh?

A cubic yard of standard normal-weight ready-mix weighs about 3,915 lb (1,776 kg). That's the figure to use for load planning when concrete is being delivered or moved.

What waste factor should I use?

10% for production-scale pours on simple rectangular forms. 15% for small residential slabs under 2 yd³. 20% for complex shapes with lots of corners, columns, or steps.