How to Calculate Concrete Volume (Formula + Examples)

Pour too little concrete and the truck leaves before the slab is full — you end up with a cold joint right through the middle of your patio, a weak spot that cracks within a year. Order too much and you’re paying for a truckload of stone-hard leftovers you have to chip out of the drum. Either mistake costs real money.

The good news? Learning how to calculate concrete volume takes about five minutes, a tape measure, and one simple formula. This guide walks you through it step by step: the slab formula that covers most DIY jobs, how to figure out bag counts for 60-lb and 80-lb bags, what to do about footings, post holes and columns, and the waste factor pros never skip. Two fully worked examples show every number, so there’s no guesswork left.

Why Concrete Is Measured in Cubic Yards

If you’ve ever phoned a concrete plant, the first thing you’ll notice is that nobody talks about litres or cubic feet. Ready-mix concrete is quoted and sold by the cubic yard — and there’s a practical reason for it. A cubic yard is the natural batch size of a mixer truck, and it’s the unit every plant prices against.

So what exactly is it? A cubic yard is a cube measuring 3 feet on every side: 3 × 3 × 3 feet. Multiply that out and you get 27 cubic feet in one cubic yard. That’s the single most important number in this whole guide to how to calculate concrete volume — nearly every concrete calculation you ever do ends with a division by 27.

One cubic yard of standard concrete weighs roughly 3,600 to 4,000 pounds — close to two tons. That’s why a “small” patio slab can need a full mixer truck. Understanding the scale helps you see why accurate measuring matters: concrete is heavy, it’s not cheap, and once it sets, your mistakes are permanent.

Bagged concrete, the kind you buy at the hardware store, plays by the same math — but there it pays to think in cubic feet. As a rule of thumb, projects under about one cubic yard are usually bag jobs, while anything bigger makes more sense as ready-mix delivered by truck. We’ll unpack that 1-yard rule later.

How to Calculate Concrete Volume: The Slab Formula (Length × Width × Thickness ÷ 27)

Most home projects — patios, shed pads, sidewalks, garage floors — are simple slabs, which are just flat rectangles. Here’s how to calculate concrete volume for a slab:

Volume (cubic yards) = Length (ft) × Width (ft) × Thickness (ft) ÷ 27

Here’s how to use it in three steps:

1. Measure all three dimensions in feet. Length and width are easy with a tape measure. For thickness, convert inches to feet by dividing by 12. A 4-inch slab is 4 ÷ 12 = 0.333 ft. A 5-inch slab is 5 ÷ 12 ≈ 0.417 ft. A 6-inch slab is 0.5 ft.
2. Multiply the three numbers together. That gives you the volume in cubic feet.
3. Divide by 27. That converts cubic feet into cubic yards — the unit the plant sells.

That’s the whole trick. If you’re working with the numbers in inches instead, there’s a shortcut: Length (in) × Width (in) × Thickness (in) ÷ 46,656 gives cubic yards directly, since 36³ = 46,656. Most people find feet easier, though, so stick with the ÷ 27 version.

Worked Example 1: A 10×10 ft Patio Slab

Let’s say you’re pouring a square patio slab, 10 feet by 10 feet, 4 inches thick — a very common residential job. Here’s how to calculate concrete volume for this patio, line by line:

1. Convert thickness: 4 inches ÷ 12 = 0.333 ft
2. Multiply: 10 ft × 10 ft × 0.333 ft = 33.33 cubic feet
3. Convert to yards: 33.33 ÷ 27 = 1.23 cubic yards

So you need about 1.23 cubic yards of concrete for that patio. Notice where most beginners go wrong: they multiply 10 × 10 × 4 (using inches for thickness) and get a meaningless number. Always convert thickness to feet first — that single step is the difference between 1.23 yards and a fantasy figure.

Because 1.23 yards is just over the 1-yard line, this is borderline territory. You could mix bags (about 56 eighty-pound bags — see the bag section below), or you could call the plant for a small load. There’s no wrong answer at this size; it comes down to price, labour, and how much mixing your back can take.

Worked Example 2: A 20×20 ft Driveway Slab

Now let’s see how to calculate concrete volume for a bigger job: a driveway section 20 feet by 20 feet, 5 inches thick (the standard for anything carrying cars).

1. Convert thickness: 5 inches ÷ 12 = 0.417 ft
2. Multiply: 20 ft × 20 ft × 0.417 ft = 166.67 cubic feet
3. Convert to yards: 166.67 ÷ 27 = 6.17 cubic yards

That’s firmly in ready-mix territory — and the bag math shows why. At 0.60 cubic feet per 80-lb bag, 166.67 cubic feet would need about 278 bags (166.67 ÷ 0.60 = 277.8, always rounded up to 278). That’s a full truckload of dry mix, and you’d be mixing for hours while the first batches start setting. A ready-mix truck at 6.17 yards is cheaper per unit, more consistent, and done in an afternoon.

How to Calculate Bags of Concrete (60-lb and 80-lb)

If your project is under roughly a cubic yard, bagged concrete from the hardware store is the practical route — no truck scheduling, no minimum-load fees. Once you know how to calculate concrete volume in cubic feet (the number from Step 2 above, before dividing by 27), bag math is simple division.

The standard yields are printed on every bag, but here they are:

• An 80-lb bag yields about 0.60 cubic feet of finished concrete — roughly 45 bags per cubic yard.
• A 60-lb bag yields about 0.45 cubic feet — roughly 60 bags per cubic yard.
• A 40-lb bag yields about 0.30 cubic feet.

For a detailed breakdown of coverage per bag size, see this guide on how to calculate concrete coverage by bag size.

The formula: Number of bags = Volume (cubic feet) ÷ yield per bag, then round UP. Always up — you can return an unopened bag, but you can’t stretch a short pour.

Let’s run the 10×10 patio from Example 1 (33.33 cubic feet):

• 80-lb bags: 33.33 ÷ 0.60 = 55.6 → 56 bags
• 60-lb bags: 33.33 ÷ 0.45 = 74.1 → 75 bags

The table below gives quick answers for common slab sizes at the standard 4-inch thickness (rounded up, waste factor not included — add that separately):

A note on bag weight vs. labour

Eighty-pound bags are cheaper per cubic foot, but sixty-pound bags exist for a reason: your back. If you’re working alone, the lighter bags are worth the small premium. Either way, keep the bags dry until mixing day — a bag that’s absorbed humidity sets into a rock in the paper.

Footings, Post Holes and Columns

Not every pour is a slab. Here’s how to calculate concrete volume for the other common shapes:

Strip footings and foundation walls are just long rectangles, so the slab formula works unchanged: Length × Width × Depth ÷ 27. A footing 20 ft long, 2 ft wide and 1 ft deep: 20 × 2 × 1 ÷ 27 = 1.48 cubic yards.

Round post holes and sonotube columns are cylinders. The formula is:

Volume per hole = π × radius² × depth (all in feet), then multiply by the number of holes.

Say you’re setting fence posts in 12-inch diameter holes, 3 feet deep. The radius is 6 inches = 0.5 ft:

π × 0.5² × 3 = 2.36 cubic feet per hole

Ten such holes: 10 × 2.36 = 23.6 cubic feet, or 0.87 cubic yards. In bags: 23.6 ÷ 0.60 = 39.3 → 40 eighty-pound bags. Now you know why fencing contractors buy concrete by the pallet.

Irregular shapes — L-shaped patios, curved walkways — just get split into rectangles (and triangles, area = ½ × base × height) and added together. Precision isn’t critical here; the waste factor below covers the small discrepancies.

The Waste Factor: Why You Add 5–15%

Here’s the step DIYers skip and professionals never do: add extra. Industry guidance says to order 5–15% more concrete than the raw calculation — the calculator on this site even suggests 10–15% — and it’s not padding, it’s physics. Spillage happens. Forms bulge under the weight. Subgrade is never as perfectly level as the plan assumed. A slab specified at 4 inches might average 4.5 in spots you can’t see.

Once you know how to calculate concrete volume, the waste-factor math is a simple percentage increase: Adjusted volume = raw volume × 1.10 (for 10%).

For our 10×10 patio: 1.23 yd³ × 1.10 = 1.35 cubic yards. In bags: 33.33 ft³ × 1.10 = 36.7 ft³ → 36.7 ÷ 0.60 = 61.2 → 62 eighty-pound bags.

Think about the economics for a moment. On a ready-mix order, the cost of adding half a yard to the first truck is small; the cost of ordering a second truck because you came up short — with its own delivery fee and a cold joint in your slab — is enormous. This is one of those places where a “wasteful” margin is actually the cheapest option. If you’d like to double-check the percentage arithmetic, our percentage calculator handles it in seconds — and for the full project math, use our concrete volume calculator to skip the hand calculations entirely.

Bagged vs Ready-Mix: The 1-Yard Rule

The decision is simpler than it looks once you know how to calculate concrete volume. Under about one cubic yard, bagged concrete wins: no scheduling, no truck minimums, and you can pour in stages if you need to. Over a yard, ready-mix almost always wins on cost, consistency and your schedule.

The 20×20 driveway example makes the case brutally clear: 278 eighty-pound bags. That’s roughly 11 tons of dry mix to load, haul, mix and place before the first batches start curing. A mixer truck delivers the same volume in one go, mixed to spec, placed in an hour.

Practical tips either way:

• For bagged pours, recruit help or rent a mixer. Hand-mixing more than 20 bags in a wheelbarrow is a young person’s game, and concrete waits for no one.
• For ready-mix, have the site ready before the truck arrives: forms set, rebar placed, wheelbarrow paths clear. Truck time is billed by the minute once the free unloading window passes.
• Either way, order or buy the waste factor. Running out mid-pour is the single most expensive mistake in this guide.

Common Mistakes

1. Forgetting the ÷ 27. The most common error in concrete math: you multiply length × width × thickness and proudly announce you need “33 yards” for a patio that needs 1.23. Always divide cubic feet by 27.
2. Entering thickness in inches. A 4-inch slab is not 4 feet thick. Divide inches by 12 before multiplying. This one mistake inflates your order by a factor of twelve.
3. Skipping the waste factor. The ground under your forms is not a machine-shop surface. Ten percent extra is cheap insurance.
4. Trusting uneven ground. If the subgrade dips, concrete fills the dip — and your volume estimate assumed it wouldn’t. Grade and compact the base first, then measure.
5. Rounding bags DOWN. Bags come whole, and partial bags are useless. Fifty-five point six bags means fifty-six bags. Always round up.

Frequently Asked Questions

Learning how to calculate concrete volume isn’t complicated — it’s just unforgiving of skipped steps. Measure in feet, divide by 27, round bags up, and always add your waste factor. Do that, and the only surprise on pour day will be how smooth it goes. To skip the hand math on your next project, run your numbers through our free concrete volume calculator — it handles slabs, footings and post holes, and adds the waste margin automatically.