Concrete Calculator
Concrete for a slab, footing, post holes or stairs (in cubic yards and in bags).
The pour
This works out how much concrete a job needs. Pick the shape, enter the measurements, and it returns the volume in cubic yards or cubic metres.
It also gives the number of bags if you are mixing by hand, and includes a waste allowance, because concrete is one thing you do not want to run out of halfway through a pour.
How concrete is measured and ordered
Concrete is sold by volume, not by weight or area. In the US that means cubic yards, and one cubic yard is 27 cubic feet.
So every calculation is the same three numbers multiplied together: length × width × thickness. The only thing that trips people up is that thickness is almost always given in inches while the other two are in feet, so it has to be converted before anything else.
For small jobs you buy bags and mix on site. Past about half a cubic yard, ready-mix delivered by truck is usually cheaper and far less work than mixing 25 bags by hand.
Dividing the inches by 12 is the step people skip. Putting 4 straight into the sum instead of 0.333 makes the answer twelve times too big.
What to enter
- Shape
- Slab, footing, column, or round holes for posts. Each changes which measurements are asked for.
- Length and width
- The footprint of the pour, normally in feet.
- Thickness
- How deep the concrete is, usually entered in inches. 4 inches is standard for a patio or shed base; driveways that take vehicles are usually 5 or 6.
- How many holes
- For post holes, the count of identical holes. Fence work is where a small per-hole error multiplies fastest.
What a cubic yard actually covers
- At 4 inches thick
- One cubic yard covers about 81 square feet. Ten yards covers 810.
- At 5 inches
- About 65 square feet per cubic yard.
- At 6 inches
- About 54 square feet per cubic yard, the usual depth for a driveway.
- 80 lb bags
- About 45 bags fill one cubic yard. Each yields roughly 0.6 cubic feet.
- 60 lb bags
- About 60 bags per cubic yard, at roughly 0.45 cubic feet each.
What this assumes
Measurements are of the finished pour. If the ground is uneven or over-excavated, you will need more than the maths says.
A waste allowance of around 10% is normal. Spillage, uneven subgrade and slight over-dig all eat into a load.
How to calculate how much concrete you need
Four steps, and the conversion is the one that matters.
- ÷ 12
- Turns inches of thickness into feet
- ÷ 27
- Turns cubic feet into cubic yards
Convert the thickness. Divide the inches by 12. Four inches becomes 0.333 feet.
Multiply all three. Length × width × thickness gives cubic feet.
Divide by 27. That converts cubic feet into cubic yards, which is how concrete is ordered.
Add waste. Around 10% is standard. Running short mid-pour leaves a cold joint, which is a permanent weak line through the slab.
See a worked example: a 30 ft by 40 ft slab, 4 inches thick
- Area
- 30 ft × 40 ft
- Thickness
- 4 inches
Convert the thickness: 4 ÷ 12 = 0.333 ft.
Multiply: 30 × 40 × 0.333 = 400 cubic feet.
Divide by 27: 400 ÷ 27 = 14.81 cubic yards.
Add 10% for waste: about 16.3 cubic yards, so you would order 16.5 or 17.
About 14.8 cubic yards, or 16.3 with waste
Frequently asked questions
About 14.8 cubic yards, or roughly 16.3 once you add the usual 10% waste allowance.
At that size, ready-mix delivery is the only sensible option. Mixing it by hand would mean around 670 eighty-pound bags.
About 810 square feet. One cubic yard covers roughly 81 square feet at that depth, so ten covers ten times as much.
Go deeper and it covers less: at 6 inches, ten yards covers only about 540 square feet.
Multiply length by width by thickness, all in feet, then divide by 27 to get cubic yards.
The step to be careful about is the thickness: it is nearly always quoted in inches, so divide by 12 first. Putting 4 into the sum instead of 0.333 makes the answer twelve times too large.
About 45 bags of 80 lb, or 60 bags of 60 lb. An 80 lb bag yields roughly 0.6 cubic feet, and there are 27 cubic feet in a yard.
Bags make sense up to about half a yard. Beyond that the cost and the labour both climb steeply against ordering ready-mix.
4 inches is standard for patios, walkways and shed bases. 5 to 6 inches for driveways and anything carrying vehicles.
Thickness is not the only thing that matters: the compacted base underneath and proper control joints do as much to prevent cracking. Check local building rules for anything structural.
Yes. Around 10% is the normal allowance, and it is cheap insurance.
Running out mid-pour is genuinely bad, not just inconvenient. The first batch starts curing before the second arrives, leaving a cold joint, which is a permanent weak seam through the slab.
Problems people actually run into
Forgetting to convert inches to feet
This is the single most common error, and it is always in the same direction: a twelvefold overestimate.
The sanity check that catches it: a 10 × 10 patio at 4 inches needs about 1.2 cubic yards. If your answer for something that size comes out in the teens, the thickness was not converted.
Measuring the form instead of the ground
Concrete fills whatever space is actually there, not the space you intended. A subgrade dug an inch deeper than planned adds real volume across a large slab.
On a 30 × 40 pour, one extra inch of depth is nearly four more cubic yards. Check the depth in several places before ordering, not just at the edges.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 3, 2026