If you are asking how much concrete do i need, here is the honest short version: concrete is bought by volume, so every job comes down to the size of the space you are filling. Measure the length, the width and the depth, turn those numbers into a volume, and then order a little more than the maths says. This guide walks through that process for slabs, footings and post holes, with the figures for the jobs people tackle most often. If you would rather skip the arithmetic, our concrete calculator does the same sums instantly and converts the result into cubic yards, cubic feet, cubic metres and bags.
The short answer
For any flat, rectangular pour, multiply length by width by depth. Keep everything in feet and you get cubic feet. A depth given in inches must be divided by 12 first, so a 4 inch slab is 0.333 feet thick. To turn cubic feet into cubic yards, divide by 27. If you work in metric, multiply length by width by depth in metres and the answer is already in cubic metres. Whatever figure you end up with, add 5 to 10 percent on top for waste, spillage and ground that is not perfectly level. That small allowance is what stops a pour running short a wheelbarrow-load from the finish line.
Quick answers for common jobs
The table below shows net volumes before the waste allowance is added, so remember to order a little extra on top of these figures.
| Job | Size | Concrete needed |
|---|---|---|
| Small slab | 10 × 10 ft, 4 in thick | 1.23 yd³ |
| Slab | 10 × 12 ft, 4 in thick | 1.48 yd³ |
| Slab | 12 × 12 ft, 4 in thick | 1.78 yd³ |
| Driveway section | 20 × 10 ft, 4 in thick | 2.47 yd³ |
| Fence post hole | 12 in diameter, 36 in deep | About 2.4 cu ft before the post goes in |
Notice how quickly the volume climbs. Doubling the area of a slab doubles the concrete, and adding just two inches of depth to a large pour can add more material than an entire small job. That is why measuring carefully, rather than guessing from the size of the space, makes such a difference to both cost and stress on the day.
How to measure, step by step
Slabs
- Measure the inside of your formwork, not the outside. The boards themselves take up space that concrete will never fill.
- Record the length and the width in feet. If a side is not a round number, write it as a decimal, so 10 ft 6 in becomes 10.5 ft.
- Decide the thickness and convert it to feet by dividing the inches by 12.
- Multiply the three numbers together for cubic feet, then divide by 27 for cubic yards.
- Add 5 to 10 percent, and round up to a quantity your supplier will actually deliver.
Break an L-shaped or uneven slab into rectangles, work out each one separately, and add the volumes together. Odd corners and cut-outs rarely change the total much, but a missing rectangle does.
Strip footings
A strip footing is simply a long, narrow slab buried in a trench, so the same formula applies: length of the trench multiplied by its width multiplied by its depth. A trench that runs 30 ft, is 1.5 ft wide and is poured 1 ft deep holds 45 cubic feet, which is about 1.67 cubic yards before waste. Measure the trench after it is dug, because a spade-width of extra digging along the whole run quietly adds up to a lot of extra concrete.
Post holes
Round holes need the circle formula instead. Take the radius, which is half the diameter, and multiply pi by the radius squared by the depth: π × r² × depth. A hole 12 inches across is 1 ft wide, so its radius is 0.5 ft, and at 3 ft deep it holds about 2.36 cubic feet. The post itself takes up room inside that hole, so subtract the part of the post below ground if you want a tighter figure. For a fence run, work out one hole and multiply by the number of posts, then add your waste allowance once, at the end.
How deep should the concrete be?
Depth is the measurement people most often guess, and it drives the volume more than any other single choice. Use these starting points, then check local requirements for anything structural:
- Patios and walkways: 4 inches is the usual standard for foot traffic and garden furniture.
- Driveways: 4 to 6 inches, with the thicker end of that range for heavier vehicles and turning wheels.
- Shed bases: 4 inches suits most garden buildings on a well-prepared base.
- Footings: follow your local building code. They are commonly 12 inches deep or more, and width matters as much as depth because the footing spreads the load.
Whatever depth you choose, measure it inside the finished formwork or trench rather than trusting the plan. A base that dips an extra inch in the middle will swallow that concrete whether you ordered it or not.
Worked example: a 10 × 10 patio slab
Say you are laying a patio that measures 10 ft by 10 ft, poured 4 inches thick. The depth in feet is 4 divided by 12, which is 0.333 ft. Multiply 10 × 10 × 0.333 and you get 33.3 cubic feet. Divide by 27 and that is 1.23 cubic yards net. Add roughly 10 percent for waste and uneven ground and you are at about 1.36 cubic yards, so ordering 1.5 cubic yards of ready-mix gives you a sensible margin without paying for a mountain of surplus.
Prefer to mix it yourself? The same slab works out to about 56 bags of 80 lb concrete mix, which is a serious amount of lifting and mixing for one weekend. Run your own slab size through the concrete calculator to see the yardage and the bag count side by side before you decide.
Ready-mix or bags?
Ready-mix suits anything much over half a cubic yard, awkward shapes that must be poured in one go, and jobs where your time matters more than the delivery charge. Bags suit small, stop-start work: a couple of post holes today, a small repair next week, or a site a truck simply cannot reach. The catch with bags is scale. Every cubic yard is dozens of heavy bags to buy, carry, open and mix, and the concrete has to stay consistent from the first mix to the last.
If your numbers land anywhere near the bag side of that line, read our companion guide on how many bags of concrete do i need, which breaks down bag sizes, yields and a full fence-post example. And if you want to understand the arithmetic itself rather than just the answer, our concrete calculator formula guide explains every conversion in plain language.
Before you order
Measure twice, convert the depth to feet before you multiply, and add your waste allowance at the end rather than rounding every measurement up as you go, which quietly inflates the order. Then do one final check with the concrete volume calculator: enter the same dimensions and confirm the two answers match. If they do not, the mistake is almost always a depth left in inches or a footing measured on the outside of the trench. Fixing either takes a minute now and saves a second delivery later.
Frequently asked questions
How much extra concrete should I order?
Add 5 to 10 percent to your net volume. Ground is rarely perfectly level, forms flex a little under wet concrete, and some material always stays in the barrow or on the chute. Running short mid-pour is the expensive mistake, because a second delivery costs far more than a small surplus.
How do I work out concrete for a round slab?
Use pi × radius² × depth. The radius is half the diameter, so for a round slab 10 ft across and 4 in thick the sum is 3.1416 × 25 × 0.333, which is about 26.2 cubic feet, or 0.97 cubic yards before waste.
What if my slab has uneven thickness?
Measure the depth at several points, including the deepest spots, and use a realistic average that leans towards the deeper readings. Then add the full waste allowance, because low areas always take more concrete than the neat plan suggests.
How much does a cubic yard of concrete weigh?
About 4,000 lb, which is roughly 2 tons. That weight is worth keeping in mind when you plan access, barrow runs and how much a trailer or pickup can legally carry in bagged form.