How much concrete do I need?

1. The formula behind every concrete calculator

Concrete fills a three-dimensional space, so the only thing that matters is volume. For any flat, rectangular pour — a slab, a pad, a path, a footing — the volume is simply length multiplied by width multiplied by thickness. The trick that trips people up is units: all three measurements have to be in the same unit before you multiply. Length and width are usually in feet or metres, but thickness is the odd one out because slabs are thin, so it is normally given in inches or millimetres. You cannot multiply 10 ft by 10 ft by 4 inches and get a sensible answer — the 4 inches first has to become 0.33 ft (4 ÷ 12), or in metric the 100 mm becomes 0.1 m.

Once everything is in feet you get a volume in cubic feet; once everything is in metres you get cubic metres. Concrete in North America is ordered by the cubic yard, and there are 27 cubic feet in a cubic yard, so the last step is to divide your cubic feet by 27. In the UK, Australia and New Zealand concrete is ordered by the cubic metre, so you simply read the cubic-metre figure directly. The calculator handles all of these conversions, but the underlying maths is this one line: volume = length × width × thickness.

2. Measuring your pour, step by step

1. Measure the length and width of the area at ground level, in feet or metres.
2. Decide the thickness — how deep the concrete will sit — in inches or millimetres.
3. Put length and width in the same unit (feet or metres), and convert thickness to match.
4. Multiply the three together to get the raw volume.
5. Convert to the unit you order in: divide cubic feet by 27 for cubic yards, or read cubic metres directly.
6. Add roughly 10% for waste, then round up — concrete is bought whole, not to the litre.

For an L-shaped patio or a path with a step in it, break the shape into rectangles, work out each one separately, and add the volumes together. For a round footing or column, the area of the circle is π × radius², then multiply by depth — but most home pours are rectangular, which is why the calculator focuses on the length × width × thickness case.

3. Cubic yards, cubic feet and cubic metres

These three units describe the same thing — how much space the concrete fills — and confusion between them is the single biggest cause of ordering the wrong amount. One cubic yard equals 27 cubic feet, and it also equals about 0.765 cubic metres. Put the other way round, one cubic metre is about 1.31 cubic yards or 35.3 cubic feet. If a supplier quotes you in “yards” they mean cubic yards of volume, not a length. Always confirm which unit a quote is in before you compare prices, because a price “per yard” and a price “per metre” are not the same quantity of concrete.

For a sense of scale: a single cubic yard of concrete covers about 81 square feet at 4 inches thick, or 54 square feet at 6 inches thick. A cubic metre covers roughly 10 square metres at 100 mm, or about 6.7 square metres at 150 mm. Keeping those rules of thumb in mind makes it easy to sanity-check whatever number the calculator gives you.

4. How many bags of concrete?

Bagged pre-mix is the easiest option for small jobs. Each bag lists the volume of finished concrete it yields once water is added, and you simply divide your total volume by that yield. The figures below are typical for common bag sizes — always check the actual yield printed on the bag, as it varies a little by brand and mix.

Roughly 45 bags of 80 lb mix make a cubic yard. That is a lot of mixing by hand, which is why bags make sense for footings, post holes and small pads, while anything approaching a full yard is usually easier and cheaper to order as ready-mix delivered by the truck.

5. Common slab sizes at a glance

All figures at 4 inches (100 mm) thick, before adding a waste margin.

A thicker slab scales the volume directly: a 6-inch slab needs 1.5× the concrete of a 4-inch slab of the same footprint, and a 5-inch slab needs 1.25×. Use the calculator for your exact dimensions rather than interpolating from the table.

6. How much extra to order for waste

Always order a little more than the calculator says. A typical allowance is 5–10%. Some of the mix is lost to spillage off the barrow, some disappears into an uneven or over-excavated sub-base, and edges and form gaps soak up more than you expect. The bigger reason is timing: ready-mix concrete starts to cure as soon as it is mixed, so you cannot pause a pour, drive back to the supplier and top up — the first batch will have set. Running 5% over costs a few dollars; running short can ruin the whole pour with a cold joint where fresh concrete meets concrete that has already begun to harden. When in doubt, round up.

7. Bags versus ready-mix

The right choice is mostly about volume. Under roughly half a cubic yard — fence posts, a small pad, a few footings — bagged mix wins on convenience: you buy only what you need, mix it as you go, and there is no minimum delivery. Between half a yard and a full yard it becomes a judgement call between the labour of mixing and the cost of a small delivery. Past about one cubic yard, ready-mix delivered by the truck is almost always cheaper per yard, far faster, and gives a more consistent mix than batching dozens of bags by hand. Remember that mixing 45 eighty-pound bags to make a single yard is several hours of heavy work, so the “cheaper” bag option has a real labour cost attached.

8. How thick should the slab be?

Thickness drives both the volume and whether the slab will last. As a general guide, 4 inches (100 mm) is common for patios, walkways, shed bases and other foot-traffic areas, while 5–6 inches (125–150 mm) is common where vehicles drive or park, such as a driveway. These are rules of thumb, not rules of law: structural slabs, footings, and anything supporting a building are governed by your local building code and, often, by an engineer's design — including reinforcement, sub-base preparation and the concrete strength. Always confirm the requirement for your specific project and jurisdiction before you pour. This guide and calculator help you estimate quantity; they do not replace the code or a professional design.

Common questions

How much concrete do I need for a 10x10 slab?

A 10 ft × 10 ft slab at 4 inches thick is 100 ft² × 0.33 ft = 33.3 ft³, which is about 1.23 cubic yards (0.94 m³). Add ~10% for waste and order about 1.36 cubic yards.

How many bags of concrete make a cubic yard?

A cubic yard is 27 ft³. At roughly 0.6 ft³ per 80 lb bag you need about 45 bags; at 0.45 ft³ per 60 lb bag about 60 bags; at 0.30 ft³ per 40 lb bag about 90 bags. These are typical pre-mix yields, not exact for every brand.

Is it cheaper to buy bags or ready-mix concrete?

For small jobs under about half a cubic yard, bagged mix is convenient and economical. Past roughly one cubic yard, ordering ready-mix delivered by the truck is usually cheaper per yard and far less labour than mixing dozens of bags.

How much extra concrete should I order?

Add about 5–10% to your calculated volume. Spillage, an over-excavated or uneven sub-base, and the fact you cannot return wet concrete all mean a slight surplus is far safer than running short mid-pour.