Working out how much concrete you need comes down to one multiplication, length × width × thickness, plus a sensible allowance for waste. The tricky parts are mixing units, forgetting the waste and choosing between bags and a truck. This guide walks through each step with real numbers. If you just want the answer, use our concrete calculator, which does all of this for you.
Step 1: Measure the area
Measure the length and width of the area you will pour at its widest points. For anything other than a simple rectangle, split the shape into rectangles, measure each one and add the volumes together at the end.
- An L-shaped patio becomes two rectangles.
- A slab with a cut-out (for a tree or drain) is the full rectangle minus the cut-out.
- Round footings or piers can be treated as squares with sides of 0.886 × the diameter, which have the same area.
Step 2: Decide the thickness
Thickness has the biggest effect on the order and is the easiest number to get wrong. Common figures are:
| Project | Typical thickness |
|---|---|
| Garden path | 3–4 in (75–100 mm) |
| Patio or shed base | 4 in (100 mm) |
| Driveway for cars | 5–6 in (125–150 mm) |
| Footings | As specified by your plans |
These are rules of thumb. Your building code and any engineer’s drawings always take priority. Check the depth of your excavation in several places: ground is rarely level, and an extra half inch across a large slab adds a surprising amount of concrete.
Step 3: Convert everything to the same unit
Most mistakes happen here. Multiply feet by inches without converting and the answer is out by a factor of 12.
- Imperial: convert inches to feet by dividing by 12, so 4 in = 0.333 ft. Multiply to get cubic feet, then divide by 27 to get cubic yards.
- Metric: convert centimetres to metres by dividing by 100, so 10 cm = 0.10 m. Multiply to get cubic metres.
Step 4: Add a waste allowance
Concrete is lost to spillage, uneven subgrade, formwork that bows and material left in the mixer. Estimators normally add 5 to 10 percent. Use 10% for DIY pours and hand-mixed bags. Running short in the middle of a pour is far worse than a little left over, because the join between old and new concrete is a weak point.
To add 10%, multiply the volume by 1.10. If you want to see how different allowances change your order, our percentage calculator is handy for quick what-ifs.
Step 5: Choose bags or ready-mix
Pre-mixed bags are labelled with their yield. Typical values are:
- 80 lb bag: about 0.60 ft³, so 45 bags per cubic yard
- 60 lb bag: about 0.45 ft³, so 60 bags per cubic yard
- 20 kg bag: about 0.0093 m³, so roughly 108 bags per cubic metre
Bags make sense for small jobs such as fence posts, a few footings or a short path. Above roughly one cubic yard (about 0.75 m³), price a ready-mix truck. It delivers the whole pour at once, which gives a stronger, more consistent slab, and it saves hours of heavy mixing.
Worked examples
A 12 × 10 ft patio, 4 in thick
- Volume: 12 × 10 × 0.333 = 40 ft³.
- Cubic yards: 40 ÷ 27 = 1.48 yd³.
- With 10% waste: 1.48 × 1.10 = 1.63 yd³.
- In 80 lb bags: 44 ft³ ÷ 0.60 = 73.3, so 74 bags, which means it is worth getting a ready-mix quote.
Four footings, 2 × 2 ft and 12 in deep
- One footing: 2 × 2 × 1 = 4 ft³. Four footings: 16 ft³.
- With 10% waste: 17.6 ft³, or about 0.65 yd³.
- In 60 lb bags: 17.6 ÷ 0.45 = 39.1, so 40 bags.
A 4 × 3 m shed base, 10 cm thick
- Volume: 4 × 3 × 0.10 = 1.20 m³.
- With 10% waste: 1.32 m³.
- In 20 kg bags: 1.32 ÷ 0.0093 = 141.9, so 142 bags.
Common mistakes to avoid
- Mixing units. Always convert inches to feet, or centimetres to metres, before multiplying.
- Measuring the hole, not the slab. If you have dug deeper than the finished slab and will not fill it with gravel, order for the full depth.
- Forgetting thickened edges. Many slabs have deeper edges or internal beams. Calculate them separately and add them.
- Skipping the waste allowance. Ordering the exact net volume almost guarantees a shortfall.
- Ignoring the sub-base. A compacted gravel base is a separate material order and does not reduce the concrete needed.
Next steps
Plug your measurements into the concrete calculator to check your figures, switch between imperial and metric, and get a bag count for the bag size you plan to buy. For more background on concrete materials and practice, the American Concrete Institute publishes guides for both professionals and homeowners.
How we made this guide
Drafted with AI assistance from the sources listed below, then checked line by line against those sources and the tool by a human editor. Finance, health and accounting guides are also reviewed by a qualified specialist before they are indexed.