How Much Concrete Do You Need? Slabs, Footings and Post Holes
Concrete is a one-shot material. Once you start pouring, you cannot pause while you fetch another few bags, so the quantity needs to be right before the work begins. The good news is that the maths is simple: find the volume of the space you are filling, convert it to the unit your supplier uses, and add a small allowance.
This guide covers the three shapes most home projects use: a flat slab, a long footing trench and a round post hole. It also shows how to turn a volume into bags or a ready-mix order. Slab thickness, reinforcement and footing depth depend on the load and your local building rules, so treat these figures as quantity arithmetic, not design advice.
Volume of a slab
A slab is a box, so its volume is length × width × thickness. The usual trap is mixing units: slab thickness is normally quoted in inches or millimetres, while the other two sides are in feet or metres. Convert the thickness first.
volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12Divide the result by 27 to get cubic yards.
Worked example: a patio 10 ft by 12 ft and 4 in thick. Thickness is 4 ÷ 12 = 0.333 ft, so the volume is 10 × 12 × 0.333 = 40 ft³. Divided by 27, that is 1.48 yd³. Adding a 10% allowance gives 1.63 yd³, so you would order a little over 1.6 yd³.
Metric version: a slab 3 m by 4 m and 100 mm thick is 3 × 4 × 0.1 = 1.2 m³. With 10% extra, order about 1.32 m³. The concrete calculator does the unit conversion for you, and the volume of a box calculator is handy for checking the raw figure.
Bags or ready-mix?
Pre-mixed bags state how much concrete they make once water is added, called the yield. Typical figures are in the table below, but the label on your product is the number to trust.
| Bag size | Typical yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
For the 40 ft³ patio above, 80 lb bags at 0.6 ft³ each need 40 ÷ 0.6 = 66.7, so 67 bags. With the 10% allowance it is 44 ÷ 0.6 = 73.3, so 74 bags. That is a lot of mixing and lifting, since each bag weighs 80 lb.
Past about a cubic yard, many people price ready-mix delivery instead. It is ordered by the cubic yard or cubic metre, and some suppliers charge extra for small loads, so ask about minimum quantities before deciding.
Footings and post holes
A footing trench is another box: length × width × depth. A trench 40 ft long, 16 in wide and 8 in deep holds 40 × 1.333 × 0.667 = 35.6 ft³, or 1.32 yd³.
A post hole is a cylinder, so use the circle formula.
volume = π × radius² × depthKeep radius and depth in the same unit.
Worked example: a hole 10 in across and 24 in deep has a radius of 5 in. The volume is 3.1416 × 25 × 24 = 1,885 in³, which is 1,885 ÷ 1,728 = 1.09 ft³. If a 4×4 post (actually about 3.5 in square) sits in the hole, it displaces 3.5 × 3.5 × 24 ÷ 1,728 = 0.17 ft³. The concrete needed is 0.92 ft³, which is about 1.5 bags of 80 lb mix, so buy 2 bags per post.
For many holes, multiply by the post count before rounding up to whole bags. The volume of a cylinder calculator checks a single hole quickly.
How much extra to order
Your measured volume is the ideal case. Real ground is uneven, forms bow slightly outward under the weight of wet concrete, and some mix is lost in the barrow or the mixer. A planning allowance of 5% to 10% is common for slabs and footings, and the more irregular the excavation, the closer to 10% you should go.
- Order slightly over rather than short: a second batch poured later may not bond or match the first.
- Measure the depth at several points on the sub-base and use the average.
- Remember that gravel or sand beneath the slab is a separate material with its own volume calculation.
Slab thickness, reinforcement, joints and footing depth depend on soil, climate, load and local codes. Confirm requirements with your local building authority or a qualified professional before you pour. This article covers quantity arithmetic only.
- Concrete Calculator: Choose what you are pouring, enter its dimensions in any mix of units, and see how much concrete it takes as a volume, a weight and a bag count. Add a price and the same figures become a cost estimate.
- Cylinder Volume Calculator: Enter the radius of the circular base and the height of the cylinder, pick a unit, and the calculator returns the volume in cubic units. The guide below explains the formula and shows how to work backwards.
- Volume of a Box Calculator: Enter the length, width and height of a rectangular box, pick the unit, and the calculator returns the volume in the matching cubic unit. The guide covers the formula, unit conversions and shipping uses.
This article is general information, not professional advice. See our Disclaimer and Editorial Policy.
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