How much concrete do I need? Slabs, footings, posts and steps

Guide · Updated 2026-11-10

Turn the size of a slab, footing, post hole or flight of steps into cubic metres or yards, add a waste share and count the bags of premix to buy.

Open the Concrete calculator Free, in your browser, nothing uploaded.

Buy too little concrete and the pour stops halfway; buy too much and you are left with heavy bags. The concrete calculator turns the size of a slab, a footing, a set of post holes or a flight of solid steps into a volume, adds a share for waste and tells you how many bags of premix that is. It is for homeowners, gardeners and anyone pricing a small building job. Before the pour, the gravel calculator works out the stone base underneath, and if you would rather blend cement, sand and stone yourself, concrete mix calculator gives the proportions.

What it is

Concrete is bought by volume, so every job comes down to one question: how much space will the wet mix fill? The tool answers it with two shapes from school geometry.

A slab or footing is a box:

• V = L × W × T, where L is the length, W the width and T the thickness or depth, all in metres (or feet).

A post hole is a cylinder with the post standing in the middle, so the post’s own volume is taken off:

• V = π ÷ 4 × (D² − d²) × H, where D is the hole diameter, d the post diameter and H the depth.

Solid steps are a staircase of boxes. The first step is one rise tall, the second is two rises, and so on, because each step is filled right down to the ground. With n steps, width W, rise R and tread depth G:

• V = W × R × G × (1 + 2 + … + n) = W × R × G × n × (n + 1) ÷ 2

The answer shows in m³, yd³ and ft³. Then the waste share is added and the total divided by one bag’s yield, rounding the bags up.

How to use it

1. Press Metric (m, mm) or Imperial (ft, in). Anything you have typed is converted, so you can measure in one and read in the other.
2. Pick a shape: Slab, Footing, Post holes or Steps. Each one opens its own boxes with a sample job filled in.
3. Type your sizes. Long sides go in metres or feet; thicknesses and diameters go in millimetres or inches.
4. Set Extra for waste (%). It starts at 10.
5. Choose the bag size from 20, 25 or 30 kg, or 40, 60 or 80 lb. The yield box fills in with a typical label figure; change it to match your bag.
6. Read the large total, the volume, the volume with waste and the bag count. Reset example brings back the sample for that shape, and Clear empties the size boxes.

A worked example

Sam is building a garden shed on a concrete base, with solid steps up to a raised path.

The base is 2.4 m long, 1.8 m wide and 100 mm thick:

• Volume: 2.4 m × 1.8 m × 0.1 m = 0.432 m³ (about 0.57 yd³)
• With 10% waste: 0.432 m³ × 1.1 = 0.4752 m³
• 25 kg bags at 0.012 m³ each: 0.4752 ÷ 0.012 = 39.6, so 40 bags

The steps are 4 steps, 1.0 m wide, with a 170 mm rise and a 300 mm tread:

• 1 + 2 + 3 + 4 = 10
• Volume: 1.0 m × 0.17 m × 0.3 m × 10 = 0.51 m³ (about 0.67 yd³)
• With 10% waste: 0.561 m³
• 25 kg bags: 0.561 ÷ 0.012 = 46.75, so 47 bags

The steps need more than the base, because the fourth step is four rises deep.

The tool does one shape at a time, so Sam adds the two: 0.4752 + 0.561 = 1.0362 m³, or 87 bags. At that size the tool’s note suggests ready-mix from a truck, and Sam decides to get a quote.

Where it is used

• Garden and home jobs: shed bases, patios, paths, fence and pergola posts, and the pads under a hot tub or a water tank.
• Small builders and handymen, who need a quick bag count for a quote before visiting the supplier.
• Landscapers, who mix shapes on one job and add the totals, as Sam did.
• Ordering ready-mix: suppliers sell by the cubic metre or yard, so the “with waste” figure is the number to give them on the phone.

Common mistakes

• Mixing up millimetres and metres. A 100 mm slab typed as 100 m in the wrong box gives a huge answer. The tool asks for thickness in millimetres (or inches) for this reason; check the unit beside each box.
• Measuring the plan, not the dig. Holes and trenches end up wider and deeper than drawn. Measure after digging, or raise the waste share to 15%.
• Forgetting the post. For a post set in concrete, type its diameter so it is taken off. Type 0 only for a solid pier with nothing inside.
• Trusting the default yield. Bags of the same weight can yield different volumes. Read the bag and type its figure into the yield box.
• Counting the gravel as concrete. A 100 mm slab on 100 mm of stone needs 100 mm of concrete, not 200. The stone is a separate order.

Questions people ask

Why does the tool say 10% extra and not 0%? Ground is never perfectly flat, forms bow a little, and some mix stays in the barrow. Running short mid-pour is far worse than having a bag left over, which keeps for the next small repair if stored dry.

How do I work out a round slab or an L-shaped base? Split an L into two rectangles and run each as a slab, then add the volumes. For a round pad, use Post holes with one hole, the pad’s diameter, its thickness as the depth and a post diameter of 0.

Is a cubic yard the same as a “yard of concrete”? Yes. When a supplier in the US or Canada says a yard, they mean a cubic yard, which is 27 ft³ or about 0.765 m³.