Concrete calculator

How the volume is calculated

slab = length × width × (thickness ÷ 100) · strip = length × (width ÷ 100) × (height ÷ 100) · wall = length × height × (thickness ÷ 100) · post = π × (diameter ÷ 200)² × depth · to order = net × (1 + reserve ÷ 100)

Concrete is measured by volume: every shape is reduced to cubic meters and then multiplied by the number of identical elements. A slab, a strip footing and a wall are plain boxes, while a post hole is a cylinder — that is why you enter a diameter, and the radius in meters is the diameter in centimeters divided by 200.

If you need the area of the slab or wall first, work it out in the area calculator, then enter the length and width of that same surface here, plus the thickness in centimeters.

Finally the reserve is added to the net volume to give the quantity to order. Cubic meters are shown to two decimals, and the number of bags is always rounded up — you cannot buy half a bag.

Slab, strip footing, wall and post — which measurements to enter

Lengths and heights of elements go in meters, while slab thickness, the width and height of a footing section and the post diameter go in centimeters. Every field repeats its unit — the most common mistake is entering the slab thickness in meters instead of centimeters, which gives a result a hundred times too small.

For a strip footing enter the total length of the strip, meaning the sum of all runs around the building, not the length of one side. For post holes enter the hole diameter and the depth, and put the number of holes in the “Number of identical elements” field — the calculator multiplies the volume of one element by that number.

The “Number of identical elements” field works in every mode: if you are pouring three identical slabs or twelve identical holes, enter the measurements of one element and how many there are — the result also shows the volume per element.

Net volume and concrete to order

The net volume is pure geometry — how many cubic meters fit into the shape you described. The quantity to order is that same volume plus the reserve, and it is the number that goes into the order, which is why the two are shown separately.

The reserve exists because some concrete is lost in practice: spillage while unloading and spreading, an uneven subgrade that takes more than the measurements suggest, what stays in the pump and the line, and small differences between the excavation and the numbers on paper.

The suggested 5% is a starting planning convention and the field is free to change — it is not a prescribed amount. Use less on a level, prepared subgrade and more on a rough excavation. The material cost is calculated on the ordered quantity, because concrete is paid for as ordered.

Worked examples

A 5 × 4 m terrace slab 12 cm thick: 5 × 4 × 0.12 = 2.4 m³ net, and with a 5% reserve you order 2.52 m³. If a cubic meter costs 180 KM, the material comes to 453.60 KM — prices differ by plant and distance, so enter your own.

Twelve fence post holes 30 cm across and 0.8 m deep: one hole is π × 0.15² × 0.8 ≈ 0.057 m³, about 57 litres. Twelve holes come to roughly 0.68 m³ net, or about 0.71 m³ with a 5% reserve.

A strip footing 24 m long in total, 40 cm wide and 50 cm high: 24 × 0.4 × 0.5 = 4.8 m³ net, and 5.28 m³ with a 10% reserve.

What this calculator does not do

The calculator works out volume and the quantity to order. It does not choose a slab thickness, size reinforcement or mesh, check load-bearing capacity or say how deep a foundation goes — those are decisions a structural designer or licensed engineer makes based on the soil, the loads and the location.

The same goes for the concrete type and class, placing and curing — the calculator assumes nothing about them. The result is a planning estimate: if the excavation differs from the drawing, change the measurements and calculate again.