Worldwide
Concrete calculator
Concrete is ordered by volume, in cubic metres, so the first job on any pour is turning a length, a width and a depth into a cubic-metre figure you can hand to a merchant or type into a ready-mix order.
Enter the slab length and width in metres and the thickness in centimetres.
This calculator does that for a rectangular slab and then takes it one step further, counting the 25 kg bags of dry pre-mix the job would take if you are mixing it yourself. Enter the slab length and width in metres and the thickness in centimetres; the tool multiplies the three together to get the bare volume, adds a waste margin, and divides the result by the mixed yield of one bag to give the bag count. The bare volume is the number that matters for a delivery, the bag count for a wheelbarrow-and-mixer job. Thickness drives the volume more than people expect, because it scales the whole area: doubling a slab from 75 mm to 150 mm doubles the concrete and the cost, so it pays to settle the depth before working out anything else. A garden shed base or a path is usually 75 to 100 mm thick, a domestic driveway 150 mm or more over a compacted sub-base, and fence-post footings deeper and narrower. The waste margin covers the concrete that never reaches the form: spillage off the barrow, an uneven or over-dug sub-base that swallows extra depth, and the bit left clinging to the mixer drum. Five percent suits a tidy slab with firm edges; raise it for footings, sloping ground or a rough dig. The bag figure assumes a 25 kg bag of general-purpose dry mix makes about 0.011 m3 (roughly 11 litres) of wet concrete, a typical manufacturer figure, though it shifts with the product and how wet you mix it, so always check the bag. Bags suit small pours; once a job runs past about a third of a cubic metre, the sums usually favour a ready-mix delivery or a trailer of mixed concrete over hauling and emptying forty-odd bags by hand. The tool estimates the concrete alone, not the reinforcing mesh, the timber formwork that holds the wet pour, or the hardcore sub-base underneath, all of which you work out and buy separately.
How it works
- Measure the slab length and width in metres and enter them.
- Enter the thickness in centimetres: about 100 mm for a slab, 150 mm for a driveway.
- Set a waste allowance, around 5 percent for a firm-edged slab and more for footings or rough ground.
- The tool multiplies length, width and thickness to get the volume, then adds the waste.
- It divides the volume with waste by the yield of one 25 kg bag and rounds up to whole bags.
bags = ceil( length x width x (thickness / 100) x (1 + waste) / yield per bag )
Length times width gives the slab area in square metres. Multiplying by the thickness in metres (the centimetre figure divided by 100) gives the volume in cubic metres. Multiplying by one plus the waste fraction adds the concrete lost to spillage and an uneven base. Dividing by the mixed yield of one bag gives the bags in exact terms, and rounding up reflects that bags are sold whole. For a ready-mix order, read the volume-with-waste figure instead and round to the supplier increment.
- length, width
- slab dimensions in metres
- thickness
- slab depth in centimetres, divided by 100 for metres
- waste
- fraction added for spillage and an uneven base, 0.05 typical
- yield per bag
- cubic metres of wet concrete from one bag, about 0.011 for 25 kg
Typical slab thickness
| Garden path or shed base | 75 to 100 mm | light foot traffic |
| Patio slab | 100 mm | over a firm sub-base |
| Domestic driveway | 150 mm | cars, on compacted hardcore |
| 25 kg bag yield | about 0.011 m3 | roughly 11 litres mixed |
Worked example
A 3 by 2 metre shed base, 100 mm thick, with a 5 percent waste allowance and a 0.011 m3 yield per 25 kg bag: the slab area is 6 m2, which at 0.1 m thick is 0.6 m3 of concrete, becoming 0.63 m3 once the 5 percent waste is added. At 0.011 m3 a bag that is 58 bags of dry mix, nearly 1.5 tonnes to carry and turn by hand. At that size a ready-mix or trailer load of mixed concrete is the easier buy. Drop the base to 75 mm and the volume falls to 0.47 m3, around 43 bags, which shows how much the depth alone moves the order.
Key facts
- Concrete is ordered by volume in cubic metres, the length times width times depth.
- Thickness scales the whole order, so 150 mm uses half as much again as 100 mm.
- A 25 kg bag of dry mix makes only about 0.011 m3, so volume adds up fast.
- Past roughly a third of a cubic metre, ready-mix usually beats bags.
Tips
- Settle the slab thickness before anything else, since it drives the volume.
- Order ready-mix to the volume-with-waste figure, rounded to the supplier increment.
- For footings and rough ground, lift the waste allowance to 10 percent or more.
- Have the formwork fixed and the sub-base ready before the concrete arrives.
Frequently asked questions
How thick should the slab be?+
A shed base or garden path is usually 75 to 100 mm, a domestic driveway 150 mm or more over a compacted sub-base, and a slab carrying heavy loads thicker again with reinforcement. Thickness scales the whole volume, so decide it first: going from 100 to 150 mm adds half as much concrete again to the order.
When should I order ready-mix instead of bags?+
Bags are fine for footings, a small base or a short path, roughly up to a third of a cubic metre. Past that, the bag count climbs fast (0.6 m3 is already around 58 bags), and a ready-mix delivery or a trailer of mixed concrete saves a lot of lifting and gives a more consistent mix poured in one go before it sets.
What does the yield per bag mean?+
It is how much wet concrete one bag of dry mix makes once water is added. A 25 kg bag of general-purpose mix yields roughly 0.011 m3, about 11 litres, but the figure varies by product and how wet you mix, so check the bag. The tool uses your entered yield to turn the volume into a bag count.
Why add a waste allowance?+
Some concrete never reaches the form. It spills off the barrow, fills an uneven or over-dug sub-base, and coats the mixer drum. A 5 percent margin covers a tidy slab with firm shuttering; for footings, trench fill or sloping ground where the dig is rough, raise it to 10 percent or more so you are not caught a barrow short mid-pour.
How do I handle an L-shaped or stepped slab?+
Split the shape into rectangles, work out the volume of each (length times width times thickness), and add them before applying the waste. For footings of different depths, treat each run as its own block. A circular pad is its radius squared times 3.14 times the depth, then add waste the same way.
Does this include reinforcement or the sub-base?+
No. It counts the concrete volume only. Reinforcing mesh or rebar, the timber formwork that shapes the pour, and the compacted hardcore sub-base beneath the slab are all separate. Work the hardcore out as its own volume to a depth of about 100 to 150 mm, the same way the gravel estimate does.
Things to watch
- Bag yield varies by product and mix wetness, so check the figure on the bag.
- This counts the concrete only, not reinforcement, formwork or the sub-base hardcore.
- Running short mid-pour leaves a cold joint, so order with a margin and mix in one go.
Last updated: 2026
This is an estimate for general guidance, not financial, tax, legal or medical advice. Figures can change and individual circumstances vary. Always confirm with an official source before making decisions.
Built and maintained by Vikas Dulgunde. Editorial standards.