Concrete Calculator UK · metric

Cubic metres, bags of mix, and cost for slabs, footings, and bases. Metric units and £ costs, updated for 2026.

Last reviewed July 2026 · Free · Nothing you enter is stored

How this calculator works

Volume is length × width × thickness in metres, plus a 10% waste allowance. Bag counts use ~0.0095 m³ per 20 kg bag and ~0.012 m³ per 25 kg bag. Ready-mix pricing uses a UK average near £110/m³ delivered, excluding VAT; small-load surcharges of £50–£100 apply below a supplier's minimum (often 2–6 m³).

The waste allowance

This calculator adds 10 per cent to the raw volume, and that allowance earns its keep on almost every job. Hardcore and sub-base are never perfectly level, timber formwork bows outward once several hundred kilograms of wet concrete lean on it, and a slab poured five millimetres deep across a twenty square metre patio quietly swallows an extra tenth of a cubic metre. Running short in the middle of a pour is the expensive failure. The first batch stiffens while you wait, and the join between it and whatever arrives later is a cold joint, a permanent line of weakness that usually shows on the surface and often cracks along its length. A second delivery to make up a shortfall also attracts a part-load charge that can cost more than the concrete in the lorry. Ready-mix is generally ordered in half cubic metre steps, so round up to the next half metre rather than trying to shave the order down.

Bags or ready-mix, and where the line sits

Blue Circle publishes the figure that matters here. One 20 kg bag of its ready-to-use concrete produces around 0.01 cubic metres of fresh concrete, roughly ten litres. That is about 100 bags to the cubic metre, or two tonnes of bagged material to fetch, carry and mix one bag at a time. The calculator above works on a slightly more cautious 0.0095 cubic metres a bag, which gives 105, and we have left it that way deliberately: bag yields vary a little with how wet you mix them, and erring towards one extra bag costs a few pounds where erring the other way costs you a trip back to the merchant.

Merchants also sell 25 kg bags, and the honest position is that the yield for those is not published as consistently as the 20 kg figure. Working pro rata gives roughly 0.0125 cubic metres a bag, about 80 bags to the cubic metre. That is a derived number rather than one printed on a data sheet, so treat it as an estimate and read the bag in front of you before ordering forty of them.

For a fence post, a step or a small shed base, bags are obviously right. Somewhere around half a cubic metre the arithmetic turns. Hand mixing means every batch is a slightly different consistency, and each one starts to set while you mix the next, so a large slab poured that way is really a series of small ones. Ready-mix arrives in a single load at a consistent, certified strength and is placed inside an hour.

The obstacle is the minimum load. Most plants want three to four cubic metres, more in London, and charge a part-load surcharge below it, commonly quoted at £15 to £30 for each cubic metre you are short. Volumetric lorries, which carry the raw materials and mix on site, take orders from around half a cubic metre with no shortfall charge, at a premium of roughly fifteen to twenty-five per cent per cubic metre. For a domestic job of one to three cubic metres that is very often the cheapest way to get proper concrete on the ground.

Which mix to ask for

British suppliers sell designated mixes under BS 8500, and asking for one by name is far more reliable than describing what you are building. GEN1, at 10 N/mm², is a fill and blinding mix. GEN3, at 20 N/mm², suits internal floor slabs and light domestic work. PAV1, at 35 N/mm², is the designated mix for domestic driveways, hardstandings and external paving.

The difference with PAV1 is not only strength. It is air entrained, which means deliberately introduced microscopic bubbles that give freezing water somewhere to expand into. On an exposed British drive that takes rain, repeated frosts and the occasional scattering of de-icing salt, that air content matters as much as the extra newtons. A shed base is perfectly happy on C20 or C25; a drive that will be salted in January is not.

If a supplier quotes C25/30 rather than a GEN or PAV designation, they are using the strength class notation instead, where the two figures are the characteristic cylinder and cube strengths. C25/30 is a common domestic order and sensible for a patio or a base. What you should not do is take whatever happens to be left on the lorry from the previous job.

Thickness, sub-base and where the mesh goes

One hundred millimetres is the conventional thickness for a patio, a path or a garden shed base, laid over 75 to 100 mm of compacted MOT Type 1. A domestic driveway is usually 150 mm over 150 mm of Type 1, and heavier again where the ground is soft clay or a van will park. Workshops carrying machinery are commonly taken to 150 mm with A142 mesh, and genuinely heavy-duty drives to A252.

The sub-base is not a formality. Concrete is strong in compression and weak in tension, so a slab fails when it is asked to bridge a soft spot: the underside goes into tension and cracks. A well compacted Type 1 layer removes the soft spots. A thicker slab over poorly compacted ground still cracks, just later and more expensively.

Mesh only works if it sits inside the slab rather than under it. Laid on the sub-base and poured over, which is what happens on most domestic jobs, it ends up at the very bottom where the stresses it is meant to carry are not. It needs setting on spacers before the pour, or lifting into the concrete as it goes. Reinforcement does not prevent cracking in any case. It holds cracks tight and keeps both sides in the same plane, and movement joints decide where they appear.

Where the slab is inside a building, or will be one, a polythene damp-proof membrane goes underneath and is lapped into the wall damp-proof course. That is a Building Regulations matter rather than a preference, and it cannot be added afterwards.

Curing, and the British weather

Concrete does not dry out to gain strength. It hydrates, a chemical reaction that needs water to continue, so a slab that dries early stops gaining strength and stays permanently weaker than the mix it came from. Blue Circle's own guidance is to cover finished concrete and protect it from drying out for a minimum of seven days. Full design strength is quoted at 28 days.

In this country the more common threat is cold rather than heat. Concrete Society guidance is that the temperature of the concrete should not fall below 5°C until it has reached 5 MPa, that surfaces in contact with fresh concrete need to be a few degrees above freezing, and that exposed surfaces should be covered with insulating material once cast. Concrete should not be placed against formwork or steel carrying ice or snow. If the water in young concrete freezes, the damage is permanent and the slab is unfit for use.

Rain is the other hazard, and it matters most in the hour or two after finishing. Heavy rain on a surface that has not yet set washes out the fines and leaves a weak, dusty skin that never hardens properly. A sheet of polythene held clear of the surface on battens deals with drying, frost and rain at once.

As a working timeline, foot traffic after a day or two, a car at about a week, and nothing heavy until the month is up. A slab feels hard long before it is strong, which is precisely why new drives get damaged.

Bag yields and coverage

Yields are per bag of ready-to-use concrete, the sort sold in merchants with the aggregate already in it. Coverage assumes a 100 mm slab and excludes the 10 per cent waste allowance.

The 20 kg yield is from Blue Circle's technical data sheets for Multi-Purpose Concrete and High Strength Concrete (40N), both of which state that one bag produces around 0.01 m³ of fresh concrete. The 25 kg figure is derived pro rata from that and is not a manufacturer-published yield, so it is an estimate rather than a specification. The calculator itself uses marginally lower yields of 0.0095 and 0.012 m³, which round the bag count up rather than down. The ballast and cement quantities are a widely quoted trade rule of thumb for a nominal 1:6 mix, not a published standard. Coverage figures are arithmetic.

What goes wrong on domestic pours

Mesh laid straight onto the sub-base. Reinforcement at the very bottom of a slab does almost nothing. Set it on spacers, or lift it into the pour as you go.

Ordering exactly what the maths says. Formwork bows and the sub-base is never flat. Coming up short mid-pour buys you a cold joint and a part-load charge.

Buying bags for a cubic metre. That is roughly a hundred 20 kg bags and two tonnes of lifting. A volumetric lorry will deliver mixed concrete from about half a cubic metre.

Ordering the wrong designated mix. A drive that takes frost and de-icing salt wants air-entrained PAV1. A fill mix such as GEN1 will surface-scale within a few winters.

Pouring into a cold snap. Below 5°C the reaction stalls, and frost in fresh concrete does permanent damage. Watch the week's forecast, not the day's, and cover the slab.

No movement joints. The slab will crack. Joints decide where, and on a large bay they need forming rather than hoping for.

Frequently asked questions