How Much Concrete Do I Need? A UK Guide in Cubic Metres

Concrete volume is length times width times depth, with everything in metres. The answer comes out in cubic metres.

Volume (m³) = Length (m) × Width (m) × Depth (m)

The only trick is the depth, because you'll have it in millimetres. Divide by 1,000. A 150 mm slab is 0.15 m. A 100 mm slab is 0.1 m. Get that conversion wrong by a factor of ten and you'll either order a teaspoon or bankrupt yourself.

A 4 m × 3 m patio at 100 mm: 4 × 3 × 0.1 = 1.2 m³.

Takeaway: millimetres divided by 1,000 gives you metres. Everything else is multiplication.

Slabs, bases and patios. Length × width × depth. If the shape isn't a rectangle, chop it into rectangles, work each one out, and add them up. An L-shaped patio is two rectangles, not a headache.

Trenches and strip footings. Length × width × depth, same formula, different orientation. A 12 m run of footing at 600 mm wide and 300 mm deep is 12 × 0.6 × 0.3 = 2.16 m³. Measure the total run along the centre line, not around the outside, or you'll double-count the corners.

Posts, columns and pads. These are cylinders, so it's π × radius² × depth. A 300 mm diameter post hole at 600 mm deep is 3.14 × 0.15² × 0.6 = 0.042 m³. Remember to knock off the volume of the post itself if it's a big one.

Steps. Treat every step as its own rectangular block, work out each, add them together. It's tedious and it's also the only way to get it right.

Takeaway: every concrete job is rectangles and cylinders. Break it down and add it up.

Every calculator on the internet tells you to add 10% for wastage. It's repeated so often that nobody stops to ask what the 10% is actually for.

Here's the problem. Spillage is not what makes your volume wrong. If you're pouring into a formed-up slab, you might lose a few litres down the sides and off the barrow. That's not 10%. That's about 1%.

The thing that makes your volume wrong is the depth. You've calculated a 150 mm slab. Your sub-base is not 150 mm below the top of the formwork everywhere. It dips. It's been whacked down harder in some places than others, there's a soft spot where the digger tracked over it, and the far corner is a bit low because it was raining when you levelled it. And concrete, being a liquid, fills every one of those dips.

This matters because the error only goes one way. Concrete fills the void it's poured into. It cannot fill less than the void. So your actual volume is always equal to or greater than your nominal volume, never less. It's not random scatter you're covering with a safety margin. It's a systematic, one-directional, and entirely measurable bias.

So measure it. Before you order, run a straightedge across the formwork and drop a tape at eight or ten points across the slab. Average the readings. If your nominal 150 mm is actually averaging 158 mm, that's your real number, and it's a 5% uplift you know about rather than a 10% you guessed at.

In practice, on a decently prepared base, the honest figure is usually 3% to 6%. On a rough dig it can be 15%. A blanket 10% is simultaneously too much on a good job and nowhere near enough on a bad one.

Takeaway: measure your actual depth at multiple points and average it. Don't guess a percentage.

There's a second reason the wastage question is badly framed: running over and running short are not remotely the same size of mistake.

Order half a metre too much and you've spent about £60 and you've got a barrow of spare concrete. Stick it in a spare bit of formwork, make a shed base, bin it. Annoying, cheap.

Run short mid-pour and you have a cold joint. The first pour starts going off while you wait for a second load that might be ninety minutes away, and you end up with a plane of weakness straight through your slab. On a structural element that's a defect. On a garage floor it's a crack waiting to happen. The fix is breaking it out.

I've watched a gang stand around for over an hour because a 3.6 m³ slab was ordered as 3.2. The concrete cost of getting it right first time was about £50. The cost of getting it wrong was four blokes on day rates doing nothing, plus a joint through the middle of the slab forever.

Takeaway: always round up to the next 0.1 m³, and if you're within a whisker of the next half metre, take it.

A single garage base, 6.0 m × 3.5 m, at 150 mm thick. Internal floor, sheltered, so GEN3 is the right designation.

Step 1, the nominal volume: 6.0 × 3.5 × 0.15 = 3.15 m³

Step 2, measure the real depth. Tape dropped at nine points across the base gives readings between 151 mm and 168 mm. The average comes out at 158 mm. The sub-base has a dip near the back corner where it was hard to get the whacker in.

Step 3, the real volume: 6.0 × 3.5 × 0.158 = 3.32 m³

Step 4, round up: order 3.5 m³.

At a Midlands price of roughly £120 per m³ delivered for GEN3, that's about £420 before VAT. The 3.5 m³ order also clears the 3 m³ threshold that most suppliers use for short-load surcharges, which matters more than the £40 of extra concrete, as the next section explains.

Now compare. The same 3.5 m³ in 25 kg bags from Travis Perkins would be roughly 336 bags, because a cubic metre of concrete weighs about 2,400 kg. At £6 a bag that's over £2,000 and about three days of mixing. It isn't a close call.

Takeaway: measure, round up, and check where the short-load threshold sits before you commit.

Here's where the standard advice actively costs people money.

Most UK suppliers apply a short-load surcharge on orders below about 3 m³, and some set it at 4 m³. In 2026 that surcharge typically runs £80 to £150. A full drum on a standard mixer truck is around 6 m³ at the major suppliers, and the per-m³ price steps down noticeably once the wagon is properly loaded.

So picture a 10 m² patio at 100 mm. That's 1.1 m³. Add your dutiful 10% and you're ordering 1.25 m³. At £120 per m³ the concrete is £150, and the short-load surcharge is £120. You are paying nearly as much in surcharge as you are for the material.

At that point "how much do I need" is the wrong question entirely. The right question is "what does this actually cost me per cubic metre once the surcharge lands", and the answer might be that you're better off bringing a second small job forward, doing the patio and the shed base on the same load, and paying for 3 m³ you'll fully use rather than 1.25 m³ you'll be surcharged for.

Two UK options the American calculators never mention. Mini-mix services run smaller vehicles and will do 1 to 2 m³ with a lower surcharge. Volumetric mixers, the ones that batch on the back of the wagon, charge you for what actually comes out of the chute. If your volume is genuinely uncertain, a volumetric wagon removes the entire over-or-under problem, because you stop when the formwork's full.

Takeaway: on anything under 3 m³, ring the supplier and ask for the short-load threshold and the surcharge before you decide your volume.

There are three ways to buy concrete in the UK and the price gap between them is enormous.

Bagged concrete looks convenient right up until you do the division. A cubic metre needs roughly 96 bags of 25 kg. Nobody is mixing 96 bags. The practical ceiling for bags is about 0.2 to 0.3 m³, which is a few post holes or a small pad.

The crossover point where ready-mix beats site-mixing is somewhere around 0.8 to 1 m³ once your own time is worth anything at all.

One thing that isn't about money: if the pour is a foundation covered by Building Regulations, Building Control will want evidence the concrete meets the specified grade. That evidence is the delivery ticket, which only comes with certified ready-mix batched to BS 8500 and BS EN 206. You cannot produce that from a mixer in your driveway, whatever the mix looked like.

Takeaway: over 1 m³, order ready-mix. On regulated foundations, order ready-mix regardless.

Volume is only half the order. The supplier also needs the designation, and BS 8500-1 is the standard that governs it.

The one to get right is PAV1 for anything external. Air entrainment puts microscopic bubbles into the mix that give freezing water somewhere to expand into. Without it, a UK winter will scale the surface off your driveway within a few years. Plenty of guides will happily tell you GEN3 is fine for a driveway. It isn't, and the difference is a few pounds per cubic metre against relaying the whole thing.

Takeaway: internal slab, GEN3. Anything outside that sees frost, PAV1 minimum.

Have these ready and the call takes two minutes:

One rule that matters more than all of it: never let anyone add water to the mix on site. It's the oldest trick on a hot day, it wrecks the strength, and it voids the compliance certificate you may need for Building Control.

Takeaway: ask for the delivery ticket and keep it. It's your proof of grade.