Concrete is sold by the cubic metre (m³). To order the right amount you need three measurements of the space you're filling: how long it is, how wide it is and how deep it is. Multiply them together and you have your volume. The maths is easy. Getting the three numbers right is the part people get wrong, and after years of pouring bases, slabs and footings across Surrey we've seen most of the ways it goes wrong. That's what this guide is about.
Length (m) × Width (m) × Depth (m) = Volume (m³)
All three measurements must be in metres before you multiply. If you measure in centimetres, divide by 100 first. In millimetres, divide by 1,000.
Step 1: Measure the length and width
Measure the inside of your formwork or the edge of the excavation, not the outside of the timber. For a rectangular slab that's two measurements. Take each one at both ends and use the larger figure. Trenches and hand-dug bases are rarely straight, and we've never turned up to one that was narrower than the drawing.
If the shape isn't a rectangle, split it into rectangles you can measure separately, calculate each one and add them together. An L-shaped extension footing is just two rectangles. A driveway with a curved edge can be measured as a rectangle to the widest point. You'll be slightly over, which doesn't cost you anything with volumetric supply (more on that below).
Step 2: Measure the depth properly
Depth is where most estimates go wrong. It's the smallest of the three numbers, so a small slip has a big effect. A shed base at 125mm instead of 100mm is a quarter more concrete.
Measure from the top of the sub-base (the compacted hardcore or blinding you're pouring onto) up to the finished surface level. Take readings at several points, particularly in the corners and the middle. Ground is rarely level, and a lot of Surrey sits on clay that moves. If the base is 100mm at one end and 140mm at the other, use the average, about 120mm.
Step 3: Do the maths (a worked example)
Say you're laying a base for a 4m × 3m garden room in Leatherhead at 125mm deep.
4.0 × 3.0 × 0.125 = 1.5 m³. If you'd taken the depth as 100mm instead of 125mm you'd have ordered 1.2 m³ and run 0.3 m³ short. With a drum mixer that means a second delivery and a cold joint across your slab. With a volumetric truck the driver just keeps going.
Mixed units are fine, as long as you convert
Plans are often in millimetres, tape measures in centimetres and old drawings in feet. Convert everything to metres before multiplying:
- Centimetres ÷ 100 = metres (150cm = 1.5m)
- Millimetres ÷ 1,000 = metres (125mm = 0.125m)
- Feet × 0.3048 = metres (10ft = 3.05m)
- Inches × 0.0254 = metres (6in = 0.15m)
Or skip the conversions. The calculator below lets you pick a unit for each measurement, so you can type 4m × 3m × 125mm and read off the answer.
Dimensions of the pour
Measure the area you're filling - a slab, footing, trench or base. Mixed units are fine.
Suggested mix: Concrete grades explained
Fill in all three dimensions to see the volume. We include a 5% wastage allowance.
Minimum delivery of 1m³ not met. We'll still quote 1m³ for the pour. Call 0800 669 6500 if you're unsure.
Anything else for the job?
Call us or send an enquiry
Tell us the volume and postcode and we'll confirm the right mix, the price and a delivery slot.
Uneven ground, sloping sites and trenches
For a slab on ground that falls across the site, take depth readings on a grid (every metre or so) and average them. For a trench footing, measure the depth at several points along its length and do the same. Where a trench widens or the sides have collapsed, measure the width at the widest point; concrete fills whatever void is there.
If you're filling around existing pipes or services, estimate the volume they take up and subtract it. On domestic jobs this rarely shifts the order by more than 0.1 m³, so don't lose sleep over it.
Why you don't need to over-order with volumetric concrete
With drum-mixed (ready-mix) concrete, the load is batched at a plant before it leaves. Order too little and you wait hours for a second, minimum-charge load while the first one goes off. Order too much and you pay for concrete you then have to get rid of. The usual advice is to add 5 to 10% "for safety". That's a polite way of saying you pay for waste every time.
A volumetric mixer carries the raw materials (cement, aggregate, sand and water) separately and mixes them on your site, to the quantity you actually need. Our drivers pour until the formwork is full, then stop. You pay for what came off the truck, not what you guessed on the phone. So the number you work out above tells us how much material to load at our Effingham yard. It isn't a figure you're held to.
Tell us the volume and the postcode
We'll confirm the right grade, the price and a two-hour delivery window on the call. Same or next day across Surrey and the surrounding areas.
Quick answers
How much concrete for a 3m × 3m shed base?
At 100mm deep: 3 × 3 × 0.1 = 0.9 m³. We'd bring materials for 1 m³ and pour what the base takes.
How many cubic metres in a typical driveway?
A 6m × 3m single driveway at 150mm is 2.7 m³. A double driveway of 6m × 6m at the same depth is 5.4 m³.
Should I round up?
Round your estimate to the nearest 0.1 m³ and tell us the real figure. Because we mix on site, we carry a margin of materials anyway, so you don't need to pad the number.
What if I've under-measured on the day?
The driver keeps pouring. A volumetric truck usually carries materials for 8 to 10 m³, so a domestic job running 10 or 20% over is covered from the same load.
Who delivers volumetric concrete in Surrey?
We do. Surex Concrete is based in Effingham, a few minutes from Leatherhead, and delivers across Surrey and the surrounding areas, same or next day, from 1 m³. Call 0800 669 6500 with your volume and postcode.