How to Calculate Concrete
Concrete is sold by the cubic metre (m³) in the UK. To find your volume, multiply length by width by depth with everything in metres.
Volume (m³) = Length (m) × Width (m) × Depth (m)
So a 3m × 3m shed base at 100mm deep works out as 3 × 3 × 0.1 = 0.9 m³.
Depth trips most people up because it is usually quoted in millimetres. Just divide by 1,000 to convert. 100mm becomes 0.1m and 150mm becomes 0.15m.
For circular post holes or columns, use π × radius² × depth. A 300mm wide post hole dug 600mm deep needs π × 0.15² × 0.6 = 0.042 m³ per post.
For odd shapes, split the area into rectangles, calculate each one separately and add the volumes together.
How Much Concrete for Common UK Projects
These figures show typical volumes at standard depths. Always measure your own site because ground is rarely perfectly level.
Project
Typical size
Depth
Volume
Shed base
3m × 2m
100mm
0.6 m³
Garden room base
4m × 3m
100mm
1.2 m³
Single garage floor
6m × 3m
150mm
2.7 m³
Driveway (one car)
5m × 3m
100mm
1.5 m³
Patio
4m × 4m
100mm
1.6 m³
Fence post hole
300mm dia
600mm
0.042 m³ per post
Strip footing
5m run
600mm × 300mm
0.9 m³
Driveways need a minimum of 100mm of concrete for cars and 150mm if vans or heavier vehicles will use them.
Bags or Ready Mix?
This is the decision that matters most for your budget and your back.
Bagged concrete suits jobs under roughly 0.5 m³. Think fence posts, repairs, small pads and path edges. A 25kg bag of premixed concrete yields about 0.012 m³, so you need around 85 bags for a full cubic metre. At £5 to £8 per bag that works out near £200 per m³ once mixed, and mixing that many bags by hand is a full day of hard graft.
Ready mix concrete becomes cheaper and far easier once you pass about 1 m³. A lorry delivers the exact grade you ordered, batched to British Standards and the pour is done in minutes. Most suppliers have a minimum load of 1 to 2 m³ and small loads under 4 m³ often carry a surcharge.
Volumetric (site mixed) concrete is the middle ground UK tradespeople often recommend. The truck mixes on site and you only pay for what you use, which removes the risk of over ordering. It is a smart choice when your volume is uncertain or access makes waste likely.
For fence posts specifically, postcrete is quicker than standard concrete. One or two 20kg bags fill a typical post hole and it sets in around ten minutes.
UK Concrete Grades Explained
Concrete strength is graded by its compressive strength in newtons. The higher the number, the stronger and more expensive the mix. Ordering the right grade matters because under specifying risks cracking and over specifying wastes money.
Grade
Also known as
Best for
C10
GEN 1
Blinding, kerb bedding, non structural fill
C20
GEN 3
Shed bases, garage floors, lightly loaded slabs
C25
ST 2 / RC25
House foundations, footings, extensions
C30
ST 3 / RC30
Driveways, paths, external slabs
C35
RC35
Commercial floors, heavy duty driveways, agricultural yards
C20 is too weak for a driveway that takes regular vehicle loads. Most UK suppliers recommend C25 or C30 for domestic driveways and C35 where HGVs or farm machinery are involved.
For foundations covered by Building Regulations, use certified ready mix rather than hand mixed bags. Building Control may ask to see the delivery ticket confirming the specified grade.
Ready Mix Concrete Prices UK
Standard domestic grades cost roughly £100 to £135 per m³ excluding delivery and VAT, with London and the South East typically 15 to 25% higher. Guide prices for 2026:
Grade
Price per m³
C20
£100 to £125
C25
£105 to £135
C30
£110 to £142
C35
£118 to £145
On top of the base price, budget for delivery charges, a small load surcharge of £40 to £80 if you order under 4 m³ and pump hire from around £275 per day if the lorry cannot reach the pour. Weekend deliveries usually cost more too.
Concrete Mix Ratios for Mixing by Hand
Mixing your own suits small jobs where a lorry is overkill. The two ratios UK DIYers use most:
1:2:4 mix (general purpose, roughly C20): 1 part cement, 2 parts sharp sand, 4 parts 20mm aggregate. Suits shed bases, paths and small slabs.
1:4 ballast mix: 1 part cement to 4 parts all in ballast. Ballast is sand and gravel already blended, which makes it the easiest option for beginners. Use 1:6 for lighter duty work like bedding.
Add water gradually until the mix is workable but not sloppy. Too much water weakens concrete, so aim for a consistency that holds its shape when you drag a shovel through it. Once water hits cement you have about two hours to pour and level, so prepare everything before you start mixing. One cubic metre equals roughly 20 wheelbarrow loads, which is worth knowing before you commit to borrowing it across the garden.
Before You Order
Add 5 to 10% for waste: Ground is never perfectly flat and some concrete always sticks to the mixer, the barrow and your boots. Running short mid pour creates a weak joint, so calculate from your maximum measurements and round up.
Never add water on site: It weakens the mix and invalidates the supplier's grade guarantee. If you need a wetter, more workable mix for pumping or trench fill, ask for a higher slump when ordering.
Check access: A fully loaded mixer lorry weighs up to 32 tonnes. Confirm gate widths, overhead cables and whether the truck can get close enough, otherwise you may need a pump or barrow crew.
Book ahead: Suppliers typically need 2 to 5 working days notice, longer in summer.
Let it cure: Keep foot traffic off for 24 to 48 hours and vehicles off for at least 7 days. Concrete reaches its design strength after 28 days.
FAQs
How do I calculate cubic metres of concrete?
Multiply length × width × depth in metres. A 4m × 3m slab at 100mm deep is 4 × 3 × 0.1 = 1.2 m³.
How many 25kg bags of concrete are in 1 cubic metre?
Around 85 bags. Each 25kg bag of premixed concrete yields roughly 0.012 m³. Check the yield printed on your bag as brands vary slightly.
How much concrete do I need for a 3m × 3m shed base?
0.9 m³ at 100mm deep. Order 1 m³ to cover waste or roughly 80 to 90 bags of 25kg premix if mixing by hand.
What is the difference between cement and concrete?
Cement is the binding powder. Concrete is cement mixed with sand, aggregate and water. A bag of cement alone will not set into usable concrete.
How much does a cubic metre of concrete weigh?
About 2.4 tonnes when wet. That is why depth errors get expensive fast and why barrowing large volumes is serious work.
What grade of concrete do I need for a driveway?
C25 or C30 at 100mm minimum depth for cars. Go to C35 and 150mm if heavy vehicles will use it.
How long before I can walk or drive on new concrete?
Walk on it after 24 to 48 hours. Keep vehicles off for at least 7 days. Full design strength takes 28 days.
Related Calculators
- Concrete Weight Calculator UK
- Shed Base Calculator UK
- Post Mix Calculator UK
- Building Garden Wall Cost Calculator UK
- Self Levelling Compound Calculator UK
- Tarmac Driveway Cost Calculator UK
References
- Cemex UK, Readymix Concrete Calculator:
- Heidelberg Materials UK, Ready Mixed Concrete Volume Calculator and Pricing Guide:
- RMS Concrete, Ready Mix Concrete Prices 2026:
- MyBuilder, Ready Mix Concrete Prices in 2026:
- BSI, BS 8500 Concrete Specification (UK complementary standard to BS EN 206)
- SoftwareApplication
