Concrete Calculator
Find how many cubic metres or bags of concrete you need, and what it will cost.
Measure your project
Your answer
That is about 35 wheelbarrow loads. We added 10% extra.
If you buy bags
One 20 kg bag makes about 0.01 m³, says Blue Circle. So about 100 bags make 1 m³.
If you order a lorry (ready-mix)
Ask for 1.00 m³. One supplier's smallest order for homes is 1 m³, so we price at least that.
This answer is a guide. For footings, drives or anything that holds weight, check your local building rules or ask a builder or engineer.
Which costs less?
You also skip mixing 99 bags by hand. Have the edges ready before it comes.
UK middle prices from October 2026, with 20% VAT. Ask your supplier for their price.
Quick answers
- A 3 × 3 m slab, 100 mm thick
- 0.9 m³. With 10% extra, 0.99 m³, or 99 bags of 20 kg.
- Bags in 1 m³
- About 100 bags of 20 kg. Each bag makes about 0.01 m³.
- How thick?
- At least 100 mm for a house floor. A trade guide says 100 mm for a car drive, and 150–200 mm for heavier use.
- Ready-mix price
- About £128 per m³ with VAT. That is the middle of 7 prices from 4 UK suppliers.
How to work it out
- Measure the length and width in metres.
- Measure the thickness in mm. Divide by 1,000 to get metres.
- Multiply length × width × thickness. This gives m³.
- Add 10% extra.
- For 20 kg bags, divide the m³ by 0.01. Round up.
3 × 3 × 0.1 = 0.9 m³
0.9 + 10% = 0.99 m³
0.99 ÷ 0.01 = 99 → 99 bags
Bags or a ready-mix lorry?
A 20 kg bag is easy to carry and mix. Per m³, though, bags cost far more. You need about 100 bags of 20 kg for 1 m³. At £11 a bag, that is about £1,100.
Ready-mix costs about £128 per m³, with VAT. So bags cost about 9 times as much. Our bag price is the middle of 4 prices from only 3 stores.
Small loads
Ready-mix comes in a mixer lorry. A full lorry holds 7–8 m³, says Heidelberg Materials. Less than that is a part load. Suppliers treat part loads in different ways:
- Rock Solid Concrete takes orders from 1 m³ for homes.
- EasyMix says it has no part-load charges. It does charge for delivery.
- Heidelberg charges for part loads, but does not list the amount.
- Mister Concrete has a minimum charge of £625 plus VAT. That is £750 with VAT.
Check the minimum charge first. At £750 with VAT, it would buy about 68 bags, or 0.68 m³. Without such a charge, 1 m³ of ready-mix costs less once you need 12 bags.
Checkatrade, a cost guide, puts delivery at £100–£200 and a minimum order at £500. Some prices include delivery and some don't. Add any delivery fee in "Change prices".
Think about moving it, too. 1 m³ is about 35 wheelbarrow loads, says Heidelberg.
Concrete for fence posts
Blue Circle's Postcrete data sheet says how many 20 kg bags fill each hole.
| Hole (wide × deep) | Post | Bags of 20 kg |
|---|---|---|
| 15 × 60 cm | 5 cm round post | ½ |
| 20 × 60 cm | 5 cm round post | 1 |
| 25 × 75 cm | 10 cm square post | 1⅔ |
| 30 × 75 cm | 5 cm round post | 2¾ |
A 20 kg bag of post mix costs about £7.50. That is from only 2 UK stores, so check your own.
For other hole sizes, pick "Round holes" in the calculator. It counts ordinary 20 kg concrete bags.
How thick should it be?
- House floor: at least 100 mm, says Approved Document C, the building rules for England. NHBC also asks for at least 100 mm. The hardcore under it should be no more than 600 mm deep.
- Garage floor: at least 100 mm on 100 mm of hardcore, says NHBC.
- Car drive: 100 mm. Use 150–200 mm for heavier use, such as large vans. These come from Pavingexpert, a trade website. We found no official UK figure for drives.
- Patio slabs: a concrete bed of 50–80 mm. It sits on 50–80 mm of hardcore, says Cemex.
- Shed base: we found no rule for how thick. See the mix below.
For a floor, drive or anything that holds weight, check your local building rules. Or ask a builder or engineer.
Which mix to ask for
Ready-mix suppliers sell mixes by name. Heidelberg Materials suggests these for home jobs. Cemex says what goes in each one.
| Mix | Use it for | Cement : sand : stone | 25 kg cement bags per m³ |
|---|---|---|---|
| ST2 | Foundations | 1:3:6 | 10.6 |
| ST3 | Floor slab with a smooth top | 1:2:4 | 11.8 |
| ST4 | Shed or garage base | 1:1.5:3 | 13.2 |
Cement from the British Standard table (BS 8500-2), as copied by Pavingexpert. For ST2 and ST3, Cemex gives 15–34% less.
For a drive, ask for PAV1. The Concrete Society says it is for house drives and domestic parking.
Mixing by hand? Heidelberg's cement data sheet says 1 part cement to 4 parts ballast. Our cement calculator works out the bags of cement and ballast.
Before you order
- Work out the m³, with about 10% extra.
- Ask for the price per m³, and if it includes VAT. Some price lists leave VAT out.
- Ask about part-load fees, delivery and any minimum charge.
- Ask how long the lorry can wait. Heidelberg gives 30 minutes to unload for free.
- Check the lorry can get close. If not, line up barrows and helpers.
Questions people ask
How much concrete do I need?
Multiply the length, width and thickness, all in metres. That gives m³. Add 10% extra. The calculator above does it for you.
How many bags of concrete make 1 m³?
About 100 bags of 20 kg. Blue Circle says one bag makes about 0.01 m³.
How much is a cubic metre of concrete?
About £128 from a ready-mix supplier, with VAT. That is the middle of 7 prices from 4 suppliers. In 20 kg bags, 1 m³ costs about £1,100.
Is it cheaper to use bags or ready-mix?
Per m³, ready-mix costs far less. But one supplier has a minimum charge of £750 with VAT. A cost guide puts delivery at £100–£200. So for a small job, bags can still win.
How many wheelbarrows of concrete are in a cubic metre?
About 35, says Heidelberg Materials. A full lorry of 7–8 m³ is about 245–280 barrow loads.
How much concrete do I need for a fence post?
Blue Circle's data sheet gives bags per hole. A 20 × 60 cm hole with a 5 cm round post takes 1 bag of Postcrete. A 25 × 75 cm hole with a 10 cm square post takes 1⅔.
How much does a cubic metre of concrete weigh?
About 2.4 tonnes, says Cemex.
What concrete mix do I need for a shed base?
Heidelberg Materials suggests ST4 for shed and garage bases. Cemex says ST4 is about 1:1.5:3 cement to sand to stone.
Where our numbers come from
- Bag size: Blue Circle Multi-Purpose Concrete data sheet (20 kg makes about 0.01 m³)
- Bag price: the middle of 4 prices from 3 UK stores for 20 kg concrete (Oct 2026)
- Ready-mix price: the middle of 7 prices from 4 UK suppliers (Oct 2026, with VAT)
- Lorry size and mixes by job: Heidelberg Materials merchant guide
- Part loads: Heidelberg Materials ready-mix pricing guide
- Smallest order: Rock Solid Concrete price guide (1 m³ for homes)
- No part-load fee: EasyMix Concrete price page
- Minimum charge: Mister Concrete price page (£625 + VAT)
- Post holes: Blue Circle Postcrete data sheet (bags per hole)
- Post mix price: 2 UK stores (Oct 2026)
- House floors: Approved Document C (at least 100 mm)
- Floor slabs: NHBC Standards 2024, chapter 5.1 (at least 100 mm)
- Concrete drives: Pavingexpert (a trade guide; 100 mm for cars, 150–200 mm for heavier use)
- Patio build-up: Cemex guide to garden walls and paving
- Cement in each mix: Cemex ST mixes (kg of cement per m³)
- Cement and stone per m³: BS 8500-2 Table 10 (as copied by Pavingexpert)
- Mix ratios and 25 kg bags: Heidelberg General Purpose cement data sheet
- Drive mix: Concrete Society (PAV1 for house drives)
- Concrete weight: Cemex concrete FAQs (about 2.4 tonnes per m³)
- Ready-mix delivery and minimum order: Checkatrade cost guide (a third party)
- Garage floors: NHBC Standards 2026, chapter 10.1 (at least 100 mm)
- Extra for spills: Sakrete slab calculator (10%)
Made by BuildersCount. Last checked 3 Oct 2026.
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