Concrete calculator: how much will I need?
Free concrete calculator: cubic metres, then bags, post-fix or a lorry order. No sign-up, nothing stored.
CONCRETE TO ORDER
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Turn on JavaScript and this works out your concrete as you type. The formulas are below if you'd rather do it by hand.
Your sections, read back
Two ways to buy the same concrete
Bags of concrete
Post-fix
Lorry
Still to confirm before you buy
This sheet adds up the sizes you gave it. It has not checked whether they are right for the job.
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How the concrete calculator works
You type the sizes the way you measured them. A shed sold as 10 by 8 feet, a thickness in millimetres off the base instructions, a post hole in centimetres: each field keeps its own unit, and we read anything in feet or inches back to you in metres before adding it up. That readback is where a thickness typed in metres instead of millimetres gives itself away. Typed that way, the base would need a thousand times the concrete, which is one way to meet the neighbours.
The sums are short. A slab, base or footing is length times width times thickness. A round hole or pier is pi times half the diameter, squared, times the depth. Anything the concrete won't fill, like the post standing in each hole, comes off before we multiply the section by how many you've got. Feet and inches convert by their exact definitions in the Weights and Measures Act 1985, where a yard is 0.9144 metres, so nothing drifts on the way to cubic metres.
Next comes the bit you actually pay for. Your allowance goes on top of the net volume, and we round the total up once, to something you can actually buy. Take that 10 by 8 foot shed base at 100 mm. It holds 0.74 m³, or 743 litres. Add 5% and you're at 0.78 m³. At 0.01 m³ a bag, the figure on Tarmac's Blue Circle premix sheet, that's 79 bags. A ready-mix supplier selling in quarter cubic metres sends 1.00 m³, which is 34.5% more than the base holds, and you've paid for every bit of it. A mix-on-site lorry mixes what you use, so its 0.78 m³ stays an estimate: you pay for what's mixed on the day.
Post-fix gets counted differently, because the maker counts it differently. The bag prints bags per post by post size and hole size, and that table doesn't follow the hole's volume: Tarmac's Postcrete sheet gives the same 3/4 bag for a 75 mm post in a hole 20 cm across, whether it's 45 or 60 cm deep. So a post-fix section is posts times the figure on your bag. We never turn it into concrete, or concrete into it.
A total that lands on a whole bag stays there. Three metres by one metre at 100 mm is 0.3 m³, and at 0.01 m³ a bag that's 30 bags, not 31. And the weight on the front never gives a count: 20 kg tells you what you're lifting, not what it makes.
Who this is for
Homeowners with a shed base, a hot tub pad, a path or a row of fence posts marked out, and one purchase ahead: bags from the merchant, a lorry, or post-fix for the posts.
Anyone holding a shed or base kit sized in feet while the supplier sells by the cubic metre.
People choosing between ready-mix, mini-mix and mix-on-site who'd like to see what each one's rounding costs before ringing round.
Homeowner questions about concrete, bags and post-fix
How many 20kg bags of concrete make a cubic metre?
Divide by what one bag makes. Tarmac's Blue Circle premix sheet says a 20 kg bag makes around 0.01 m³, so a cubic metre is about 100 bags before anything for spills. The same sheet's worked block is a touch bigger, about 10.9 litres, which would make it nearer 92. Which is why we count from the figure on your own bag. If your bag says 25 kg, check it says concrete: Blue Circle's General Purpose Cement, for one, comes in 25 kg bags and prints no yield at all.
How much concrete do I need for a 10x8 shed base?
At 100 mm thick, a 10 by 8 foot base holds 0.74 m³. At 0.01 m³ a bag that's 75 bags, or 79 once you add 5% for spills. An 8 by 6 base at the same thickness holds 0.45 m³ (45 bags), and a 6 by 4 holds 0.22 m³ (23 bags). The thickness is your shed maker's call, or your plans', and not ours. The hardcore and sand blinding underneath aren't part of the concrete either.
How many bags of Postcrete do I need per post?
Read it off the table on the bag. Tarmac's sheet for its Blue Circle Postcrete gives 3/4 bag for a 75 mm square post in a hole 20 cm wide and 60 cm deep, and 1 1/2 bags for the same post in a hole 25 cm wide. Multiply by your posts and round up once. The same sheet says it's for domestic and decking posts, rotary washing lines and posts for gates under a metre wide, and not for anything that needs a specified strength.
How much concrete do I need for fence posts?
The hole, minus the post. A hole 200 mm across and 600 mm deep holds 18.8 litres, and a 75 mm square post takes 3.4 litres of it, so each hole needs about 15.5 litres. Eight holes come to 124 litres: 13 bags at 0.01 m³ a bag. Choose Post holes and the calculator takes the post out of every hole for you. Using post-fix instead? Switch the holes to post-fix and type the figure off the bag.
How much is a cube of concrete?
There's no single price, and we don't put one on it. Heidelberg Materials says plainly there's no flat rate for ready-mix: the price moves with the strength, the delivery distance and the size of the load, and a part load can carry its own charge. If a quote says per cube, ask whether that means a cubic metre, 1,000 litres: we searched UK makers' and suppliers' pages on 8 October 2026 and none defined the word. Put your own quote into Add your own prices and the total builds line by line.
Should I use C20 or C25 concrete?
That's for your plans, building control or an engineer, not for a calculator. What we can do is read the names. Written in full they're C20/25 and C25/30. The first number is the minimum strength of a test cylinder and the second of a test cube, both in newtons per square millimetre, so C25/30 is the stronger of the two. The Concrete Centre's guide to BS 8500 says to quote the class in full to avoid confusion, so if someone just says "C20", ask for the class in full.
What is GEN1 or ST2 concrete?
Two different ways of naming concrete under BS 8500. GEN is one of four designated families, alongside FND for foundations, PAV for paving and RC for reinforced concrete, and only certified ready-mix producers can supply them: a QSRMC or BSI Kitemark logo on the delivery ticket confirms it. ST is standardised prescribed concrete, meant for small sites mixing by hand or in a mixer under 150 litres. The number picks a grade set in the standard. We don't print a table matching them to C classes, because the guide we read gives none.
How much extra concrete should I order?
We don't pick that number for you. Concrete goes missing in spills, in the mixer and the barrow, and in a base that isn't quite level, and the bag sheet itself says "around". What we can show you is what your choice costs: the allowance you type, then what rounding adds. On a 10 by 8 foot shed base, a 5% allowance turns into 34.5% more than the base holds once a supplier rounds to the next quarter cubic metre, and 7.6% more if it sells in tenths.
Do I need planning permission for a concrete base?
This calculator doesn't check, and the rules differ between England, Wales, Scotland and Northern Ireland. In England, impermeable paving over five square metres in a front garden needs planning permission unless rainwater drains to a permeable area, such as a lawn or border. Foundations can also bring in building regulations, or building standards in Scotland. Some small detached buildings are exempt. Your council's planning and building control teams can tell you before you dig.
Concrete names on an order form
We built this decoder from The Concrete Centre's guide to BS 8500 (February 2024), read on 8 October 2026, not from the paid standard itself. It tells you what a name on the form means. It never tells you which one your job needs.
| On the order | What it means |
|---|---|
| C20/25, C25/30 | A compressive strength class. C is normal-weight concrete; the first number is the minimum strength of a test cylinder, the second of a test cube, in MPa (N/mm²). Quote it in full to avoid confusion, so "C20" on its own is worth a question. |
| FND, GEN, PAV, RC with a number (GEN1, FND2, RC28/35) | Designated concrete: foundations, housing and non-structural building, paving, reinforced. Supplied only by certified ready-mix producers; a QSRMC or BSI Kitemark logo on the delivery ticket confirms it. The number picks a grade set in BS 8500. |
| ST with a number (ST2) | Standardised prescribed concrete, for small sites mixing by hand or in a mixer under 150 litres, with no requirement to demonstrate strength. The guide says to avoid it where ready-mix can be used. |
| Designed concrete | The designer sets the strength class and the other limits; the producer works out the mix. |
| Prescribed or proprietary concrete | Prescribed: the specifier sets the mix itself, rarely used. Proprietary: a producer's own product, sold on its properties. |
| S1 to S4 | Consistence (slump) classes, picked by how the concrete will be placed. S3 is the class normally used. |
How much concrete do I need?
Measure, convert, multiply, then buy in whole units. A slab or base is length times width times thickness, all in metres. A hole or pier is pi times the radius squared, times the depth. A cubic metre is 1,000 litres, and a base 100 mm thick takes a tenth of a cubic metre for every square metre it covers.
That's the net volume, and you'll buy more than that. Bags come whole, counted from the yield on the bag. Ready-mix and mini-mix come in a supplier's steps above a minimum, and some suppliers add a part-load charge to a small load. Mix-on-site charges for what's mixed, and post-fix goes by the post. Add your allowance, round up once at the very end, and you have a figure to say on the phone. A concrete slab calculator and a concrete bag calculator are the same sums underneath, so one set of measurements answers both.
Sources and last checked
Sources checked 8 October 2026. Data version concrete-uk-2026-10-07.
- Unit definitions: Weights and Measures Act 1985, Schedule 1 (legislation.gov.uk, up to date to 6 October 2026).
- Bag figures quoted above: Tarmac, Blue Circle Multi-Purpose Concrete, General Purpose Cement and Postcrete product data sheets (December 2023).
- Lorry terms: Heidelberg Materials UK, ready-mixed concrete pricing guide and Supamix small loads pages; Eastern Concrete, mix-on-site concrete page.
- Concrete names: The Concrete Centre, "BS 8500 for building and civil structures" (February 2024, for BS 8500:2023).
- Front gardens: the Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class F; GOV.UK, "Permeable surfacing of front gardens: guidance".
- Building rules: GOV.UK, GOV.WALES, nidirect and gov.scot guidance on building regulations and building standards.
How we made it: the calculator is arithmetic over the numbers you type. It holds no bag yields, no thicknesses and no prices of its own, and the makers and suppliers above are named for what they publish, not recommended.
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