Concrete Calculator: How Much Concrete Do I Need?
Work out how much concrete you need for a slab, a strip footing, fence posts or steps, in cubic metres and in what to buy: bags of cement and ballast (or sand and gravel), ready-mix bags, Postcrete or a ready-mixed lorry load.
No sign-up required. Free to use, with every formula shown on this page. Last reviewed October 2026.
Work out your concrete
Concrete Calculator
Work out the concrete for a slab, a footing, fence posts or steps, and what to buy: bags of cement and ballast, ready-mix bags, Postcrete or a lorry load.
1 What you're pouring
Length
Width
Thickness
Paths and patios usually 75–100 mm, shed bases 100 mm, car standing 150 mm, all on firm, compacted ground or hardcore.
Length
Width
Concrete depth
For garden walls and similar. Footings for extensions and other structural work must follow Building Regulations and building control.
A rule of thumb is a hole about three times the post's width and a quarter of its height deep, but check the post maker's advice.
Solid steps built on the ground, each one on top of the last.
2 How you'll make it
3 Extras
Uneven ground and spills use more than you'd think. A mixer makes about 70% of its drum size in concrete per load.
Prices (optional)Concrete you need
0 m³
Before the extra
0 m³
Weight, mixed
0 t
Mixer loads
–
What you'll need
How it's worked out: volume from your measurements, plus the extra for waste. Mixing your own: the dry ingredients take up about 1,54 times the volume of mixed concrete, split by the mix ratio; cement 1.440 kg per m³ in 25 kg bags, ballast 1,4 t per m³ (about 1,8 t per m³ of concrete, as UK merchants quote), sharp sand 1,6 and gravel 1,5 t per m³, 850 kg bulk bags. Ready-mix: a 20 kg bag makes about 0,01 m³ (Tarmac). Postcrete: bags per post from Blue Circle's own table. Mixed concrete weighs about 2,4 tonnes per m³.
Estimates only. Real quantities vary with the ground, the aggregate and how wet the mix is. For structural work, follow Building Regulations and your engineer's or building control's specification.
How it works
Volume, mix and what to buy
The calculator works out the volume of concrete, adds a little extra, then turns it into a shopping list for the way you’ll make it. The sources and assumptions behind it are on our How we calculate page.
Work out the volume
A slab or footing is length × width × thickness. For fence posts it takes the hole’s volume less the post. Solid steps on the ground stack up, so three steps hold six steps’ worth of concrete. It adds 10% for uneven ground and spills.
Split it into the mix
Cement, sand and stone shrink by about a third when you add water, so a cubic metre of concrete needs about 1.54 m³ of dry ingredients, split by the mix ratio. At 1:5 that’s about 15 bags of cement and 1.8 tonnes of ballast per cubic metre.
Turn it into bags
Cement comes in 25 kg bags and ballast, sand and gravel in 850 kg bulk bags. A 20 kg bag of ready-mix makes about 0.01 m³. Postcrete uses the maker’s table of bags per post. A ready-mixed lorry is ordered in cubic metres, rounded up to the next 0.25 m³.
Examples
How much concrete for a shed base, patio or fence?
The same calculator, a few common jobs. Small jobs add up quickly: a modest patio already takes over 130 bags of ready-mix.
Illustrative only, from the calculator above. All include 10% for waste. Cement in 25 kg bags, all-in ballast in 850 kg bulk bags, ready-mix in 20 kg bags (about 0.01 m³ each), mixer loads for a 130 litre mixer. Steps have a 300 mm going. The six fence posts need 12 bags of Postcrete instead (2 bags a post, from Blue Circle’s table).
Tips
Before you mix and pour
Good concrete is mostly preparation: a firm base, the right mix and slow drying.
Prepare the base
Dig out soft soil and lay a compacted sub-base of hardcore, usually about 100 mm. Build timber formwork to the finished height and check it’s level, with a slight fall on patios and paths so rain runs off. Measure the depth inside the formwork for the calculator.
Choose how to make it
For a few posts, Postcrete or ready-mix bags are easiest. For a shed base or path, mixing cement and ballast in a mixer is usually cheapest. Over about 1 m³, ready-mixed concrete delivered saves a lot of work, but check the supplier’s minimum load and access for the lorry.
Let it cure slowly
Concrete keeps getting stronger for weeks. Cover it to stop it drying too fast in sun or wind, and protect it from frost. You can usually walk on it after a day or two, but wait about a week before building on it. For structural work, follow Building Regulations and building control.
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Common questions
Frequently asked questions about how much concrete you need
Common questions about bags, ballast, mixes and thickness.
How many bags of cement do I need for a cubic metre of concrete?
About 15 bags of 25 kg for a general 1:5 mix of cement and ballast, about 13 for a 1:6 mix (footings) and about 18 for a stronger 1:4 mix. Add 10% for waste. The calculator works out the bags for your own job.
How much ballast do I need for 1 m³ of concrete?
About 1.8 tonnes of all-in ballast for each cubic metre of concrete, which is just over two 850 kg bulk bags. That’s for a 1:5 mix, before adding anything for waste.
How many bags of Postcrete per post?
It depends on the post and the hole. Blue Circle’s table gives 2 bags for a 100 mm square post in a 300 × 600 mm hole, ¾ bag for a 75 mm post in a 200 × 600 mm hole and ½ bag for a 50 mm round post in a 150 × 600 mm hole. Always check the bag.
What mix ratio should I use for a shed base?
A 1:5 mix of cement and all-in ballast is the usual choice for shed bases, paths and patios. If you buy sand and gravel separately, use 1 part cement, 2 parts sharp sand and 3 parts 20 mm gravel. Footings often use a slightly weaker 1:6 mix.
How thick should a concrete slab be?
About 75–100 mm for paths and patios, 100 mm for a shed base and about 150 mm where a car will park, each on a compacted sub-base of hardcore. Larger buildings and anything structural need a design from a builder or engineer.
Is it cheaper to mix my own concrete or buy ready-mix?
Mixing your own cement and ballast is usually cheapest per cubic metre. Ready-mix bags cost more but are handy for small jobs. For bigger pours, a ready-mixed lorry can be good value and saves hours of mixing, though some suppliers charge extra for small loads. Enter your local prices in the calculator to compare.
Are the results exact?
No. They are estimates: the real amount depends on how even the ground is, the aggregate and how wet the mix is. The calculator adds 10% for waste, but buying a little extra is cheaper than running out halfway through a pour.