Concrete calculator slabs, footings and posts

How the calculator gets to the answer

Every shape comes down to one number: cubic metres. A slab or a strip footing is length × width × depth. A round post hole is π × radius² × depth, times the number of holes, less the space the post takes up if you enter a post size. The calculator adds your wastage allowance, then converts that volume into whatever you are buying.

Ready-mix is rounded up to the next 0.5m³, with a 1m³ minimum load. The minimum is the suppliers'; the half-metre step is our assumption, and you can change both. Pre-mixed bags use a yield of 0.009m³ per 20kg bag, the figure on Hanson's Multipurpose Concrete data sheet. A site mix gives 25kg bags of cement and tonnes of all-in ballast for the ratio you choose.

Here is the worked example the calculator opens with: a shed base 3m by 2.4m, 100mm thick, with 5% wastage and a 1:4 site mix.

• Volume: 3 × 2.4 × 0.1 = 0.72m³. Plus 5% = 0.756m³.
• Ready-mix: 0.756m³ rounds up to 1m³, which is also the usual minimum load.
• 20kg bags: 0.756 ÷ 0.009 = 84 bags, about 1.7 tonnes to carry in from the van.
• Site mix, 1:4: ballast 0.756 × 1.65 = 1.25 tonnes. Cement 0.756 ÷ 4 × 1,300kg = 245.7kg, so 10 bags of 25kg.
• Weight once mixed: about 1.8 tonnes at 2.4 tonnes per m³.

Ordering ready-mix: minimums and timing

Order ready-mix in cubic metres, rounded up, and expect a minimum load of about 1m³. Tarmac's minimum loads depend on the plant in the North, Scotland, Central and Wales, and are 1m³ for standard mixes in the South and Anglia, rising to 2m³ for waterproof or lightweight mixes. Breedon Minimix sets 1m³ as its minimum and 4m³ as the most one of its small trucks carries.

No supplier we checked publishes a fixed step between those sizes, so the calculator rounds to the nearest 0.5m³ above your figure, the size Tarmac's delivery page starts its small loads at. Ask your plant whether it will batch a quarter of a cubic metre more or less, and change the setting to match.

The shed base above needs 0.756m³ but the lorry brings 1m³. That gap is real money. Either use the spare quarter of a cubic metre on a path or a pad, or price the job in bags or a site mix instead.

• Have barrows and hands ready. Breedon reckons 25 full wheelbarrows make 1m³, and gives you 30 minutes to unload a minimix.
• Know the weight. Tarmac puts standard concrete at 2.3 to 2.4 tonnes per m³. A full 4m³ truck is heavy, and Breedon warns that any surface it crosses must take a heavy goods vehicle.
• Say what it is for. Plants sell concrete by use and strength. Breedon, for example, names GEN 1 its single-storey foundation mix but says it is not for structural loads, and rates GEN 3, its two-storey mix, for load-bearing work. For house foundations, building control or your engineer sets the mix, not a calculator.

Mixing your own: cement and ballast

For a mixer on site, 1 part cement to 4 parts all-in ballast by volume is the general-purpose mix. That is the ratio on Hanson's General Purpose Cement data sheet, which puts it alongside 1:2:3 of cement, concrete sand and 20mm aggregate. B&Q's guide gives its mixes in those three parts: 1:2:3 for shed and garage bases, 1:2.5:3.5 for light wall foundations and fence posts, and 1:1.5:2.5 for steps, paving and driveways. Read across on Hanson's basis, that is roughly 1:4, 1:5 and 1:3.5 in all-in ballast. The conversion is ours, not B&Q's. Heidelberg Materials says 1 cement to 5 ballast is usually enough for a shed base.

Turning a ratio into bags takes two assumptions, both from published figures. First, a cubic metre of ballast with cement makes about a cubic metre of concrete, the rule of thumb in Wickes' project guide, because the cement and water fill the gaps between the stones. Second, cement is taken at 1,300kg per m³, the point where Breedon's cement data sheet puts the line between aerated (1,000 to 1,300kg) and settled (1,300 to 1,450kg) cement.

For the ballast weight, Wickes says its 800kg jumbo bag covers about 9.7m² at 50mm, which is 1.65 tonnes per m³. Per cubic metre of concrete, before wastage, that gives:

Strip footings and post holes

A strip footing is a long, narrow slab, so it uses the same sum. Take a garden wall footing 4m long, 400mm wide and 200mm deep, with 5% wastage. That is 4 × 0.4 × 0.2 = 0.32m³, plus 5% = 0.336m³. In 20kg bags it is 0.336 ÷ 0.009 = 37.3, so 38 bags. As a 1:5 site mix it is 87kg of cement, so 4 bags, and 0.55 tonnes of ballast. A ready-mix lorry would still bring 1m³, so a footing this size is usually mixed on site.

Post holes are where the bag count surprises people. Six fence posts, 100mm square, in holes 250mm across and 600mm deep: each hole holds π × 0.125² × 0.6 = 0.0295m³, less 0.006m³ for the post, which leaves 0.0235m³. Six of those plus 5% is 0.148m³, or 17 bags of multi-purpose concrete, nearly three a post.

Rapid post mixes are a different product with their own tables. Tarmac says one 20kg bag of its Postmix Concrete does a standard fence post, and the Postcrete data sheet gives 2 bags for a 100mm square post in a 300mm by 450mm hole, and 3½ bags at 750mm deep. The bag count here does not apply to them, so use the maker's table, or the fence calculator, which takes Postcrete bags per post.

How deep to go is the job's call, but makers give starting points. Hanson recommends at least 75mm for paths that take foot traffic and 100mm for driveways on a well-compacted base. Heidelberg Materials puts a shed base at 75mm or more over a similar depth of sub-base. Tarmac's Postmix sheet sets a post hole at about a quarter of the post's height.

Bagged concrete: what a bag really fills

A 20kg bag of ready-to-use concrete makes about 0.009 to 0.01m³ once water goes in. Hanson's Multipurpose Concrete sheet says 0.009m³, or 0.09m² at 100mm thick. Tarmac's Multi-Purpose Concrete data sheet says around 0.01m³, which it describes as 330mm by 330mm by 100mm. The calculator starts on Hanson's lower figure so you are not a bag short. Switch it to 0.010m³ if that is what your bag says, or choose Other bag and type in the weight and yield from a 25kg bag or another brand.

That means a cubic metre is about 111 bags. Bags make sense for a few post holes, a step or a small pad. Past a few tenths of a cubic metre, the shed base example shows the problem: 84 bags at 20kg each is a lot of lifting and a lot of plastic, where a 1:4 site mix needs 10 bags of cement and 1.25 tonnes of ballast.

The calculator only offers those two 20kg yields. For a 25kg bag or any other product, divide the volume by the yield printed on the bag. Do not assume a 25kg bag fills a quarter more than a 20kg bag without checking the label.

Before you pour

Most concrete orders go wrong on the depth, not the sums. Heidelberg Materials points out that being 1cm out over 25m² adds a quarter of a cubic metre. Check the levels across the whole area, not at one corner, and enter the average depth. Wastage covers spills and a bit of uneven ground. It does not cover a dig that went 30mm too deep.

Once it is down, look after it. Hanson's cement sheet says to keep exposed concrete from drying out for at least the first seven days, and to place it between 10°C and 30°C as a general rule. Blue Circle's General Purpose Cement sheet says not to use it below 5°C or above 30°C.

Quoting a customer for the base? Put the concrete, the hire of a mixer and your labour as separate lines on the quote template, so a change in depth is easy to re-price. The stone underneath is a separate order, and the gravel calculator handles the sub-base.

• Outside its scope: structural design, foundation sizes, reinforcement, the sub-base, formwork or the damp proof membrane. New buildings and extensions come under the Building Regulations, so their foundations need approval from a building control body.
• It is an estimate. Bag yields and ballast densities vary by product and by how wet the ballast is. Check the figure on your bag or delivery note.
• It does not price anything. Ready-mix, cement and ballast prices vary by area and load size, so ask your supplier.

Common questions