How Many Bags of Concrete Do I Need? UK Guide

A standard 25kg bag of general purpose concrete mix yields approximately 0.0125m³ of mixed concrete, once water's been added and it's been compacted into place. That's the figure to work from, not the bag weight.

Worked back the other way, 1m³ of concrete needs roughly 80 bags of 25kg mix. That number surprises people every time, because it sounds like a lot for a cubic metre, until you remember that a cubic metre is a thousand litres and a mixed 25kg bag is only about twelve and a half.

For post-mix concrete specifically, the ratio is close enough to use the same 80 bags per m³ figure, though postcrete manufacturers quote quantities in a different way, covered below.

Takeaway: always convert through volume, never estimate bags from weight directly.

Trade guidance for bagged concrete typically allows 5 to 10% wastage on top of the calculated volume, and for DIY or first-time pours, the higher end is the sensible one. Spillage during mixing, over-excavation, and formwork that's slightly bigger than planned all eat into the margin. A tight calculation that assumes a perfect pour rarely survives contact with an actual mixing tub.

Our concrete bag calculator applies a wastage allowance automatically, but if you're doing the sums by hand: calculate the volume, convert to bags, then add 10%, and round up. Always round up. A part-used bag is a part-used bag either way, so there's no saving in rounding down.

Takeaway: 10% wastage on a DIY job isn't pessimism, it's what actually happens.

A straightforward slab, 100mm thick, for a garden shed.

Volume: 3m × 2m × 0.1m = 0.6m³

Bags needed on volume alone: 0.6 / 0.0125 = 48 bags

With 10% wastage: 48 × 1.1 = 52.8, round up to 53 bags

At a typical 2026 trade price of around £4.50 to £5 per 25kg bag from a UK builders' merchant, that's roughly £240 to £265 in bagged concrete. Worth checking that against a ready-mix quote once the volume passes about half a cubic metre, because the crossover point is closer than most people think, and it's covered below.

Takeaway: for a job this size, bags are still the practical choice, but only just.

A run of six fence posts, each set in a hole 300mm × 300mm × 600mm deep, using post-mix concrete rather than general purpose bags.

Volume per hole: 0.3m × 0.3m × 0.6m = 0.054m³

Total for six posts: 0.054 × 6 = 0.324m³

Postmix products like Blue Circle Postcrete are sold by bag count for a stated post size rather than by weight-to-volume conversion, and the packaging typically states 2 to 3 bags per standard fence post hole depending on post size and hole depth. For a hole this size, budget on 2 bags per post, so 12 bags for the run, then check that figure against the pack guidance for the specific post dimensions, because postcrete bag counts are given per hole, not per cubic metre, and running it through the 80 bags per m³ conversion will give you a different and less useful number.

Takeaway: postcrete is priced and packaged per post hole, not per cubic metre, so don't run it through the general 80-bags-per-m³ conversion.

This is the part most bag calculators skip, and it's worth saying plainly: bags stop being the cheaper option well before most people switch.

Below roughly 0.5m³, bagged concrete usually wins on total cost once you factor in that ready-mix suppliers charge a minimum load fee, and a small pour doesn't fill enough of a truck to make sense. Above about 1m³, ready-mix from a supplier like Tarmac or a local independent plant typically works out cheaper per cubic metre, and it saves the physical labour of mixing 60-plus bags by hand or in a small mixer.

Between 0.5m³ and 1m³ is the genuinely competitive range, and the right call depends on access for a truck, whether you can use the concrete fast enough once it arrives, and what a local minimum load actually costs. It's worth getting an actual ready-mix quote at 0.6m³ rather than assuming bags are automatically cheaper just because the volume feels small.

Takeaway: don't assume bags are cheaper just because the job feels small. Get a ready-mix price once you're above about half a cubic metre.

Pre-bagged general purpose concrete is supplied ready-blended, so there's no ratio to get right, but it's worth knowing what's actually in the bag. A typical general purpose mix corresponds to roughly a 1:2:3 cement to sand to aggregate ratio by volume, which lands close to a C20 equivalent strength once cured, suitable for sheds, paths and non-structural slabs.

For anything load-bearing, a structural footing or a base intended to take a vehicle, check the bag's stated compressive strength against BS 8500, and use a foundation-grade or higher-strength mix rather than general purpose. Approved Document A covers foundation depths and specification for domestic work, and it's worth a look before pouring anything that's actually holding a structure up.

Takeaway: general purpose bags are fine for sheds and paths. Anything structural needs a foundation-grade mix checked against BS 8500.

These figures use standard 25kg bags at 0.0125m³ yield each, with 10% wastage included. Run the exact dimensions through the concrete bag calculator rather than relying on this table for anything the job actually depends on.

These figures are based on standard 25kg general purpose bagged concrete and typical postcrete yield guidance. Yield per bag varies slightly between manufacturers and mix types, so check the bag for its stated yield before ordering a large quantity. For anything structural, footings, retaining structures, or a base carrying significant load, get the mix specification confirmed against BS 8500 and, where relevant, Approved Document A, rather than relying on general purpose figures.

Takeaway: the numbers here get you a shopping list. For structural work, confirm the mix grade before you order.