How do I calculate how many bags of concrete I need for a slab?

Multiply the length, width and thickness of the slab in metres to get the volume in cubic metres. Then divide by the yield of one bag (check the packaging — a 25 kg bag typically yields around 0.012 m³). Round up to a whole number of bags and add at least 5% for waste.

How many 25 kg bags of concrete do I need per cubic metre?

A 25 kg bag of general-purpose concrete mix typically yields around 0.012 m³, so you need roughly 83 bags per cubic metre. This varies slightly by product, so always check the stated yield on the packaging from your supplier.

Should I use bagged concrete or ready-mix for a shed base?

Bagged concrete is practical for small pours up to around 0.5–1 m³. For a typical 3 m × 2.4 m shed base at 100 mm thick (0.72 m³), bags are manageable but time-consuming. For larger bases, ready-mix from a local supplier is usually more cost-effective and consistent.

What concrete mix should I use for a garden slab UK?

For most domestic garden slabs, a C20 general-purpose mix is suitable. For driveways or areas with vehicle loading, a C25 or stronger mix is recommended. Both specifications are covered under BS 8500. Most bagged general-purpose concrete mixes sold at UK builders merchants are suitable for C20 applications.

Why does concrete bag count vary by bag size?

Different bag sizes contain different amounts of dry mix, so the yield per bag changes. A 20 kg bag yields less concrete than a 25 kg bag. Always use the yield figure stated on the specific product you are buying rather than a generic estimate.

Should I add extra concrete bags for waste?

Yes. A 5% waste allowance is a reasonable minimum for most site conditions. On awkward pours, uneven ground, or when mixing manually, 10% is safer. It is better to have one or two spare bags than to run short mid-pour.

Concrete Bag Calculator UK

Use this concrete bag calculator to work out how many bags of concrete you need before you buy materials. Enter the slab length, width, and thickness in metric measurements, select your bag size, and get a bag count based on the volume of the pour and the yield of each bag. It is useful for shed bases, garden slabs, fence post pads, and any small concrete job where bagged concrete is more practical than ordering ready-mix. A 25 kg bag of general-purpose concrete mix typically covers a small portion of a cubic metre, so this tool takes care of the maths and adds a waste allowance so you order the right amount first time.

Calculate Concrete Bags

Units

Imperial (feet)

Metric (meters)

Length (m)

Width (m)

Thickness

Bag Size

How this calculator works

This calculator works out the volume of your slab by multiplying length, width, and thickness after converting the measurements into consistent units. It then applies any waste allowance used by the tool and divides the required volume by the yield of the selected bag size. The result is rounded up so you get a whole number of bags to buy rather than a theoretical partial bag.

Formulas used

Slab volume

volume = length × width × thickness

Adjusted volume

adjusted volume = volume × (1 + waste % / 100)

Bag count

bags needed = ⌈adjusted volume / yield per bag⌉

Estimated cost (if price entered)

estimated cost = bags needed × price per bag

Bag yield varies by product and bag weight, so final ordering should always be checked against the manufacturer's stated coverage or yield.

Worked example

Take a shed base measuring 3.0 m by 2.4 m, laid at 100 mm thick. The volume works out at 3.0 × 2.4 × 0.1 = 0.72 m³. Using 25 kg bags with a yield of 0.012 m³ per bag, you need 0.72 ÷ 0.012 = 60 bags before any waste allowance. Adding 5% waste brings that to 63 bags — so in practice you would order 65 bags to allow for spillage and top-up. At a typical 2026 price of around £5–£6 per 25 kg bag from Travis Perkins or Jewson, that puts materials at roughly £325–£390 before delivery.

When to use bagged concrete

Bagged concrete is usually most practical for smaller slabs, patch repairs, posts, and areas where mixing on site is manageable. For larger pours, ready-mix may be more efficient and more consistent. This calculator helps you sense-check whether the job still feels practical in bags before you buy materials.

Concrete mix and bag sizes for UK jobs

In the UK, bagged concrete is sold in 20 kg and 25 kg bags at builders merchants including Travis Perkins, Jewson, and Buildbase. Ready-to-use general-purpose mixes are suitable for most domestic applications — shed bases, small slabs, and post holes. For structural applications, the concrete specification should follow BS 8500 and the relevant design mix class. A C20 mix (general domestic use) and C25 mix (light structural use) are the most common for garden and groundwork jobs. Always check the bag for the stated yield, as this can vary slightly between manufacturers and mix types.

Frequently asked questions

Related tools

Dry Pour Concrete Calculator — volume, bags and cost for dry-pour slabs
Concrete Volume Calculator — calculate concrete volume for slabs, footings, and columns
Concrete Truck Load Calculator — estimate ready-mix delivery loads
Weight of Concrete Slab Calculator — estimate the total weight of a concrete slab

Guides worth reading

How Much Concrete Do I Need? UK Guide — grades, wastage, worked example and 2026 ready-mix prices
how many bags of concrete do I need? — yield per bag, postcrete comparison and crossover point with ready-mix

Disclaimer: This calculator is for planning and estimating only. Actual bag counts can vary with product yield, mixing method, waste, and site conditions. Always check the manufacturer's information before ordering.