How much concrete do I need for a slab?

Multiply the slab’s length by its width by its thickness (all in the same unit) to get the volume, then add a 5-10% waste allowance. For example, a 4 m x 3 m slab at 100 mm thick needs 4 x 3 x 0.1 = 1.2 m³ before waste, or about 1.29 m³ once a 7.5% allowance is added. This calculator does that conversion and waste calculation automatically from your inputs.

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

Divide your total required volume (including waste) by the yield of one bag. A typical 25 kg bag yields around 0.017 m³ mixed, and a 60 lb (27.2 kg) bag yields roughly 0.0128 m³ (0.45 cu ft). For a 1.29 m³ order using 25 kg bags, that is roughly 76 bags — always round up, since partial bags are not sold.

What is the standard thickness for a concrete slab?

Most residential slabs — shed bases, patios, garden paths — are poured 100-150 mm (4-6 in) thick. Driveways and garage floors that need to carry vehicle loads are usually thicker, 125-150 mm (5-6 in) or more, and often reinforced with mesh. Always check local building guidance for load-bearing or structural slabs rather than relying on a general rule of thumb.

How much does a concrete slab cost?

Cost depends on volume and your local ready-mix or bagged-concrete price. This calculator multiplies your calculated volume by a per-m³ material cost you can adjust to your local supplier’s quote, giving a total material estimate — labour, formwork, reinforcement and site preparation are additional costs on top of the raw material.

Should I use bagged concrete mix or ready-mix for my slab?

For small slabs — roughly under 1 m³, such as a shed base or small patio — bagged dry mix is usually more practical since you mix only what you need. For anything larger, a ready-mix delivery is typically both cheaper per unit volume and far less labour, since mixing dozens of bags by hand for a driveway or garage floor is impractical.

What concrete mix should I use for a shed base in the UK?

A GEN1 designated mix (a general-purpose mix roughly equivalent to a 1:2:4 ratio) is typically sufficient for a shed base or garden slab under BS 8500; for driveways or anything carrying vehicle weight, a stronger mix such as ST2 or PAV1 (paving-grade) is more appropriate. Check with your supplier or a builder for anything load-bearing.

Concrete Slab Calculator — How Much Concrete Do I Need? | UK

Concrete Slab Calculator: How Much Concrete Do I Need?

Pouring a slab — a shed base, a patio, a garage floor, a driveway — comes down to one question that is easy to get wrong: how much concrete do you actually need to order? Get the volume wrong and you either run short mid-pour (a genuine problem once concrete starts setting) or you pay for a wasted extra truck. This concrete slab calculator works out the exact volume from your slab's length, width and thickness, then converts that into either the number of bags to buy or a cement:sand:aggregate mix breakdown, plus a real material cost estimate.

It is built for anyone pouring a rectangular concrete slab: DIYers laying a shed or garden-room base, homeowners planning a patio or driveway, and small contractors who need a fast, defensible number for a quote. Unlike a plain volume-only tool, this one also shows the waste allowance and the bag-yield or mix-ratio math openly, so you can see exactly how the final number was built rather than trusting a hidden constant.

Enter your slab dimensions in whichever unit suits you (metres, centimetres, feet or inches), tell it how many identical slabs you are pouring, and pick whether you want the answer in bags of dry mix or as a custom cement:sand:aggregate ratio for site-mixed concrete. The calculator handles the unit conversion, the waste allowance, and the split for you.

How to Use the Concrete Slab Calculator

Enter the slab length and choose its unit (m, cm, ft or in).
Enter the slab width and choose its unit.
Enter the slab thickness (also called depth) and choose its unit — most slabs are 100–150 mm (4–6 in) thick.
Set the number of identical slabs you are pouring (leave at 1 for a single slab).
Adjust the waste allowance percentage if needed — 5–10% is standard to cover spillage, uneven subgrade and over-excavation.
Choose your calculation mode: "Bags" for pre-mixed dry bags, or "Custom mix" for a site-batched cement:sand:aggregate ratio.
If using bags, check or edit the bag yield (the volume one bag produces once mixed) to match your bag size.
If using a custom mix, select a mix ratio preset (1:2:4, 1:1.5:3 or 1:3:6) or enter your own.
Read off the total volume needed, the bag count or mix-component volumes, and the estimated material cost.

The Math Behind the Calculation

A slab is a rectangular prism, so its volume is simply length x width x thickness. Because thickness is usually given in a much smaller unit than length and width (millimetres or centimetres against metres), the most common real-world error is forgetting to convert thickness into the same unit before multiplying — this calculator does that conversion for you automatically.

If you are pouring more than one identical slab, that per-slab volume is multiplied by the quantity to get the raw total. A waste allowance is then applied on top — typically 5-10% — to account for spillage, subgrade irregularities and a minor over-ordering safety margin, giving the final volume you should actually order.

From there, the calculator splits the answer two possible ways. In "bags" mode, the total volume (already inflated by waste) is divided by the bag yield — the volume of mixed concrete one bag produces — and rounded up to a whole number of bags, since you cannot buy a fraction of a bag. In "custom mix" mode, the total volume is instead split into cement, sand and aggregate volumes in proportion to your chosen ratio (e.g. for a 1:2:4 mix, the total parts are 1+2+4=7, so cement gets 1/7 of the volume, sand 2/7, and aggregate 4/7).

Worked example: a 4 m x 3 m slab at 100 mm (0.1 m) thick. Volume = 4 x 3 x 0.1 = 1.2 m³ per slab. For a single slab (quantity = 1), the raw total is 1.2 m³. Applying a 7.5% waste allowance: 1.2 x 1.075 = 1.29 m³ to order. If using 25 kg bags with a yield of about 0.017 m³ per bag, that is 1.29 / 0.017 = 75.9, rounded up to 76 bags. If instead mixing on site at a 1:2:4 ratio, the 1.29 m³ splits into roughly 0.184 m³ cement, 0.369 m³ sand and 0.737 m³ aggregate.

Common Concrete Mix Ratios (Cement : Sand : Aggregate, by Volume)

1:2:4 — general-purpose slabs and reinforced concrete (RCC), roughly 15 MPa compressive strength
1:1.5:3 — driveways, garage floors and other higher-load surfaces, roughly 20 MPa
1:3:6 — footings, mass fill and non-structural backing concrete, roughly 10 MPa
Source: standard cement:sand:aggregate volumetric mix-ratio tables used across residential concrete practice; always check your local building code or an engineer’s spec for load-bearing or structural slabs.

UK Concrete Standards

Concrete in the UK is specified under BS 8500 (which implements BS EN 206 nationally), with designated mixes such as GEN1 or ST2 commonly used for domestic slabs, foundations and shed bases.
Aggregate is typically sold and priced by the tonne rather than by volume, with "ballast" (a sand-gravel mix) the standard term for concreting aggregate — source: DfT Specification for Highway Works; UK builders' merchant terminology.

Pro Tips and Common Mistakes

At a typical material cost of around £125 per m³ of ready-mix concrete, a 4 m x 3 m slab at 100 mm thick (1.29 m³ including 7.5% waste) costs roughly £161 in raw material — formwork, reinforcement mesh and labour are additional if you are not doing the pour yourself.

Always round up your bag count, never down — you cannot buy a partial bag, and running short mid-pour is far worse than having a little extra.
Use the higher end of the 5-10% waste range (closer to 10%) if your subgrade is uneven, if you are hand-mixing, or if this is your first pour — spillage and over-excavation add up fast.
For anything structural (a slab that will carry a building, a heavy vehicle, or is part of a foundation), use ready-mix concrete from a supplier rather than bagged mix — it is far more consistent in strength and far more practical in volume for anything over about 1 m³.
Concrete reaches most of its working strength in about 7 days but is not considered fully cured until around 28 days — avoid heavy loads (parking a car, building on top) before that point.
A common mistake is entering thickness in the wrong unit (e.g. treating 100 as metres instead of millimetres) — double check your thickness unit specifically, since it is the dimension most often mis-entered.
If you are unsure whether to use bags or ready-mix, the rough crossover point is about 1 m³: below that, bagged mix is usually more practical; above it, a ready-mix delivery is usually cheaper and faster.