How much concrete do I need for an 8 × 6 ft shed base?

At 4 inches thick, an 8 × 6 ft slab is 16 cubic feet, or about 0.59 cubic yards, before waste. At 100 mm thick, a 2.4 × 1.8 m slab is 0.432 m³. Add a waste allowance on top, and enter your own thickness because the right one depends on your ground and shed.

Should the base be bigger than the shed?

Many people make the base slightly larger than the shed's floor so the walls sit fully on concrete, but the shed manufacturer's base instructions take priority. Enter the base size you intend to pour, not the shed's roof size, which usually includes overhangs.

Is a 100 mm or 4 inch slab thick enough?

This calculator cannot tell you. The thickness buttons are common examples only. Whether a slab is suitable depends on the ground, the shed's weight and use, drainage, frost and any local rules. Check with the shed supplier, a builder or an engineer.

Bags or ready-mix for a shed base?

Small bases are often mixed from bags, while larger ones can be quicker and more consistent with ready-mix. Compare the bag count here with your supplier's minimum order and delivery charge. Bag counts use the yield printed on the packaging, or an estimated standard yield where one is available.

Is the sub-base included in the concrete volume?

No. The optional MOT Type 1 sub-base is calculated separately and shown in its own section. It is aggregate, not concrete, and is never added to the concrete you order.

Shed Base Calculator

Work out how much concrete a garden shed, summerhouse or small outbuilding base needs, as ready-mix or bags of pre-mixed concrete, with an optional MOT Type 1 sub-base estimate kept separate. It estimates quantities only; it does not decide whether a base design is structurally suitable.

Base size

Example shed sizes (fills length and width in feet)

Examples only. Enter your actual base size below, which may differ from the shed's nominal size.

Length

Width

Concrete slab thickness

Common thickness examples (not recommendations)

Confirm the thickness with your shed supplier or builder for your ground and use.

Waste and ordering

Waste allowance

Custom

Waste is added after the net volume is calculated, covering spillage, over-dig and uneven ground.

Show results for

Bag weight (kg)

Manufacturer-stated yield (optional, replaces the estimate)

Read this from the bag or the product data sheet.

Yield unit

Estimated standard yield: 10 L per 20 kg bag of ready-to-use pre-mixed concrete (blue circle multi-purpose ready-to-use concrete, uk), from the Blue Circle Multi-Purpose Concrete product information (Tarmac). Bag counts based on it are estimates — confirm the yield on the packaging of the product you buy.

Bag counts are for ready-to-use pre-mixed concrete (cement, sand and stone in one bag), not pure cement powder. Bag weight alone does not determine yield: check the wet yield stated on the product packaging or manufacturer's technical information before buying.

Optional sub-base (MOT Type 1 or similar)

Also estimate a sub-base layer under the slab

Results

Enter your project measurements to see your results.

What this calculator does

A shed base is usually a simple rectangular slab, so the concrete is length × width × thickness. This page adds the details that matter when you go to buy it: a waste allowance, a ready-mix figure, a bag count based on the yield of the product you choose, and, if you want one, a separate estimate for the compacted stone layer underneath.

Measuring a shed base

Find the base size in your shed's instructions; it is often the floor size, not the roof size.
Mark out the slab and measure the length and width of the area you will pour.
Check the diagonals are equal so the base is square.
Measure the formwork depth in several places, because sloping ground changes the thickness. See the measurement guide.

Calculating concrete volume

All measurements are converted to metres before any maths, so mixing feet for the size and millimetres for the thickness is fine. The volume is then shown in cubic metres and litres for metric users, or cubic yards and cubic feet for imperial users.

Understanding slab thickness

The 75, 100 and 150 mm (3, 4 and 6 in) buttons are thicknesses people commonly ask about. They are not recommendations. A heavy workshop, soft or sloping ground, or a base that will carry a vehicle may need a different design, so treat your supplier's or builder's guidance as the authority.

Allowing for waste

Waste covers spillage, uneven ground under the slab and formwork that bows slightly. It is applied once, after the net volume. Small jobs are often given 5–10%, but the figure is yours to choose.

Bags versus ready-mix

Bag counts use the yield printed on the packaging when you enter it. Without one, an estimated standard yield is used for bags where a manufacturer figure is on record (the UK 20 kg Blue Circle product and US 40–90 lb general-purpose mix). Bags are always rounded up. For ready-mix, compare the volume with your supplier's minimum load using the ready-mix calculator.

Understanding optional sub-base quantities

A sub-base is crushed stone, not concrete, so it is reported in its own rows and never added to the concrete volume. You enter the finished compacted depth; the compaction factor converts it into the larger loose volume that is delivered. Weight uses the density type you choose, so compaction is never counted twice. For a sub-base on its own, use the MOT Type 1 calculator. Paving around the shed? Try the paving sand calculator.

Metric example

A 3 m × 2.4 m base at 100 mm is 7.2 m² and 0.72 m³. With 10% waste that is 0.792 m³. Using 20 kg bags with a 10 L stated yield, 0.792 m³ is 79.2 bags, so you would buy 80.

Imperial example

An 8 × 6 ft base at 4 in is 16 ft³, or 0.59 yd³. Using 80 lb bags at 0.6 ft³ each and no waste, that is 26.67 bags, rounded up to 27.

Common mistakes

Measuring the roof overhang instead of the floor or base size.
Entering the slab thickness in metres when the unit is set to millimetres.
Assuming every bag of the same weight gives the same yield.
Adding the sub-base volume to the concrete order.

When to get professional advice

Ask a builder or engineer for large or heavy buildings, poor or sloping ground, bases near trees or drains, or anything that may need planning or building approval.

Thickness, waste, density and compaction values on this page are editable examples, not standards.

Frequently asked questions

How much concrete do I need for an 8 × 6 ft shed base?: At 4 inches thick, an 8 × 6 ft slab is 16 cubic feet, or about 0.59 cubic yards, before waste. At 100 mm thick, a 2.4 × 1.8 m slab is 0.432 m³. Add a waste allowance on top, and enter your own thickness because the right one depends on your ground and shed.
Should the base be bigger than the shed?: Many people make the base slightly larger than the shed's floor so the walls sit fully on concrete, but the shed manufacturer's base instructions take priority. Enter the base size you intend to pour, not the shed's roof size, which usually includes overhangs.
Is a 100 mm or 4 inch slab thick enough?: This calculator cannot tell you. The thickness buttons are common examples only. Whether a slab is suitable depends on the ground, the shed's weight and use, drainage, frost and any local rules. Check with the shed supplier, a builder or an engineer.
Bags or ready-mix for a shed base?: Small bases are often mixed from bags, while larger ones can be quicker and more consistent with ready-mix. Compare the bag count here with your supplier's minimum order and delivery charge. Bag counts use the yield printed on the packaging, or an estimated standard yield where one is available.
Is the sub-base included in the concrete volume?: No. The optional MOT Type 1 sub-base is calculated separately and shown in its own section. It is aggregate, not concrete, and is never added to the concrete you order.