Shed Base Calculator - Concrete, Paving Slabs & Timber Frame Bases

How to Calculate a Shed Base

Working out a shed base comes down to five steps. Here is a worked example for a standard 8 × 6ft shed (2.44m × 1.83m):

1. Add the overhang – add 50mm to each side of the shed's length and width, so the base is slightly bigger than the shed footprint. 2.44m + 100mm = 2.54m long, 1.83m + 100mm = 1.93m wide.
2. Calculate the base area – multiply the two figures: 2.54 × 1.93 = 4.90 m².
3. Work out the sub-base – multiply the area by your sub-base depth (100mm is standard). 4.90 × 0.1 = 0.49 m³ of compacted MOT Type 1, plus 10% waste, which is about 1.04 tonnes to order. Compaction changes the material's volume, not its mass, so ordering by weight at the compacted density already accounts for it – no separate compaction allowance is needed on top.
4. Work out the concrete – multiply the area by the slab depth (100mm standard). 4.90 × 0.1 = 0.49 m³, plus 10% waste, which is about 44 x 25kg bags of ready-mix, or roughly 8 bags of cement plus 1.18 tonnes of ballast if you are mixing from scratch (5:1 ballast:cement by volume for a proper C20 mix, allowing for the dry materials bulking up before mixing).
5. Add the DPM and formwork – the damp proof membrane needs to lap up the sides of the formwork by at least 100mm, so add that to each dimension: 5.84 m² of DPM. The formwork (shuttering) around the perimeter comes to 8.94m, which is 3 lengths of 3.6m timber.

This calculator does all of this automatically for concrete, paving slabs or a timber bearer frame, and scales to any shed size.

What Size Should a Shed Base Be?

A shed base should be slightly larger than the shed itself, not the exact same size. The extra margin around the edge is the overhang. Most UK guidance puts this at 50-75mm per side.

• 50mm (2 inches) per side – the low end of the standard range, and this calculator's default. It gives you room to position the shed accurately and stops the roof overhang dripping water straight onto bare ground next to the base.
• 75mm per side – the top end of the standard range, giving a bit more positioning margin on a larger shed without the edge becoming a trip hazard.
• 0mm – only really suitable when the base itself is going to be hidden by cladding or a skirt, since any misalignment during positioning will be visible.
• Up to 150mm – useful on larger workshops or where you want a small path of hardstanding around the building, but avoid going much wider than this or the exposed slab edge becomes a trip hazard and a place for weeds to establish.

Whatever overhang you choose, keep it consistent on all four sides so the shed sits centrally on the base.

How Thick Should a Shed Base Be?

100mm of concrete over 100mm of compacted MOT Type 1 sub-base is the standard specification for a typical timber garden shed up to about 10 × 8ft. That is 200mm of total dig depth.

• 75mm concrete – workable for small, light structures (up to about 6 × 4ft) on firm, well-drained, well-prepared ground, but it has less margin for error and is more prone to cracking without reinforcement.
• 100mm concrete – the standard depth for most domestic sheds. Suitable for timber and metal sheds up to a fairly large size.
• 125-150mm concrete – worth the extra for workshops, log cabins, anything with significant stored weight (machinery, a workbench), or ground that is soft, uneven or prone to standing water.

Adding steel mesh reinforcement lets you get away with a thinner slab for the same strength, but for a straightforward DIY base, increasing the depth is simpler than detailing reinforcement correctly.

For a standard garden shed, an unreinforced slab on a properly compacted sub-base is adequate and reinforcement isn't necessary. For a workshop taking heavy machinery, a base that will ever see vehicle weight, or any base over about 4m in either dimension, consider adding A142 steel mesh laid mid-depth in the slab. This calculator does not include mesh in its material list, so add it separately if your base falls into one of those heavier categories.

Concrete Mix for a Shed Base

A shed base does not need a structural engineer's mix design, but it does need to be stronger than the very lean mixes sold for general garden use. The standard specification for a shed base is C20 (also called GEN 3), which as a site-mix is roughly 5 parts ballast to 1 part cement by volume (equivalent to 1:2:3 cement:sand:aggregate if you are buying sand and aggregate separately rather than pre-blended ballast). A weaker path or patio mix, sometimes quoted around 6:1 or leaner, is too weak for a structure that has to stay flat and crack-free under a shed's weight for years.

When you mix dry ballast and cement with water, the finished concrete takes up noticeably less space than the loose dry materials did, because the water fills the gaps between the aggregate particles. As a rule of thumb, you need about 1.5 times the loose dry volume for every cubic metre of finished, compacted concrete. This calculator applies that factor automatically when it works out your cement and ballast quantities.

Bags vs Ready-Mix: Where's the Crossover?

For a small base like a shed, bagged materials almost always work out cheaper and easier than ordering a ready-mix lorry, which usually has a minimum load (often around 1m³ or more) and a delivery charge. As a guide:

• Under about 1m³ (most single garden sheds) – bag it yourself, either pre-blended ready-mix bags (just add water) or separate ballast and cement.
• 1-2m³ (large sheds, workshops) – compare the cost of the bags this calculator gives you against a local ready-mix supplier's minimum load charge. It can go either way depending on your area.
• Over 2m³ – ready-mix usually becomes cheaper and far less labour, and mixing that much by hand or in a small mixer takes a very long time.
• Where to buy – Wickes, B&Q and Travis Perkins all stock pre-blended ready-mix concrete in 20kg and 25kg bags, plus ballast and cement separately if you would rather mix your own. Screwfix is a better bet for the mixing tools (a mixing hoe, buckets or a small cement mixer) than for the bagged materials themselves.

Which Base Type Should I Choose?

Base Type	Best For	Effort
Concrete slab	Any shed, especially heavier metal, apex or workshop sheds. The most durable, level, long-term option.	High – formwork, mixing or ordering ready-mix, curing time
Paving slabs	Small to mid-size timber sheds. Quicker than concrete, no curing time, and easier to lift and relevel later.	Medium – sub-base and sand bed still need to be right
Timber bearer frame	Light timber sheds on reasonably level, well-drained ground. The fastest, cheapest option.	Low – but the least durable and needs a damp-free site

Our advice: if the shed is going to be a workshop, hold anything heavy, or sit on ground that gets wet, build a proper concrete slab. For a straightforward garden storage shed on firm ground, paving slabs or a bearer frame on compacted gravel will do the job for a fraction of the effort. Jewson and Travis Perkins are both good sources for pressure-treated bearer timber in the lengths this calculator recommends, cut to size at the counter if you ask.

Planning Permission for a Shed Base

A shed base on its own does not normally need planning permission, since it is assessed as part of the outbuilding it supports. Most garden sheds in the UK fall under permitted development and do not need an application, provided:

• The outbuilding is single-storey, with a maximum eaves height of 2.5m if it is within 2m of a boundary.
• Maximum overall height is 4m with a dual-pitched roof, or 3m for any other roof type (when not within 2m of a boundary).
• Outbuildings do not cover more than 50% of the total area around the original house, including any other extensions.
• It is not positioned forward of the principal elevation (the front of the house facing the road).

Most sheds are also exempt from Building Regulations if the floor area is no more than 15m² and it contains no sleeping accommodation. Sheds between 15m² and 30m² are usually still exempt provided they are at least 1m from any boundary or built substantially from non-combustible materials. Listed buildings, conservation areas and AONBs can have tighter restrictions, so check with your local planning authority if you are unsure, and note that the detail of these rules can vary slightly between England, Wales, Scotland and Northern Ireland.

Common Shed Sizes: Quick Reference

Approximate materials for each common UK shed size, using a 50mm overhang, 100mm concrete slab and 100mm MOT Type 1 sub-base, with 10% waste included:

Shed Size	Base Size	MOT Sub-base	Concrete (25kg bags)	Cement + Ballast (from scratch)
6 × 4ft	1.93m × 1.32m	0.54 tonnes	23 bags	5 bags + 0.61 tonnes
8 × 6ft	2.54m × 1.93m	1.04 tonnes	44 bags	8 bags + 1.18 tonnes
10 × 6ft	3.15m × 1.93m	1.28 tonnes	54 bags	10 bags + 1.46 tonnes
10 × 8ft	3.15m × 2.54m	1.69 tonnes	71 bags	13 bags + 1.93 tonnes
12 × 8ft	3.76m × 2.54m	2.02 tonnes	85 bags	16 bags + 2.30 tonnes
14 × 10ft	4.37m × 3.15m	2.91 tonnes	122 bags	22 bags + 3.31 tonnes

These are guide figures for the standard 100mm/100mm concrete specification. Use the calculator above for exact quantities based on your own overhang, depth and base type.

Frequently Asked Questions

How thick should a shed base be?

100mm of concrete over 100mm of compacted MOT Type 1 sub-base is the standard specification for a typical timber garden shed, giving 200mm of total dig depth. 75mm can work for small, light sheds on firm ground, while 125-150mm is worth using for workshops, heavier structures or soft ground.

How much ballast and cement do I need for an 8x6 shed base?

For an 8x6ft shed (2.44m x 1.83m) with a 50mm overhang and 100mm concrete slab, the base works out to roughly 2.54m x 1.93m (4.90 m2), needing about 0.49 m3 of concrete. Mixed from scratch at a 5:1 ballast to cement ratio for a C20 mix, that is approximately 8 bags of cement (25kg) and 1.18 tonnes of ballast, including 10% waste.

How much gravel do I need for a 6x4 shed base?

For a 6x4ft shed (1.83m x 1.22m) with a 50mm overhang, the base area is approximately 2.55 m2. At a standard 100mm depth, that needs roughly 0.54 tonnes of MOT Type 1 or gravel sub-base to order, including 10% waste. This is the weight of material at its compacted density: since compacting doesn't remove mass, ordering by weight already accounts for it. At a lighter 75mm depth, it drops to around 0.40 tonnes.

Is 3 inches of concrete enough for a shed base?

3 inches (about 75mm) can be enough for a small, light shed (up to around 6x4ft) on firm, well-prepared, well-drained ground. For anything larger, or for a workshop or heavier structure, most tradespeople recommend at least 100mm of concrete over a compacted sub-base, since a thinner slab has less margin for error and is more prone to cracking.

What size should a shed base be?

A shed base should be larger than the shed itself by a consistent overhang on all four sides. Most UK guidance puts this at 50-75mm (2-3 inches). This gives room to position the shed accurately and helps stop roof runoff dripping straight onto bare ground at the edge. Wider overhangs up to 150mm are sometimes used on larger sheds, but going much beyond that creates a trip hazard and a place for weeds to grow.

Do I need planning permission for a shed base?

A shed base on its own does not normally need planning permission, since it is assessed as part of the shed it supports. Most garden sheds fall under permitted development rules covering height, coverage of the garden and position relative to the house. Listed buildings, conservation areas and AONBs can have tighter restrictions, and the exact rules vary slightly between England, Wales, Scotland and Northern Ireland, so check with your local planning authority if you are unsure.

Concrete, paving slabs or timber bearers - which shed base is best?

Concrete is the most durable and best choice for workshops, metal sheds or heavier structures. Paving slabs are quicker to lay with no curing time and suit small to mid-size timber sheds. A timber bearer frame on compacted gravel is the fastest and cheapest option, best suited to light timber sheds on firm, well-drained ground.

Do I need a DPM under a shed base?

A damp proof membrane is not a legal requirement for a garden shed base, but it is good practice under a concrete slab, since it reduces moisture rising through the concrete into a timber shed floor and helps the slab cure more evenly. It is less commonly used under paving slabs or a timber bearer frame, where drainage is handled by the sub-base itself.

Can I put a shed straight onto gravel?

Not directly onto loose gravel. Without a frame to spread the load, a shed will settle unevenly into loose material over time. The minimum acceptable approach is a timber bearer frame sitting on a compacted, levelled gravel or hardcore bed, which spreads the shed's weight across the bearers rather than a few points of contact.