How to Calculate Concrete for a Shed Base

How Much Concrete Do You Need for a Shed Base?

Getting the concrete quantity right for a shed base saves you money and hassle. Order too little and you're left with a half-finished pour. Order too much and you've got a wheelbarrow of expensive waste setting in your drive. Here's how to work it out properly.

Step 1: Measure Your Shed Base

Before anything else, you need three measurements:

• Length — the longest side of your base
• Width — the shortest side of your base
• Depth — how thick the slab needs to be

For a standard garden shed, a 100mm (0.1m) thick slab is usually sufficient. If you're building a workshop, storing heavy equipment, or housing a vehicle, go with 150mm (0.15m).

Always add 50–100mm to the length and width beyond your shed footprint. This gives you a lip around the edge for drainage and stops the shed timber sitting right on the slab edge where water pools.

A Note on Sub-Base

Your concrete slab should sit on a compacted sub-base of MOT Type 1 or hardcore, typically 100mm deep. This isn't included in your concrete calculation but don't skip it — without it, your slab will crack as the ground shifts.

Step 2: Calculate the Volume

The formula is straightforward:

Volume (m³) = Length (m) × Width (m) × Depth (m)

For example, a 3m × 2.4m shed base at 100mm deep:

3.0 × 2.4 × 0.1 = 0.72 m³

Add 10% for Waste

Always add 10% to your calculated volume. Ground is never perfectly level, formwork shifts slightly, and you'll lose a bit in the mixer. For our example:

0.72 × 1.10 = 0.79 m³

Rather than doing this by hand every time, you can plug your dimensions straight into the Concrete Calculator to get your volume, number of bags, and ready-mix quantity instantly.

Common Shed Sizes: Concrete at a Glance

Here's a quick reference table for standard shed sizes with a 100mm slab depth (10% waste included):

| Shed Size (ft) | Base Size (m) | Volume (m³) | 25kg Bags (Approx.) | Ready-Mix (m³) | |---|---|---|---|---| | 6 × 4 | 2.0 × 1.4 | 0.31 | 15 | 0.31 | | 8 × 6 | 2.6 × 2.0 | 0.57 | 27 | 0.57 | | 10 × 6 | 3.2 × 2.0 | 0.70 | 34 | 0.70 | | 10 × 8 | 3.2 × 2.6 | 0.91 | 44 | 0.91 | | 12 × 8 | 3.8 × 2.6 | 1.09 | 52 | 1.09 | | 12 × 10 | 3.8 × 3.2 | 1.34 | 64 | 1.34 | | 16 × 10 | 5.0 × 3.2 | 1.76 | 84 | 1.76 |

Bag counts assume pre-mixed concrete at approximately 0.021 m³ per 25kg bag.

If you need to convert between units or check material weights, the Weight Calculator can help.

Mix Ratios: Getting It Right

For a shed base, you want a C20 mix (also called Gen 3 or ST2 if ordering ready-mix, as classified in BS 8500). This is suitable for domestic foundations, shed bases, and paths.

C20 Mix Ratio by Volume

| Material | Parts | |---|---| | Cement (OPC) | 1 | | Sharp sand | 2 | | Gravel (20mm aggregate) | 4 |

So for every bucket of cement, you need two buckets of sharp sand and four buckets of gravel. Add water gradually — the mix should hold its shape when squeezed but not be crumbly or soupy.

How Much Cement, Sand, and Aggregate?

For 1 m³ of C20 concrete, you'll need roughly:

• 6.5 bags of 25kg cement (about 160kg total)
• 600kg of sharp sand
• 1,200kg of 20mm gravel
• 150 litres of water (adjust to conditions)

Ready-Mix vs Hand Mixing

Hand Mixing (Belle Mixer or by Hand)

Best for smaller bases up to about 0.5 m³. Beyond that, you'll be mixing all day and your arms will know about it.

• Pros: Cheaper for small jobs, no minimum order, work at your own pace
• Cons: Physically hard, slower, more risk of inconsistent mix

A 25kg bag of pre-mixed concrete costs around £5–£6 from a builders' merchant (2026 prices). For a 6×4 shed base, that's roughly £75–£90 in materials.

Ready-Mix Delivery

For anything over 0.5 m³, ready-mix is worth it. Most suppliers have a minimum order of 1 m³ (some do mini-mix from 0.5 m³ in smaller vehicles).

• Pros: Consistent quality, much faster, less labour
• Cons: Minimum order charges, need to be ready when the truck arrives, access can be an issue on tight sites

Expect to pay £90–£130 per m³ for ready-mix delivered, depending on your area and the mix specification. Mini-mix services tend to charge a premium — around £150–£200 per m³ — but are worth it for awkward access sites.

The Golden Rule

Have everything prepped before your concrete arrives. Formwork in place, sub-base compacted, reinforcement mesh laid if you're using it, and a path cleared from the truck to the pour site. Ready-mix waits for no one.

Tips for a Good Shed Base

• Use formwork. Timber boards (25mm × 150mm) pegged into the ground keep your edges clean and level.
• Compact the sub-base. Hire a whacker plate — it's £25–£35 a day and makes a massive difference.
• Lay mesh reinforcement. A sheet of A142 mesh costs around £15 and prevents cracking. Sit it on spacers so it's in the middle of the slab, not on the bottom.
• Cure it properly. Cover the slab with polythene sheeting for at least 3 days. Don't let it dry out too fast, especially in summer — rapid drying causes surface cracks.
• Check your levels. Use a long spirit level or a laser level. A slight fall (1:60) away from any adjacent building helps with drainage.

Planning Permission

Good news: most garden sheds don't need planning permission under permitted development rights, provided:

• The shed is single storey with a maximum eaves height of 2.5m
• It's not forward of the principal elevation
• It covers no more than 50% of the garden area
• In designated areas (conservation areas, AONB, etc.), it's no more than 10 m³

However, Building Regulations don't typically apply to small detached buildings under 15 m² that contain no sleeping accommodation. Always check with your local authority if you're unsure.

Quick Summary

1. Measure your base dimensions and depth (100mm for sheds, 150mm for workshops)
2. Calculate volume: Length × Width × Depth
3. Add 10% for waste
4. Choose your method: hand mix for under 0.5 m³, ready-mix for anything larger
5. Use a C20 mix (1:2:4 ratio)
6. Prep everything before you pour

Need to run the numbers quickly? The Concrete Calculator does all the maths for you — just plug in your dimensions and it'll tell you exactly what you need.