I made a proper mess of my first shed base. I ordered 20 bags of ready-mix concrete for a 3-metre by 2.4-metre base, poured them out and barely covered half the area. What I had failed to account for - apart from the basic maths - was that I had measured the shed footprint rather than the base footprint, and I had added no wastage whatsoever.
I had to make two emergency trips to the builder's merchant, mixing concrete in the late afternoon sun while my wife watched from the kitchen window with what I can only describe as disappointed patience.
This guide is everything I wish I had known before I picked up that first bag.
Why shed bases need careful measuring
A concrete shed base does not need to be complicated. But getting the quantity wrong is genuinely inconvenient - and embarrassing if it happens in front of a neighbour who builds things for a living. Too little and you end up with a thin, weak slab or a visible gap at the edge. Too much and you have wasted money and a pile of setting concrete in the barrow.
The formula itself is simple. The errors creep in at the edges.
The formula
Volume (m3) = Length x Width x Depth
All measurements need to be in metres. If your depth is in millimetres (which it usually is), divide by 1000.
So for a 3m x 2.4m shed base at 100mm deep:
- Volume = 3.0 x 2.4 x 0.1 = 0.72 m3
- Add 10% wastage: 0.72 x 1.10 = 0.79 m3
That is about 64 bags of 25kg concrete mix, or 0.75-1.0 m3 ready-mix depending on your supplier's minimum.
Use the Concrete Base Calculator to get the exact figure for your dimensions.
How deep should a shed base be?
For a standard garden shed, 100mm is the most commonly used depth. Here is a rough guide:
| Use | Depth | Notes |
|---|---|---|
| Light wooden garden shed | 75-100mm | On firm, prepared sub-base |
| Larger shed, summerhouse | 100mm | With 100mm compacted sub-base below |
| Workshop, heavy machinery | 100-150mm | Consider mesh reinforcement |
| Garden room, home office | 100-150mm | Check supplier guidelines |
For most garden sheds, 100mm is fine. Going thinner than 75mm is a false economy - the slab will be more prone to cracking and movement, especially on clay soils. I have seen 60mm garden shed bases that were essentially rubble within five years.
Bags or ready-mix?
This is the question most people agonise over, and the answer is usually simpler than you think.
Use bagged concrete if:
- Your base is under about 0.5 m3
- You do not have ready-mix access nearby
- You want to spread the work over a weekend
Use ready-mix if:
- Your base is larger than 0.5-1.0 m3
- You have good access for a truck and chute
- You want to do it in one session and get it done
For a 3m x 2.4m base (0.79 m3), ready-mix makes sense. Mixing 64 bags of concrete by hand is genuinely exhausting - roughly two hours of solid work. Hiring a mixer for the day plus buying the bags can work out more expensive than a small ready-mix order at these volumes. Get a quote from your local supplier before deciding.
Bag counts for common shed sizes
Here are rough bag estimates at 100mm depth with 10% wastage included:
| Shed size | Volume (inc. 10% wastage) | 25kg bags | Ready-mix to order |
|---|---|---|---|
| 1.8m x 1.2m | 0.24 m3 | 20 bags | 0.25 m3 |
| 2.4m x 1.8m | 0.48 m3 | 40 bags | 0.5 m3 |
| 3.0m x 2.0m | 0.66 m3 | 55 bags | 0.75 m3 |
| 3.0m x 2.4m | 0.79 m3 | 66 bags | 1.0 m3 |
| 3.6m x 2.4m | 0.95 m3 | 79 bags | 1.0 m3 |
| 4.0m x 3.0m | 1.32 m3 | 110 bags | 1.5 m3 |
Ready-mix volumes rounded up to nearest 0.25 m3 - typical supplier minimum increment.
Do not skip the sub-base
One of the biggest mistakes with DIY shed bases is pouring concrete straight onto soil or bare earth. I have seen people do this and the results are never good - the slab settles unevenly, roots push up from below, and after a few winters of frost heave the whole thing can crack or tilt.
A compacted sub-base - typically 75-100mm of MOT Type 1 or compacted hardcore - does three important things:
- Improves drainage - water can drain away under the slab rather than pooling and freezing beneath it
- Prevents settlement - soft spots in the soil do not transfer directly into the slab above
- Reduces frost heave - a free-draining base is far less affected by the ground freezing in winter
Use our Sub Base Calculator and MOT Type 1 Calculator to work out those quantities first, before you even think about the concrete.
Wastage - how much to add?
For a straightforward rectangular shed base, 10% wastage is a reasonable starting point. This covers:
- Spills and splashes during mixing and pouring
- Slight over-excavation at the edges
- The fact that ready-mix must be ordered in 0.25 m3 increments
If you are mixing by hand, I would lean towards 12-15% wastage. It is genuinely easy to lose a surprising amount of material during hand mixing - concrete sticks to the barrow, splashes during pouring, and stiffens in the mix before you can use it.
What mix to use
For a shed base, a general-purpose concrete mix (C20, often labelled as Gen 3 or ST2 in the UK) is perfectly suitable. You do not need specialist structural concrete for a standard garden shed base.
For ready-mix, ask for C20 or C25 general-purpose mix. For bagged concrete, the pre-mixed "all-in" or "general purpose" bags are appropriate - they contain cement, sand and aggregate in the right proportions and you just add water.
Avoid using straight cement or sand - you need the aggregate in the mix for strength and bulk.
My tips for laying a shed base
After making every mistake possible on my first few concrete projects, here is what I would tell someone starting out:
Measure the base, not the shed. I got caught out by this the first time. The base should be at least 50-100mm larger than the shed footprint in each direction. Measure what you are actually forming, not the shed that will sit on it.
Set up your shuttering first. Get your timber boards level and at the right height before mixing anything. Once you start pouring, you need to move quickly - stopping to faff with formwork is how you end up with lumpy concrete.
Lay DPM before pouring. A sheet of 1000-gauge polythene (damp-proof membrane) laid before the concrete pour keeps ground moisture out of the shed floor. Overlap joins by 300mm and turn it up the sides of the shuttering slightly.
Keep the concrete damp for 3-7 days. This is called curing and it significantly improves the final strength. In warm or windy weather, lay hessian or polythene over the slab and dampen it daily.
Do not walk on it for 24-48 hours. A recently poured slab looks set but is still gaining strength underneath. Leave it alone.
Use the Concrete Base Calculator to get an accurate number for your project before you order anything.