Free cement calculator for concrete shed bases, paths and slabs. Enter your dimensions to get an instant breakdown of bags of cement, sand, gravel and estimated cost. Works in metric (kg) and imperial (lbs).
Standard 100mm (4″) depth is recommended for most garden sheds.
This cement calculator uses a standard 1 : 2 : 3 concrete mix ratio — 1 part cement, 2 parts sharp sand, 3 parts gravel (by volume). This general-purpose mix is suitable for a garden shed concrete base, paths, footings and most domestic groundwork. It's the same mix used by most builders for concrete slabs for shed bases up to 150mm thick.
| Material | Ratio | Per m³ of concrete |
|---|---|---|
| Portland Cement | 1 part | ~320 kg (13 × 25kg bags) |
| Sharp Sand | 2 parts | ~640 kg (0.64 tonnes) |
| Gravel / Aggregate | 3 parts | ~960 kg (0.96 tonnes) |
Wet concrete is about 2,400 kg per cubic metre. The calculator adds a 10% waste buffer automatically — you'll always have slightly more material than the mathematical minimum, which saves a trip to the builders' merchant.
| Application | Depth |
|---|---|
| Garden path / stepping stones | 75 mm (3″) |
| Standard shed or summerhouse | 100 mm (4″) |
| Heavy shed / workshop with machinery | 125–150 mm (5–6″) |
| Garage or vehicle-bearing slab | 150–200 mm (6–8″) |
Pouring a concrete base for a shed is one of the most satisfying weekend DIY projects you can take on. Done properly, it'll outlast the shed sitting on top of it by decades. Here's the complete process from start to finish.
Before you do anything else, decide on the exact size of your base. The base should be slightly larger than the shed footprint — add 50mm to 100mm on each side to give yourself a small margin. Mark the perimeter using wooden pegs and string lines, making sure your corners are perfectly square. Measure the diagonals — if both diagonals are exactly the same length, your corners are square.
Dig down approximately 200mm below the finished slab level. This depth allows for 100mm of compacted sub-base plus 100mm of concrete. Remove all turf, topsoil, roots and any soft ground until you reach firm subsoil. If the ground is particularly soft or wet, you may need to dig deeper and add more sub-base material.
Spread MOT Type 1 (a crushed stone aggregate) across the entire excavated area to a depth of 100mm. Compact it thoroughly using a wacker plate (you can hire one for around £40-60 a day). Compaction is critical — uncompacted hardcore will settle over time and crack your slab. The compacted surface should feel solid underfoot with no give.
Construct a wooden frame around the perimeter using treated timber (typically 100mm × 25mm boards). The top of the frame defines the top of your finished slab, so use a spirit level to make sure it's perfectly level on all sides. Drive wooden stakes outside the frame every 600mm and screw the frame to them for support — wet concrete is heavy and will push outwards.
For a standard shed base, use a 1:2:3 mix (1 part cement to 2 parts sharp sand to 3 parts gravel) by volume. For larger jobs, hire a concrete mixer (£20-30 per day) or order ready-mix delivered. Pour the concrete starting from the corner furthest from your access point. Use a shovel to push it into the corners and a rake to spread it evenly.
Once the slab is full, drag a long straight piece of timber (a "screed bar") across the top of the formwork in a sawing motion. This levels the concrete to the top of your frame. Fill any low spots and screed again. For the smoothest finish, follow up with a wooden float in circular motions, then a steel trowel for a polished finish.
Cover the finished slab with plastic sheeting or hessian and keep it damp for at least 3-7 days. This prevents the surface drying too quickly, which would cause cracking. The slab is safe to walk on after 24-48 hours, but don't place heavy loads or build the shed for at least a week. Full cure (95% strength) takes 28 days.
This is the single most common mistake. People assume concrete is so strong that you can pour it directly onto soil. But soil moves — it expands when wet, contracts when dry, and frost heaves it upward in winter. Without a properly compacted sub-base, your slab will crack within a year or two. The MOT Type 1 layer absorbs ground movement and provides drainage, both of which are essential.
The right consistency is crucial. Concrete that's too dry won't bond properly and will be weak. Concrete that's too wet ends up porous and brittle. Aim for a "stiff porridge" texture — it should hold its shape when piled but be wet enough to spread easily. The classic test: when you press a shovel into it and pull back, the surface should look smooth, not crumbly.
Even on a shed base, you want a slight slope (around 1:80, or about 12mm fall per metre) so rainwater runs off rather than pooling. Build this into your formwork from the start by setting one side of the frame slightly lower than the other. Standing water on a slab will cause moss, ice in winter, and eventual damage.
Concrete cures best between 5°C and 25°C. Below 5°C, the water in the mix can freeze before the concrete sets, which destroys it. Above 25°C, especially in direct sunlight, water evaporates too quickly and the slab cracks. The ideal seasons for pouring concrete in the UK are spring and autumn. If you must pour in hot weather, work early in the morning and cover the slab with damp hessian.
Concrete is technically safe to walk on after 48 hours, but it's still gaining strength for weeks afterwards. Building a heavy shed on a slab that's only a few days old can cause point-load damage. Wait at least 7 days before assembling a wooden shed, and at least 14 days before a heavy metal or brick-built one.
For a standard 8×6ft shed base, you'll need: a spade and shovel for digging and mixing, a wheelbarrow for moving materials, a wacker plate (hire) for compacting the sub-base, a wooden float and steel trowel for finishing, a long spirit level for levelling formwork, a tape measure and string line for marking out, a screed bar (any straight piece of timber longer than the base width), wooden stakes and screws for the formwork, and either a small concrete mixer (hire) or a large mixing tub if mixing by hand.
Total tool hire cost is typically £60-100 for a weekend, plus around £150-200 for materials on a typical 8×6ft base. Doing it yourself saves £400-600 compared to hiring a tradesman, and the finished result lasts decades.