Question 1

How much concrete do I need for a shed base?

Accepted Answer

For a standard 2.4m × 1.8m shed base at 100mm thick, you need 0.43 cubic metres of concrete. With 10% waste added, order 0.47m³, which works out to roughly 40 bags of 25kg premixed concrete.

A larger 3m × 2.4m shed base at 100mm needs about 0.72m³, or 61 bags. Use the concrete base calculator above for your exact dimensions. The shed base preset fills in standard measurements automatically.

Always round up rather than down. Running short mid-pour creates a cold joint that weakens the finished slab.

Question 2

How deep should a concrete base be for a shed?

Accepted Answer

A shed base should be 100mm (4 inches) thick as a minimum. Garage bases carrying vehicle weight need at least 100mm, and many builders use 150mm for extra durability. Garden paths can use 75mm if the sub-base is well compacted. The depth depends on ground conditions too. Clay or soft soils benefit from 125mm of concrete over a 100mm compacted hardcore sub-base. Use the calculator to see how depth changes your bag count.

Question 3

What concrete mix should I use for a shed base?

Accepted Answer

A 1:2:4 mix (cement:sand:ballast) gives C20 strength, which handles shed bases, patios, and light garden paths without trouble. For a garage base where vehicles will park, use a stronger 1:2:3 mix (C25). If you are buying premixed bags from B&Q or Wickes, the proportions are already correct inside the bag. For mixing from scratch, use the material breakdown section in the calculator above to get exact cement, sand, and ballast quantities.

Question 4

How many 25kg bags of concrete do I need for a 3x3m patio?

Accepted Answer

A 3m × 3m patio base at 100mm thick needs 0.9m³ of concrete. With 10% waste, that comes to 0.99m³, requiring around 83 bags of 25kg premixed concrete. At 75mm thick, the same 3×3m area needs about 62 bags. Use the concrete base calculator above to enter your exact depth and get a precise figure. At 83 bags, consider asking the builders merchant about pallet delivery, often free on larger orders.

Question 5

What does a concrete base cost per square metre in the UK?

Accepted Answer

Using 25kg bags at around £5 each, materials for a concrete base cost roughly £45-55 per square metre at 100mm thick. Labour adds £50-80 per square metre if you hire a contractor. A 3×3m shed base costs around £415 in concrete bags alone, not including hardcore, formwork, or tools. Ready-mix concrete costs £90-130 per cubic metre for small deliveries. The cost estimator in the calculator above updates automatically as you enter your bag price.

Question 6

How long does a concrete base take to set?

Accepted Answer

Concrete is firm enough to walk on after 24-48 hours. You can build a timber shed frame on the base after 3-5 days in summer. Allow 7 days before placing heavy loads. Cold weather slows curing: below 5°C, cover fresh concrete with hessian sacking and avoid pouring in frost. Full strength takes 28 days. Do not lay timber sole plates directly on fresh concrete before the surface is fully cured, as rising damp will rot the wood.

Question 7

Do I need a sub-base under a concrete base?

Accepted Answer

Yes, almost every shed base, patio, and garage base needs a compacted sub-base. A 100mm layer of compacted Type 1 MOT hardcore stops the concrete cracking when the ground moves. Skip the sub-base only if you are pouring over rock or compacted chalk. Clay soils, sandy ground, and made-up ground all need a proper sub-base. For enclosed structures like garages and workshops, add a damp-proof membrane between the hardcore and the concrete to keep moisture out.

Question 8

What is the difference between C20 and C25 concrete for a base?

Accepted Answer

C20 concrete (1:2:4 mix) has a compressive strength of 20 N/mm² and suits shed bases, garden paths, and patios used for foot traffic. C25 (1:2:3 mix) reaches 25 N/mm² and handles garage bases and driveways where vehicles park. For foot-traffic and light garden structures, C20 is perfectly adequate and cheaper to mix. When ordering ready-mix, specify Gen 3 for general bases or Gen 4 for garage and driveway applications.

Question 9

How much ballast and cement do I need for a concrete base?

Accepted Answer

For a 1:2:4 mix, you need roughly 320kg of cement, 700kg of sand, and 1,200kg of ballast per cubic metre of finished concrete. A 0.47m³ shed base needs around 150kg of cement, 330kg of sand, and 565kg of ballast. Ballast is often sold as all-in ballast, which combines sand and gravel in one bag. One tonne bag of ballast covers about 0.6m³ of concrete. The material breakdown in the calculator shows exact quantities for your project dimensions.

Question 10

When should I use ready-mix concrete instead of bags for a base?

Accepted Answer

Once your base exceeds 1 cubic metre, ready-mix delivery is usually faster and more cost-effective than mixing bags by hand. A 1m³ base requires 84 bags of 25kg, which takes a full day to mix. Ready-mix lorries deliver to your formwork in 15-30 minutes. Mini-mix suppliers deliver as little as 0.5m³ for smaller bases. Costs run £90-130 per cubic metre for small loads. Use the concrete base calculator to find your volume, then ring local suppliers for a quote.

Project	Size	Depth	Volume (m³)	25kg Bags
Small shed base	2.4m × 1.8m	100mm	0.47	40
Medium shed base	3m × 2.4m	100mm	0.79	67
Large shed base	4m × 3m	100mm	1.32	111
Patio base	3m × 3m	75mm	0.74	62
Patio base	4m × 3m	100mm	1.32	111
Single garage base	6m × 3m	100mm	1.98	166
Double garage base	6m × 6m	100mm	3.96	333

Wet density	2,400 kg/m³
Dry bag density	2,100 kg/m³
25kg bag yield	≈ 0.012 m³
Bags per m³	~80 × 25kg
Waste factor	+5–10%

Concrete Base Calculator UK — Volume, Bags & Cost