Concrete Calculator

How Much Concrete Do I Need?

This free concrete calculator works out exactly how much concrete you need for any garden or DIY project — shed bases, patio slabs, fence post holes, footings and paths. Enter your dimensions in metres or feet, set the thickness, and get an instant answer in cubic metres, kilograms, and the number of bags to buy.

The calculator uses a simple formula — length × width × thickness — to find the volume in cubic metres, then multiplies by the density of mixed concrete (2,400 kg/m³) to give you the weight. This matters because concrete bags in the UK are sold by weight (20kg and 25kg bags), so knowing the total weight tells you exactly how many bags to pick up.

For fence posts, the calculator works out the volume of each hole minus the post itself, then multiplies by the number of posts. This avoids over-ordering — a common mistake when concreting in fence posts by hand.

When to Use Bags vs Ready-Mix Delivery

As a rule of thumb: if you need less than 0.5 cubic metres (about 1,200kg), bags from a DIY store are practical. For anything over 1 cubic metre, a ready-mix delivery is significantly cheaper and saves hours of hand-mixing. Most UK ready-mix suppliers will deliver from 1m³ upwards, with a typical price of £100–£150 per cubic metre depending on your region and the concrete specification.

Need to calculate aggregate for a sub-base underneath? Use our gravel calculator for MOT Type 1 quantities, or the paving calculator if you're laying slabs on top.

Recommended Concrete Thickness by Project

Not sure how thick your concrete needs to be? These are the standard thicknesses for common UK garden and domestic projects.

Concrete Mix Ratios for Garden Projects

If you're mixing your own concrete rather than using pre-mixed bags, getting the right ratio is essential for strength and durability.

Tip: Always mix concrete on a hard, clean surface — a large board or existing slab works well. Add water gradually until the mix holds its shape when squeezed but isn't soupy. In warm weather, dampen the sub-base before pouring to prevent the ground from drawing moisture out of the concrete too quickly.

Frequently Asked Questions

How Much Concrete for a Shed Base? Worked Example

The shed base is the single most common reason people search for a concrete calculator, so here is a full worked example with exact quantities and costs.

Scenario: You are building a standard 8ft × 6ft (2.4m × 1.8m) garden shed and need a solid concrete base.

Step-by-Step Calculation

1. Slab dimensions: Add 100mm overhang on each side for drainage. Final size: 2.6m × 2.0m
2. Thickness: 100mm (standard for sheds under 3m × 2.4m). Use 150mm for larger workshops.
3. Volume: 2.6 × 2.0 × 0.1 = 0.52 cubic metres
4. Weight: 0.52 × 2,400 = 1,248 kg
5. 25kg bags needed: 1,248 ÷ 25 = 50 bags (round up from 49.9)
6. Sub-base: 100mm of MOT Type 1 underneath = 0.52m³ of aggregate

Shed Base Cost Estimate (2026 UK Prices)

My recommendation: For a single shed base, site-mixing is the best value if you have access to a cement mixer. If hand-mixing, buy pre-mixed bags — 50 bags is doable in a day with a wheelbarrow. Only order ready-mix delivery if you are doing additional work (a path or patio) on the same day, since most suppliers have a 1m³ minimum.

How Much Concrete for a Patio? Slab Calculator Guide

Patios need thicker concrete than shed bases because garden furniture, foot traffic and freeze-thaw cycles all take their toll. Here is how to calculate concrete for the most common patio sizes.

Common UK Patio Sizes and Concrete Needed

Key decision: Any patio over 1m³ (roughly 3m × 3m at 100mm) should use ready-mix delivery rather than bags. At £100–£150 per m³ delivered, a 4m × 3m patio costs around £180 for the concrete versus £460 in bags. The cost difference more than pays for the delivery fee.

Always pour patio concrete onto a compacted sub-base of at least 100mm MOT Type 1. Without it, the slab will crack within a year as the ground shifts underneath. If you are laying paving slabs on top rather than leaving exposed concrete, see our paving calculator for slab quantities.

Postcrete Calculator — How Many Bags Per Fence Post?

Postcrete (also sold as Postfix or post-mix) is a rapid-setting concrete designed specifically for fence posts. It sets in 5–10 minutes, so you can work your way along a fence run without waiting for each post to cure. Here is how to calculate what you need.

Postcrete Per Post

A standard fence post hole is 300mm (12 inches) in diameter and 600mm (24 inches) deep. Subtracting a 100mm (4-inch) post from the centre:

• Concrete volume per hole: approximately 37 litres (0.037m³)
• Weight per hole: roughly 40kg of postcrete
• Bags per post: 2 × 20kg bags of postcrete

Quick Reference: Postcrete for Common Fence Lengths

Tip: For more than 10 posts, standard concrete mixed on site is significantly cheaper — roughly half the cost per post. But postcrete saves hours of waiting time between posts, and for a weekend DIY job where you want the fence up in a day, that convenience is worth the premium. Calculate your full fencing materials here.

Concrete for Garden Paths — Thickness, Width and Quantities

A concrete garden path is one of the cheapest permanent landscaping features you can build. Most paths need only 75–100mm of concrete on a 75mm sub-base, and a 10-metre path typically uses less than half a cubic metre.

Path Quantities by Length

For paths longer than 5 metres, include expansion joints every 2–3 metres to prevent cracking. A thin strip of compressible material (foam board or bitumen-impregnated fibreboard) set into the concrete at intervals allows the slab to expand and contract with temperature changes without splitting.

Types of Concrete for Garden Projects

Not all concrete is the same. The type you need depends on what you are building, how much load it will carry, and whether you are mixing it yourself or ordering ready-mix. Here is what I have used across dozens of garden projects and what I would recommend for each job.

Pre-Mixed Bags (Ready-Mix)

Pre-mixed bags contain cement, sand and aggregate already blended to the correct ratio. You just add water and mix. They come in 20kg and 25kg bags from every DIY store in the UK. A 25kg bag makes roughly 0.01 cubic metres of mixed concrete — so you need about 96 bags per cubic metre. For a small shed base (2.4m × 1.8m at 100mm thick), that is around 41 bags. Pre-mixed is convenient for projects under 0.5m³, but the cost adds up quickly — at £4 per 25kg bag, one cubic metre costs around £384 in bags versus £100–£150 for ready-mix delivery.

Postcrete / Post-Mix

Postcrete is specifically designed for fence posts. You pour it dry around the post, add water, and it sets in 5–10 minutes. Two 20kg bags will fill a standard 300mm × 600mm post hole. At roughly £5–6 per bag, it is more expensive per kilogram than standard concrete, but the convenience is worth it for posts. I would never use postcrete for anything larger than a post hole — it is not designed for that, and the rapid set time means you cannot level or adjust a slab.

Site-Mixed Concrete

If you want to save money on a medium-sized project, buy the ingredients separately and mix them yourself. You need ordinary Portland cement (OPC), sharp sand and 20mm aggregate. The standard ratio is 1:2:3 by volume. Buying bulk bags of sand and aggregate from a builders' merchant works out at roughly £50–£70 per cubic metre — significantly cheaper than pre-mixed bags. The trade-off is the effort of mixing and the need for a cement mixer if you are doing more than a few barrows' worth.

Ready-Mix Delivery

For anything over 1 cubic metre, a ready-mix lorry is the most practical option. The concrete arrives fully mixed to the specification you need (C20 for general use, C25 for driveways, C30 for structural footings). Most UK suppliers deliver from 1m³ upwards, with prices typically between £100–£150 per cubic metre depending on your area and the concrete grade. You will need to be ready to pour when the lorry arrives — the driver will not wait longer than about 30 minutes.

How to Mix and Pour Concrete — Step by Step

I have poured concrete for shed bases, fence posts, paths and greenhouse pads over the years. Here is the process I follow every time, whether I am mixing two bags or twenty.

Step 1: Prepare the Ground

Dig out the area to the required depth plus 100–150mm for the sub-base. For a 100mm slab on a 150mm sub-base, you need to excavate 250mm total. Make sure the base is level — use a long spirit level or a straight-edge timber and a builders' line. Remove any roots, soft spots or organic material. If the soil is clay, dig an extra 50mm and add more sub-base to compensate for movement.

Step 2: Lay and Compact the Sub-Base

Spread MOT Type 1 hardcore in 50mm layers, compacting each layer with a plate compactor (hire one for about £30/day) or a heavy hand tamper. The finished sub-base should be firm enough that you can walk on it without leaving footprints. For a shed base, 100mm of compacted sub-base is sufficient. For a driveway, use 150mm. This step is not optional — concrete poured directly onto soil will crack as the ground settles.

Step 3: Build the Formwork

Use 25mm timber boards held in place with pegs driven into the ground outside the pour area. The top edge of the formwork sets your finished concrete level, so get this right. Apply a light coat of form oil or old engine oil to the inside face of the boards so they release cleanly once the concrete has cured. For curves, use thin plywood or hardboard bent to shape.

Step 4: Mix the Concrete

If using pre-mixed bags, cut the bag open, tip the contents into a wheelbarrow or mixer, and add water gradually. The mix should hold its shape when squeezed — like a firm snowball — but not be soupy or dry. For site-mixed concrete at a 1:2:3 ratio, measure by volume using a bucket: one bucket of cement, two of sharp sand, three of 20mm aggregate. A standard 25kg bag of cement makes roughly 0.1 cubic metres of finished concrete when mixed with the sand and aggregate.

Step 5: Pour and Level

Tip the mixed concrete into the formwork and spread it with a shovel or rake. Work it into the corners and edges. Then use a straight-edge timber (longer than the width of your pour) to screed the surface — drag it across the top of the formwork in a sawing motion to level the concrete. If air bubbles appear, tap the side of the formwork with a hammer to release them. For a smoother finish, go over the surface with a float after screeding.

Step 6: Cure Properly

This is where most DIY concrete goes wrong. After pouring, the concrete needs to stay damp for at least 7 days to cure properly. Cover it with polythene sheeting or damp hessian. In hot weather (above 25°C), mist the surface with water twice a day. Do not walk on it for at least 24 hours, and avoid putting any heavy loads on it for a full 7 days. The formwork can be removed after 48 hours in warm weather, or 72 hours in cold weather.

How Much Does Concrete Cost in the UK?

Concrete costs vary significantly depending on whether you buy bags, mix your own from ingredients, or order ready-mix delivery. Here is what I have paid across projects in 2025–2026 so you can budget accurately.

Bag Prices (DIY Stores)

A 25kg bag of pre-mixed concrete (such as Blue Circle) costs £3.50–£5.00 depending on the store. Postcrete is more expensive at £5–£7 per 20kg bag because of the rapid-set formula. At an average of £4.25 per 25kg bag, one cubic metre of concrete from bags costs approximately £408. That is fine for a small path or a few fence posts, but for anything over half a cubic metre, the cost becomes eye-watering.

Site-Mixed from Ingredients

Buying cement (£5–£7 per 25kg bag), sharp sand (£40–£50 per bulk bag) and 20mm aggregate (£35–£45 per bulk bag) separately brings the cost down to roughly £50–£80 per cubic metre. The catch is the labour — mixing a cubic metre of concrete by hand takes 4–6 hours of solid work. A hired cement mixer helps enormously.

Ready-Mix Delivery

Most ready-mix plants charge £100–£150 per cubic metre delivered, with a minimum order of 1m³. Some charge a small-load surcharge (£30–£50) for orders under 3m³. Even with the surcharge, ready-mix delivery becomes cheaper than bags at around 0.75m³ and saves enormous amounts of time. The concrete arrives already mixed to specification — you just pour and level.

Prices are typical UK 2026 retail. Ready-mix prices include delivery but check for small-load surcharges under 3m³. Always get at least two quotes from local ready-mix plants — prices vary by region.

Common Concrete Mistakes to Avoid

I have made most of these mistakes at some point, and seen neighbours make the rest. Each one is easily avoided if you know what to watch for.

1. Skipping the Sub-Base

Pouring concrete directly onto soil is the single most common mistake. Soil moves — it expands when wet, contracts when dry, and settles unevenly over time. Without a compacted sub-base of MOT Type 1 hardcore (100–150mm), your slab will crack within the first winter. I have seen a perfectly good shed base crack in half after 8 months because the builder "did not think it needed hardcore." Use our sub-base calculator to get the right amount.

2. Adding Too Much Water

Soupy concrete is easy to pour but dries weak and porous. The correct consistency is like thick porridge — it holds its shape when squeezed but is not crumbly. As a guide, a 25kg bag of pre-mixed concrete needs roughly 2.5–3 litres of water. If you can pour it like a liquid, you have added too much. Excess water creates air pockets inside the cured concrete, reducing its strength by up to 40%.

3. Not Allowing Enough Curing Time

Concrete reaches only 65% of its final strength at 7 days and 100% at 28 days. Putting a heavy shed on a 3-day-old base or driving on a week-old driveway invites cracking. Cover the surface with polythene to retain moisture, and keep heavy loads off for at least 7 days — ideally 14 for driveways. In cold weather (below 10°C), curing takes longer, so add a few extra days.

4. Under-Ordering Materials

Running out of concrete mid-pour creates a cold joint — a weak seam where the old concrete has started to set before the fresh batch goes on. Always order 10% more than the calculator shows. For bags, round up to the nearest 5. For ready-mix, round up to the nearest 0.25m³. The cost of extra concrete is nothing compared to having to break up and re-pour a failed slab.

5. Ignoring the Weather Forecast

Rain dilutes fresh concrete and washes out the cement paste on the surface. Frost stops the hydration reaction and leaves the concrete weak and crumbly. Check the forecast for the next 48 hours before pouring. You need at least two dry, frost-free days for the initial set. If rain is forecast within 6 hours of pouring, have tarpaulin ready to cover the surface immediately. The ideal pouring temperature is 10–20°C with overcast skies.

When Do You Need Reinforcement?

Not every concrete slab needs reinforcement, but getting this wrong means expensive failures. Here is my rule of thumb based on UK domestic standards.

For garden paths, small shed bases (under 2m × 2m) and fence posts, plain unreinforced concrete at the right thickness is fine. The ground does the supporting — the concrete just distributes the load.

For larger shed bases (over 2m × 2m), patios, and any slab over 100mm thick, I always lay A142 steel mesh on spacers about 50mm up from the base. The mesh prevents cracks from spreading if the ground moves or settles. A 2.4m × 1.2m sheet costs around £8–£12 and takes minutes to lay — cheap insurance for the life of the slab.

For driveways and garage floors, reinforcement mesh is essential. Vehicles create point loads that will crack unreinforced concrete within a few years. Use A193 or A252 mesh (heavier gauge), and pour to at least 150mm thick on a 150mm sub-base. Some builders also add fibre reinforcement to the mix for extra crack resistance — it costs about £8–£10 per cubic metre and gets mixed in with the concrete.

For retaining walls and structural footings, Building Regulations may require specific reinforcement details. If your project is structural (supporting a building, retaining earth over 600mm), get it checked by a structural engineer or your local building control office before pouring.

Best Concrete Products UK 2026 — Our Top Picks

The most popular concrete and cement products for garden DIY projects in the UK right now.

Links above are affiliate links — we may earn a small commission at no extra cost to you.

Where to Buy Concrete in the UK

Once you know how much concrete you need, compare prices across these suppliers. For larger projects (1m³+), ready-mix delivery is significantly cheaper than buying bags.

Links marked above are affiliate links — we may earn a small commission at no extra cost to you. We only link to reputable UK suppliers.