Calculate Concrete Bags
Select your project type, enter the dimensions, and we'll tell you exactly how many bags to buy.
Your Concrete Requirements
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Where to Buy Concrete Bags in the UK
Once you have your bag count, use these suppliers to compare prices. Postcrete and ready-mix bags vary in price — buying in bulk from a builders merchant is usually cheaper than DIY stores for large projects.
| Supplier | What They Stock | Best For |
|---|---|---|
| Wickes | Postcrete, ready-mix concrete 20kg & 25kg, sharp sand | Trade pricing, bulk orders |
| B&Q | Ready-mix concrete bags, Postcrete, ballast | UK-wide stores, same-day collection |
| Amazon UK | Tarmac Postcrete 20kg — sets in 10 mins, UK #1 for fence posts | Most popular fast-set concrete, Prime delivery |
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.
Concrete Bag Coverage Reference (2026 UK Prices)
Use this table to quickly estimate how many bags you need for common project sizes.
| Bag Size | Yield per Bag (m³) | Bags per m³ | Price per Bag (2026) |
|---|---|---|---|
| 20kg Ready-Mix | ~0.009 m³ | ~110 bags | £5.50 |
| 25kg Ready-Mix | ~0.011 m³ | ~90 bags | £6.50 |
| 20kg Postcrete (fast-set) | ~0.009 m³ | ~110 bags | £6.50 |
How to Use This Concrete Calculator
For Slabs and Patios
Measure the length and width of the area in metres. For the depth (thickness), a standard shed base is 100mm (0.10m), a driveway is 150mm (0.15m), and a garage floor is 150–200mm. Enter the depth in metres — so 100mm = 0.10m.
For Fence Post Holes
Measure or estimate the diameter of the hole you plan to dig. A standard spade-dug hole is typically 150–200mm wide. The depth should be at least one-third of the post height above ground — so for a 1.8m fence post, dig at least 600mm deep. Enter the number of holes and the calculator will give you the total bags needed.
Always Add 10% Extra
This calculator includes a 10% buffer automatically. Concrete volume calculations can be affected by uneven ground, spillage, and slight variations in mix water content. Having a spare bag or two on hand prevents a frustrating trip back to the builder's merchant mid-pour.