Concrete CalculatorConcrete Bag Calculator - Work Out How Many Bags You Need
Use this concrete bag calculator to find exactly how many 20kg or 25kg bags your project needs. Built for UK bag sizes and metric measurements, it covers shed bases, fence posts, patios, and any job where you are mixing by hand.
Concrete Bag Calculator
Pick your project type, enter dimensions, and get an instant bag count for 20kg or 25kg bags.
Results
Bags Needed
0
× 25kg bags
Total Volume
0 m³
Total Weight
0 kg
Est. Cost
£0
@ £ per bag
Tip: Results include your waste allowance. For fence posts, consider Postcrete instead. It sets in 10 minutes.
How Many Bags of Concrete Do I Need?
Start with volume. Multiply length by width by depth, all in metres. A 3m × 2m shed base at 100mm thick gives you 3 × 2 × 0.1 = 0.6 cubic metres. That number tells you everything.
Dry premixed concrete weighs about 2,100kg per cubic metre. Divide that by your bag size to get your count. For 25kg bags: 0.6m³ × 2,100 = 1,260kg, divided by 25 = 51 bags. With 10% waste added, order 56.
Choosing between 20kg and 25kg bags changes the maths. A 20kg bag gives less concrete per bag, so you need more of them. That same 0.6m³ shed base would need 69 bags at 20kg instead of 56 at 25kg. The concrete bag calculator above handles all of this for you.
Bags Per Cubic Metre: Quick Reference
One cubic metre of concrete requires 84 bags at 25kg or 105 bags at 20kg. Here is a quick reference table for common volumes:
| Volume (m³) | 25kg Bags | 20kg Bags |
|---|---|---|
| 0.1 | 9 | 11 |
| 0.25 | 21 | 27 |
| 0.5 | 42 | 53 |
| 0.75 | 63 | 79 |
| 1.0 | 84 | 105 |
| 1.5 | 126 | 158 |
| 2.0 | 168 | 210 |
For projects over 1m³, consider ordering a bulk bag (jumbo bag) at 850kg. One jumbo bag covers about 0.4m³ and saves you the hassle of opening 34 individual bags.
Common UK Projects: Bag Counts at a Glance
These figures use 25kg bags with 10% waste built in. Adjust your concrete bag calculator inputs for different thicknesses or 20kg bags.
| Project | Size | Thickness | 25kg Bags |
|---|---|---|---|
| Fence post (single) | 300mm × 600mm deep | n/a | 6 |
| Shed base 6×4ft | 1.83m × 1.22m | 100mm | 21 |
| Shed base 8×6ft | 2.44m × 1.83m | 100mm | 42 |
| Shed base 10×8ft | 3.05m × 2.44m | 100mm | 69 |
| Patio 3×3m | 3m × 3m | 100mm | 84 |
| Garden path | 5m × 0.9m | 75mm | 30 |
Fence posts are a special case. If speed matters more than strength, use Postcrete instead of standard concrete. One 20kg bag of Postcrete fills a post hole and sets in about 10 minutes. For structural posts or gateposts, stick with standard concrete and our footing calculator for deeper foundations.
Choosing Between 20kg and 25kg Bags
Both sizes use the same concrete mix. The difference is weight and where you buy them.
20kg bags are lighter and easier to handle on your own. B&Q, Wickes, and most DIY stores stock them. They cost slightly more per kilogram, but your back will thank you after mixing 50 bags.
25kg bags offer better value per kilogram. Jewson, Travis Perkins, and other builders merchants carry them. If you have a helper or a cement mixer, go with 25kg. You will need fewer bags and spend less overall.
Tips for Getting Your Order Right
Always add 10% to your calculated amount. Ground is rarely perfectly level, and some concrete sticks to the mixer, the wheelbarrow, and your boots. Running short halfway through a pour creates a weak joint in the slab.
Leftover bags keep for 3-6 months in a dry shed. You can use them for small repairs, setting a washing line post, or patching a garden wall. No waste there.
Once you pass 1 cubic metre (84 bags at 25kg), think about ready-mix delivery instead. Mixing 84 bags by hand is a full day's hard graft. A ready-mix lorry drops your concrete in minutes. Use our slab calculator to check volumes before deciding.