Concrete Base Calculator UK
Work out how much concrete you need for a shed base, garden room, garage floor or any concrete slab. Choose bags or ready-mix, set your wastage allowance and get a clear estimate in seconds.
Enter your dimensions
Typical: 75–100mm (paths), 100–150mm (shed base)
25kg bag yield ~0.0125 m³ - check product data
Covers cuts, breakages and uneven ground. 10% is a typical starting point.
Concrete estimate
Enter length, width and depth to see your concrete estimate
How to calculate concrete for a base
The calculation is straightforward: volume = length × width × depth. The tricky parts are choosing the right depth, accounting for wastage and deciding between bags and ready-mix.
Here is the step-by-step method:
- Measure the length and width of your base in metres.
- Decide on the thickness - 100mm is common for a typical shed base.
- Multiply length × width × depth (in metres) to get your volume in m³.
- Add a wastage percentage (10% is a sensible starting point).
- Divide by the bag yield to get bag count, or round up to 0.25 m³ increments for ready-mix.
This calculator does all of that for you. Enter your dimensions, pick your concrete type and the result updates instantly.
Worked example - 3m × 2.4m shed base
For this size base, ready-mix will almost certainly be more practical and cost-effective than 64 bags of bagged concrete - consider your access and mixing capacity before deciding.
Typical concrete base depths
| Use | Typical depth | Notes |
|---|---|---|
| Light garden shed | 75–100mm | On firm, prepared sub-base |
| Larger garden shed / summerhouse | 100mm | With 75mm compacted sub-base |
| Garden room / workshop | 100–150mm | Consult supplier for specific loads |
| Garage (single car) | 100–150mm | May need mesh reinforcement |
| Paths and paving base | 75–100mm | Decorative slab bedding only |
These are typical planning figures for DIY projects. Site conditions, load requirements and ground quality all affect the appropriate depth. For anything structural, get professional advice.
Common mistakes when calculating concrete for a base
- Not adding a sub-base first. Pouring concrete directly onto uncompacted soil is a common mistake. A 75–100mm compacted MOT Type 1 or hardcore layer first will make a huge difference to the longevity of the slab.
- Underestimating bags. Bag yields vary by product. Always check the yield on your specific bag and add wastage - mixing by hand wastes more than you expect.
- Ordering too little ready-mix. Having a pour interrupted mid-slab because you ran out is a nightmare. Round up and get slightly more than you think you need.
- Forgetting the depth unit. If you measure depth in millimetres but enter it as metres, you will calculate a very different volume. Always double-check your units.
- Not checking access for ready-mix. A concrete truck needs reasonable road access and often a chute run to reach the pour site. Check this before ordering.
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Frequently asked questions
Planning estimates only
These results are estimates for planning purposes only. Actual material quantities can vary based on site conditions, compaction, wastage, product size, installation method and supplier guidance. For structural, safety-critical or regulated work, always consult a qualified professional.