How Much Concrete Do I Actually Need? A DIYer's Guide to Getting It Right
Garden & Home Projects · 8 min read
Right, let me save you from making the same mistake I made the first time I laid a concrete slab. I ordered a full cubic metre for a small garden base, ended up with a mountain of leftover mix going hard in the wheelbarrow, and spent the next hour frantically throwing it around the garden hoping no one would notice. Spoiler — my wife noticed.
Calculating concrete isn't difficult, but there are a few things that'll trip you up if you don't know what to watch for. Let's go through it properly.
The basic formula
For any slab, footing, or post hole, you're working with the same simple sum: Length × Width × Depth = Volume. Always work in metres, and your answer will be in cubic metres (m³).
Example: A 3m × 2m garden shed base, 100mm thick
Length
3.0 m
Width
2.0 m
Depth
0.10 m
💡 Key tip
Always convert your depth to metres before multiplying. 100mm = 0.10m, 150mm = 0.15m. Easy to forget, and the numbers go wrong fast if you mix millimetres and metres.
How deep does it need to be?
This is where a lot of first-timers go wrong — either too thin (and it cracks) or massively over-engineered. Here's a rough guide for common garden and home projects:
Shed or summer house base
100–150mm. Go 150mm if it's on soft ground or if the building is heavy.
Driveway or parking slab
150mm minimum. Cars are heavy — don't cut corners here.
Footpath or patio
75–100mm for foot traffic only. No less than 75mm.
Fence post or gate post
Fill the hole about ⅓ of the post depth. Use a dry-mix and ram it in.
Bags or ready-mix?
For anything under roughly 0.5 m³, bagged mix from a builders' merchant makes the most sense. You can buy it, use what you need, and store the rest (if you keep it dry). Once you're getting above that volume, ready-mix starts to work out cheaper and a lot less back-breaking.
A standard 25kg bag of C20/C25 concrete mix will give you approximately 0.012 m³ when mixed. So for our shed base example above (0.66 m³ with waste), you'd need around 55 bags. That's a lot of mixing — at that point, consider hiring a small mixer or calling for a ready-mix delivery.
📦 Bag calculator shortcut
Divide your total volume (in m³) by 0.012 to get the number of 25kg bags needed. Always round up, and add 10% on top for good measure.
Things to avoid — learn from other people's mistakes
- ✕ Guessing the volume — always measure twice. Even being 20mm out on depth across a large slab means a significant difference in concrete volume.
- ✕ Skipping the sub-base — concrete on bare soil is asking for trouble. Lay at least 75mm of compacted hardcore or MOT Type 1 first, otherwise it'll settle unevenly and crack.
- ✕ Adding too much water to the mix — it makes it easier to pour, but weaker when it sets. If your mix slumps flat off a shovel, it's too wet.
- ✕ Pouring in very hot or freezing weather — below 5°C it can freeze before it sets; above 30°C it can dry too fast and crack. Cover it if you're pouring in hot sun.
- ✕ Not adding the waste percentage — always add at least 10% to your calculated volume. You'll lose some to spillage, uneven ground, and mix sticking to tools.
- ✕ Mixing too many bags at once — unless you've got a mixer, keep it to 2–3 bags at a time in a barrow. Concrete doesn't wait for you.
- ✕ Walking on it too soon — foot traffic after 24–48 hours is usually fine, but don't load it for a full week and don't drive on it for at least 28 days.
⚠️ Don't skip this
If you're pouring a slab that connects to your house, put an expansion gap between them. A thin strip of compressible foam or roofing felt does the job. Without it, movement in the ground can push the slab into the wall and cause cracking.
One last thing
Concrete work is genuinely satisfying when it goes right — and these days, a bit of careful planning beforehand makes all the difference. Measure your area properly, convert to metres, multiply out the volume, add 10%, and you'll know exactly what you need before you've picked up a shovel.
If you're unsure whether your project needs reinforcement (rebar or mesh) or a stronger concrete spec, that's a whole other conversation — leave a comment below and I'll cover it in a future post.
Good luck out there. And keep a bucket of water nearby to wash the tools before the mix goes off.
— Wayne