A friend of mine laid his own garage base a couple of years ago. He was meticulous about the sub-base - hired a plate compactor, compacted in two layers, checked his levels with a long straight edge. But when it came to the concrete itself, he ran short. He had calculated for 100mm depth and ordered accordingly, but when the ready-mix arrived and was poured, the finished depth came out at 80-85mm. He had not accounted for the fact that the sub-base level was not quite as consistent as he thought, creating low spots that needed more concrete.
The slab is fine - 85mm is not a disaster - but he tells the story to illustrate why adding a generous wastage allowance to a garage base pour is not optional.
This guide covers everything you need to get the quantity right.
Why ready-mix is almost always the answer
For a shed base you might genuinely debate between bags and ready-mix. For a garage base, that debate is usually over quickly.
A single garage base at 150mm depth is around 2.25 m3 of concrete. At a typical bag yield of 0.012 m3 per 25kg bag, that is approximately 190 bags - and that is before wastage. Mixing 190 bags by hand would take the better part of a day with a hired mixer. It would also produce inconsistent concrete as the mix changes slightly with each batch.
Ready-mix arrives already mixed to a consistent specification, is pumped or chuted directly to where you need it, and the whole pour is done in an hour. At volumes above about 1 m3, ready-mix is almost always the right choice on cost and practicality combined.
The formula
Volume (m3) = Length (m) x Width (m) x Thickness (m)
If thickness is in mm, divide by 1000 first.
Single garage example
Garage 5m x 3m, 150mm thick:
- Volume = 5 x 3 x 0.15 = 2.25 m3
- Add 10% wastage = 2.475 m3
- Round up to nearest 0.25 m3 = 2.5 m3 to order
Double garage example
Garage 6m x 6m, 150mm thick:
- Volume = 6 x 6 x 0.15 = 5.4 m3
- Add 10% wastage = 5.94 m3
- Round up = 6.0 m3 to order
For your exact dimensions, use the Concrete Base Calculator.
How thick should a garage base be?
| Use | Minimum | Recommended |
|---|---|---|
| Standard cars, domestic use | 100mm | 125-150mm |
| Heavy SUV or people carriers | 125mm | 150mm |
| Occasional van / light commercial | 125mm | 150mm |
| Workshop, heavy equipment | 150mm | 175mm |
| Regular heavy vehicle use | 175mm | Seek advice |
For most domestic garages used by family cars, 150mm is the right target. It is thick enough to handle ordinary vehicle loads, tolerates minor sub-base inconsistencies better than 100mm, and is not prohibitively more expensive than a thinner slab.
Volumes by common garage size
| Garage | Area | Volume at 100mm | Volume at 150mm |
|---|---|---|---|
| Single (4.8m x 2.7m) | 13m2 | 1.3 m3 | 1.95 m3 |
| Single (5m x 3m) | 15m2 | 1.5 m3 | 2.25 m3 |
| Single (5m x 5m) | 25m2 | 2.5 m3 | 3.75 m3 |
| Double (6m x 6m) | 36m2 | 3.6 m3 | 5.4 m3 |
| Double (6m x 7m) | 42m2 | 4.2 m3 | 6.3 m3 |
| Double (7m x 7m) | 49m2 | 4.9 m3 | 7.35 m3 |
Add 10% wastage and round up to nearest 0.25 m3 for your actual order.
Sub-base: the most important part
Before any concrete goes in, you need a proper sub-base. This is where the long-term performance of your garage floor is determined.
A typical garage floor build-up from bottom to top:
- Subgrade (natural ground, well compacted)
- Geotextile membrane (recommended, especially on clay)
- 150-200mm compacted MOT Type 1 sub-base
- DPM (1000 gauge polythene, lapped up the sides)
- Concrete slab 100-150mm
Total excavation depth: approximately 300-400mm below finished floor level.
Use the MOT Type 1 Calculator to work out sub-base quantities before you plan the concrete pour.
What concrete mix to use
For a garage base, ask for a minimum C25 (GEN4/ST4) concrete. C25 gives good strength and reasonable abrasion resistance for a floor surface.
If you want better long-term floor durability - particularly if you will be parking on it permanently and doing workshop work - C30 is worth the small extra cost per m3. It is more resistant to abrasion and freeze-thaw cycles.
Do not use C15 or C20 for a garage floor.
Do I need reinforcement mesh?
For a standard domestic car garage, steel mesh is not strictly required and is not universally specified.
However, it is worth considering if:
- Your sub-base quality is uncertain
- The slab spans more than 3-4m without a joint
- Heavy vehicles will use the garage regularly
- You want added insurance against cracking
If you use mesh, A142 fabric (6mm bars at 200mm centres) is the standard domestic spec. Use plastic spacer chairs to hold it mid-depth - not laying it on the ground before pouring.
My tips for a garage base pour
Having poured a few concrete bases and watched several more being done, here is the honest practical advice:
Book the ready-mix supplier early and have a contingency plan. Ready-mix suppliers have minimum orders and surcharges for small loads. Call at least a week in advance, confirm the volume, and be honest about the access (can they get a full-size truck to your site, or do you need a smaller truck or a pump?). Have a rough number for a second smaller order in case the pour runs slightly short.
Pour and level the same day. Unlike bags where you can stop and start, a ready-mix delivery needs to be poured, spread and roughly levelled before the driver needs to leave (typically 1-2 hours from batch). Have two people minimum.
Hire a concrete vibrating poker. Air pockets in a concrete slab are weak spots. A concrete vibrating poker (available from hire centres) pushes air bubbles to the surface. Run it slowly through the slab as you pour - it takes five minutes and significantly improves the finished strength.
Cure the slab properly. Cover with plastic sheeting or damp hessian for at least 3 days, ideally 7. In hot weather, this is critical - concrete that dries too fast in the sun is weaker and more prone to surface cracking.
Do not drive on it for 7 days. Initial set takes 24-48 hours. Full working strength takes 28 days. A week without vehicles is a reasonable minimum.
Get your volume confirmed with the Concrete Base Calculator before calling the supplier.