How to Calculate Concrete for a Shed Slab
A shed slab is one of the most common DIY concrete projects — and also one where people most often order the wrong amount of concrete. Order too little and you're left with a half-poured slab; order too much and you've wasted money. This guide walks through every step of the calculation so you get it right the first time.
Step 1: Measure the slab dimensions
Before you can calculate anything, you need three measurements:
- Length — the longest side of the slab, measured in metres or feet.
- Width — the shorter side, perpendicular to the length.
- Thickness — how deep the slab will be, measured from the finished surface down to the base material.
For a typical garden shed (3 m × 2 m), the slab footprint is simply 3 × 2 = 6 m². But thickness is where many people go wrong — read on.
Step 2: Choose the right thickness
The thickness of your shed slab depends on what you're storing and the soil conditions:
| Shed type | Recommended thickness | Notes |
|---|---|---|
| Garden tool shed (light) | 100 mm (4″) | Standard for most residential sheds |
| Workshop / heavy storage | 125–150 mm (5–6″) | Supports machinery and point loads |
| Vehicular access shed | 150 mm (6″) | If a vehicle or ride-on mower enters |
When in doubt, go thicker. The cost difference between 100 mm and 125 mm is modest — typically an extra 25% on concrete volume — but the strength gain is significant.
Step 3: Calculate the volume
The formula is straightforward:
Volume = Length × Width × Thickness
For a 3 m × 2 m shed slab at 100 mm thick:
Volume = 3 × 2 × 0.10 = 0.60 m³
In imperial units, a 10 ft × 8 ft slab at 4 inches thick:
Volume = 10 × 8 × (4/12) = 26.7 ft³ = 0.99 yd³
You can skip the manual math and use the Concrete Calculator — just enter your dimensions and it handles the unit conversions, waste allowance and bag counts instantly.
Step 4: Add a waste allowance
Concrete doesn't always go exactly where you plan. Spillage, uneven sub-base, over-excavation and formwork gaps all eat into your supply. The industry standard is to add 5–10% extra:
- 5% — for a well-prepared site with accurate formwork and experienced pourers.
- 10% — for DIY projects, uneven ground, or when mixing bagged concrete by hand.
For our 0.60 m³ example with 5% waste:
0.60 × 1.05 = 0.63 m³
Step 5: Estimate bag count (if using bagged concrete)
For small slabs, bagged concrete is often more practical than ready-mix delivery. Here's a quick reference:
| Bag size | Yield per bag | Bags for 0.63 m³ |
|---|---|---|
| 20 kg | ≈ 0.010 m³ | ≈ 63 bags |
| 40 kg | ≈ 0.020 m³ | ≈ 32 bags |
| 80 lb | ≈ 0.60 ft³ | ≈ 37 bags (for ~22 ft³) |
At 60+ bags of 20 kg, consider ready-mix delivery — it's usually cheaper and far less labour-intensive for volumes above 0.5 m³.
Step 6: Site preparation tips
A well-prepared site makes the pour go smoothly and prevents cracking:
- Excavate to depth — dig down to allow for sub-base (50–75 mm of compacted gravel) plus slab thickness. For a 100 mm slab on 75 mm of road base, excavate 175 mm below finished ground level.
- Compact the sub-base — use crushed rock or road base, compacted in layers with a plate compactor. A soft or uneven base is the #1 cause of slab cracks.
- Set up formwork — use straight, rigid timber (50 × 100 mm or 2×4) staked at 600 mm intervals. Check diagonally to ensure the slab is square.
- Lay reinforcement — for sheds, a sheet of F62 or F72 mesh on 20 mm bar chairs is sufficient. Wire the overlaps and keep the mesh in the top third of the slab.
- Order concrete — call the supplier 24–48 hours ahead. Specify the strength (25 MPa is standard for shed slabs), the total volume including waste, and the delivery time.
Common mistakes to avoid
- Forgetting the sub-base depth — your excavation depth is not the slab thickness; it's slab thickness plus sub-base.
- Ordering exactly the calculated volume — always round up or add waste allowance. Running short mid-pour creates a cold joint that weakens the slab.
- Using the wrong strength mix — a garden shed can use 20 MPa, but a workshop with heavy machinery should be 32 MPa.
- Placing mesh incorrectly — mesh sitting on the base does nothing. It must be chaired up into the top third of the slab.
Ready to calculate?
Use the Concrete Calculator to get an instant volume, bag count and mix ratio estimate for your shed slab. Enter your dimensions, choose a waste allowance, and the calculator handles the rest — including cement, sand and aggregate quantities if you're mixing on site.