Concrete foundations are one of the first things QSs are asked to measure, and one of the easiest places to make mistakes.
This guide breaks down how to calculate concrete quantities for the three most common foundation types:
- Pad foundations
- Strip foundations
- Raft foundations
The Basic Formula (Always Start Here)
For all concrete foundations, the core calculation is:
Volume (m³) = Length × Width × Depth
The difference between pad, strip and raft foundations is simply how those dimensions are defined.
1. How to Calculate Concrete for Pad Foundations
Pad foundations are usually square or rectangular bases supporting columns.
What you need
- Length (m)
- Width (m)
- Depth / thickness (m)
- Number of pads
Worked example
One pad foundation:
- Length = 1.2 m
- Width = 1.2 m
- Depth = 0.6 m
Concrete volume (per pad):
1.2 × 1.2 × 0.6 = 0.864 m³
If there are 10 identical pads:
- 0.864 × 10 = 8.64 m³
QS tips
- Check if pads vary in size — don’t assume they’re all identical
- Watch for stepped pads or thickenings
- Confirm if blinding concrete is measured separately
2. How to Calculate Concrete for Strip Foundations
Strip foundations run continuously under walls.
What you need
- Total strip length (m)
- Strip width (m)
- Concrete depth (m)
Worked example
Strip foundation:
- Length = 22 m
- Width = 0.6 m
- Depth = 0.3 m
Concrete volume:
22 × 0.6 × 0.3 = 3.96 m³
QS tips
- Measure centre-line or net length consistently
- Watch junctions and returns — don’t double count
- Check drawings for local deepening at load points
3. How to Calculate Concrete for Raft Foundations
Raft foundations are essentially large slabs covering the full footprint of the building.
What you need
- Raft length (m)
- Raft width (m)
- Raft thickness (m)
Worked example
Raft foundation:
- Length = 12 m
- Width = 8 m
- Thickness = 0.35 m
Concrete volume:
12 × 8 × 0.35 = 33.6 m³
QS tips
- Check for thickened edges or beams within the raft
- Allow separately for upstands and ground beams
- Confirm whether blinding is included or excluded
Allowances QSs Must Not Forget
1. Wastage
Typical allowances:
- Small pours: 5–10%
- Complex foundations: up to 10%
Always confirm whether the rate already includes wastage.
2. Over-excavation
Excavations are rarely perfect:
- Sides collapse
- Trenches widen
- Formation levels vary
Concrete quantities often increase slightly on site — this risk should be recognised.
3. Blinding Concrete
Blinding is usually:
- 50–75 mm thick
- Measured separately
Don’t accidentally include it in structural concrete quantities.
Common QS Mistakes to Avoid
- Mixing excavation depth with concrete depth
- Forgetting pad numbers
- Ignoring stepped foundations
- Measuring raft concrete but missing thickened edges
- Assuming drawings are perfectly coordinated
How QSs Sense-Check Concrete Quantities
A quick sense check:
- Pad foundations: usually 0.5–1.5 m³ each
- Strip foundations: roughly 0.15–0.25 m³ per linear metre
- Raft foundations: slab volume + local thickenings
If your numbers feel wildly outside this range — recheck.
Conclusion
Concrete calculations aren’t about complicated maths.
They’re about:
- Reading drawings carefully
- Applying consistent logic
- Understanding site reality
Get those right, and your quantities will stand up commercially and technically.
