Whether you’re pouring a shed base or setting fence posts, ordering the right amount of concrete saves wasted bags and half-empty mixer vans.
How it’s worked out
- Slab / footing volume = length × width × depth (all in metres)
- Post holes volume = π × (diameter ÷ 2)² × depth × number of holes
- We then add your waste percentage.
Bag estimates use typical UK trade yields: about 0.01 m³ per 20kg bag and 0.0125 m³ per 25kg bag. Ready-mix is shown in m³ rounded up for ordering.
Worked example
A 5 m × 4 m slab, 100 mm deep: 5 × 4 × 0.1 = 2.0 m³. With 10% waste → 2.2 m³ ≈ 176 × 25kg bags, or order 2.2 m³ ready-mix.
Depth tip: 100 mm = 0.1 m; 150 mm = 0.15 m.
Frequently asked questions
How many 25kg bags of concrete in a cubic metre?
As a rule of thumb a 25kg bag makes about 0.0125 m³, so you need around 80 bags per m³ before waste. Always check the bag for the manufacturer’s stated yield.
Should I allow for waste?
Yes. Add 5–10% for spillage and uneven ground. For post holes and awkward pours, 10% is common.
Disclaimer: Results are estimates for guidance only. They are not financial, tax or legal advice. Rules vary with your circumstances — check official guidance or speak to a qualified adviser before making decisions.
Sources
- Income Tax rates and Personal Allowance (checked 5 October 2026)
- Scottish Income Tax rates (checked 5 October 2026)
- Rates and thresholds for employers 2026 to 2027 (checked 5 October 2026)
- National Insurance: how much you pay (checked 5 October 2026)
- National Insurance rates and categories (checked 5 October 2026)
- 2026 to 2027: Student and Postgraduate Loan deduction tables (checked 5 October 2026)
- Tax on dividends (checked 5 October 2026)
- Corporation Tax rates and allowances (checked 5 October 2026)
- Income Tax — changes to tax rates for property, savings and dividend income (checked 5 October 2026)