Concrete Calculator
Calculate how much concrete you need in cubic metres or bags for slabs, footings, posts and columns.
By Konstantin Iakovlev · Founder, Calks.uk
Last updated: · Verified against HMRC and GOV.UK 2026/27 rates
Volume
0.90 m³
Weight
2.16 tonnes
25kg Bags
75
20kg Bags
90
Disclaimer
This calculator is provided for informational purposes only and should not be considered as financial or tax advice. All calculations are performed locally in your browser — no personal data is collected or sent to our servers. Rates and thresholds are sourced from HMRC and GOV.UK and are updated for the current tax year. Always verify results with HMRC or consult a qualified professional before making financial decisions.
How It Works
Concrete volume is calculated by multiplying length × width × depth for rectangular slabs, or πr² × depth for circular areas such as post holes. The result in cubic metres tells you how much ready-mix to order, or how many bags of dry mix you need. A standard 25 kg bag of post-mix makes approximately 0.011 m³ of concrete.
For small jobs (under 1 m³), bagged dry-mix is practical. For larger pours, ready-mix delivery is more economical and ensures consistent quality. Most UK suppliers sell ready-mix in minimum loads of 1 m³ for mini-mix and 6 m³ for standard trucks. Add 5–10% for wastage.
Enter the dimensions of your slab, footing or post holes and the calculator shows volume in cubic metres, the number of 25 kg bags needed and an approximate cost estimate. For structural work like foundations, always check Building Regulations requirements and consider getting an engineer's specification for the concrete mix design.
How much concrete do you need? Volume = length × width × depth. Sample slab 5m × 3m × 0.10m = 1.5 m³ concrete. Always add 5-10% for waste, spillage and uneven ground. Standard pre-mix bag (UK): 25 kg makes ~12 litres (0.012 m³) finished concrete — so 1 m³ = 80+ bags. Far more economical to order ready-mix above ~1 m³: typical UK price 2026 £100-£140/m³ delivered, minimum 1 m³ order (some firms 0.5 m³ minimum at premium).
Concrete mix ratios by application. By volume — Cement : sand : aggregate. C7.5 (footpaths, light foundations): 1:4:6. C20/25 (residential slabs, driveways): 1:2:4. C25/30 (structural, beams): 1:1.5:3. C30/40 (heavy industrial): 1:1:2. Domestic driveways: C25 is standard (£110-£130/m³). Garage and shed foundations: C20 (£100-£115/m³). For DIY mixing under 0.5 m³, use bagged 'general purpose' ballast (sand+aggregate pre-mixed) at 1 part cement : 5 parts ballast.
UK Building Regulations for concrete work. Foundations: minimum 1m below ground level in clay soils (heave risk); 600mm in sandy/gravel soils. Depth varies for trees nearby — within 1× tree height: 1.5m+ recommended; some species (oak, willow) require 2.5m+. Slab thickness minimums: 100mm for domestic patio; 125mm for driveway with cars; 150mm+ for vans/heavy use. Reinforcement: A142 mesh standard for driveways; A193 for vehicle-loaded areas. Building Notice or Full Plans for any structural foundation work — consult local Building Control.
Curing time and weather considerations. Concrete achieves 70% strength at 7 days, 95% at 28 days (industry standard for testing). Walk on at 24-48 hours (light pedestrian); drive cars after 7 days; full load after 28 days. Cure conditions: ideal 10-25°C, 60-90% humidity. Below 5°C: concrete won't cure properly — use accelerator additive or wait. Above 30°C: dries too fast, cracks — mist surface with water during first 3 days. Cover with plastic or wet hessian for slow, strong cure. Concrete in winter UK: avoid pours below 5°C ground temp.
Cost breakdown for typical UK concrete projects. Patio 20 m² × 100mm = 2 m³ concrete: ready-mix £220-£280 + base materials £150 (Type 1 sub-base, weed membrane) + labour £500-£800 if hired = £870-£1,230 total. Driveway 50 m² × 125mm: concrete £325-£420 + base £400 + edging £150 + labour £1,500-£2,500 = £2,400-£3,500. Shed base 3m × 2m × 100mm: 0.6 m³ — minimum order may force buying 1 m³, so consider bagged concrete (~40 bags × £6 = £240 + 4 hours work).
Example: Garden shed base, 3 m × 2.4 m × 0.1 m deep
- Volume: 3 × 2.4 × 0.1 = 0.72 m³
- Add 10% wastage: 0.72 × 1.10 = 0.79 m³
- Bags (25 kg post-mix): 0.79 ÷ 0.011 ≈ 72 bags
- Or order 1 m³ ready-mix (typical minimum load)
Frequently Asked Questions
- What does the Concrete Calculator do?
- Calculate how much concrete you need in cubic metres or bags for slabs, footings, posts and columns.
- How much concrete do I need?
- Concrete volume = length × width × depth (all metres). Add 5-10% wastage. Common uses: 0.1m deep slab = 1m³ covers 10m²; foundation 0.6m × 0.6m × 0.4m = 0.144m³ per pad. Ready-mix delivery minimum 1m³ (£90-£150). Below this, mix on site from 25kg bags (yield ~0.012m³ each — need 85 bags/m³).
- Concrete mix ratios.
- C20 / 1:2:4 (cement:sand:gravel): foundations, paths. C25 / 1:2:3.5: driveways, garage floors, garden walls. C30 / 1:1.5:3: high-strength structural use. C35 / 1:1.5:2.5: heavy-duty structural. Domestic projects rarely need above C30. DIY: 1:2:4 ballast (pre-mixed sand+gravel) easiest.
- Curing and reinforcement.
- For slabs over 4m² or carrying weight, add A142 or A193 mesh reinforcement (1.5kg/m² steel). Place mesh at 1/3 slab thickness from bottom. Curing: keep moist for 7+ days (polythene or wet hessian) — strength gains continue for 28 days. Don't pour in freezing weather or above 30°C — both compromise strength.