Concrete Bag Calculator — m, kg bags, cost comparison
How to use this calculator
Pick a preset for the most common small pours, or enter your own volume. The calculator rounds every bag size up and flags the cheapest option per bag size once prices are entered — the number you see is what to load on the pallet, not a theoretical minimum.
Direct volume — enter m³ or litres if you already know the volume.
Dimensions — enter L × W with a unit selector (m / cm / mm) plus depth.
Waste — 10% for clean formwork, 15% for footings cast against soil.
| Bag Size | Yield | Bags | Weight | Cost |
|---|---|---|---|---|
| 20 kg | 0.010 m³ | — | — | — |
| 25 kg | 0.012 m³ | — | — | — |
| 40 kg | 0.019 m³ | — | — | — |
| 50 kg | 0.024 m³ | — | — | — |
Saved Calculations
| Time | Volume | 20 kg | 25 kg | 50 kg | Best Cost |
|---|
How Many Bags of Concrete Do I Need? (Metric)
Bagged concrete is the practical choice for footings, post bases, pads, and repairs up to roughly 0.5 m³. Beyond that, a ready-mix truck or volumetric mixer is normally cheaper per cubic metre once mixing labour is included. This tool compares the 20, 25, 40, and 50 kg sizes side by side with a multi-currency cost estimate.
How to use it
Enter a known volume in m³ or litres, or switch to Dimensions and type length × width with a unit selector (m / cm / mm) plus depth. A 3 m × 3 m slab at 10 cm is the default. The waste slider adds an allowance for spillage and an uneven sub-base — 10% suits clean formwork, 15% for footings cast against soil.
Bag yield reference (EN 206)
20 kg = 0.010 m³ (10 L) · 25 kg = 0.012 m³ (12 L) · 40 kg = 0.019 m³ (19 L) · 50 kg = 0.024 m³ (24 L). Placed density is about 2,400 kg/m³ per Eurocode 2; the lower per-bag figures reflect dry mix that must be combined with water on site.
Formulas
Volume: Vm³ = L × W × D (all in m); multiply by 1,000 for litres.
Bags: N = ⌈Vm³ × (1 + waste) ÷ yieldbag⌉ (rounded up).
Cost: per size = N × price/bag in the selected currency; the lowest non-zero total is the best value.
Related concrete calculators
For structural members, size the section first with the concrete beam calculator or concrete column calculator. For larger pours use the concrete volume calculator and for vertical work the concrete wall calculator. Site work is handled by the curb & gutter calculator and the concrete stairs calculator.
FAQ
Which bag size is best? 25 kg bags are easiest to handle for DIY work. 40–50 kg bags give better value for larger pours but need more strength to carry.
How many 25 kg bags per m³? Approximately 84 bags (1 m³ ÷ 0.012 m³ per bag ≈ 84 bags). Above ~0.5 m³, ready-mix usually costs less.
Why round up every bag size? Bags are sold whole and a short pour cannot be patched cleanly after it sets, so the bag count is always ceiled after waste is applied.
Ordering tips
In 2025 EU/UK pricing, a 25 kg bag of general-purpose concrete runs about €3.50–5.50 (Mapei, Bostik, B&Q own-brand), 40 kg around €5–7, and 50 kg pallets €6–9 each. The most common estimator mistake is buying for the dry-volume figure printed on the bag and skipping the waste line — partial bags, spillage, and an over-dug post hole easily add 10–15%. Above roughly 0.5 m³ (about 42 of the 25 kg bags), price out a ready-mix truck or a volumetric mixer: per Eurocode 2 the C25/30 mix in those trucks is the same strength as most general-purpose bagged products, and the delivered €/m³ is usually lower once mixing labour is counted. For driveway and exposed slabs choose a C30/37 or higher exposure-class XC4 mix.
