Concrete bag calculator
Convert a concrete volume into the number of pre-mixed bags to buy, so you order the right amount for a small job that doesn't justify a ready-mix lorry.
Result
Bags required
How to use this calculator
- Work out your required volume first — use the concrete volume calculator if you don't already know it.
- Enter that volume and pick your bag size.
- Read the number of bags to buy.
Assumptions
- Yield figures are a standard industry approximation — always check your specific product's stated yield on the bag.
- No allowance is added for spillage; round up further for outdoor or windy mixing conditions.
Technical details: what the fields mean
- Concrete volume needed
volumeM3 - Total concrete volume required, in cubic metres.
- Bag size
bagWeightKg - The bag size sold by your supplier — 20kg or 25kg are most common.
Methodology
Bags = volume ÷ yield per bag. Yield is estimated at 0.0125m³ per 25kg bag (a widely used builders' merchant rule of thumb, roughly 80 bags per cubic metre), scaled proportionally for a 20kg bag.
Worked example
1 cubic metre of concrete using 25kg bags
Inputs: volumeM3=1, bagWeightKg=25
Result: 80 bags required
Practical guidance
- For volumes above roughly 1m³, ready-mix delivery is usually cheaper and far less labour than mixing dozens of bags by hand.
- Buy a couple of spare bags — running short mid-pour means a hard-to-hide cold joint.
Common mistakes
- Using a generic yield figure without checking the actual bag — yields vary by brand and mix.
- Not rounding up to a whole bag.
FAQs
How many 25kg bags of concrete are in a cubic metre?
Roughly 80 bags, using the standard 0.0125m³-per-bag yield estimate.
Is it cheaper to use bags or ready-mix?
For small jobs (under about 1m³) bags are usually more practical. Above that, a ready-mix lorry is typically better value and less labour.