Concrete Bag Calculator
Convert a concrete volume into the number of pre-mixed bags needed, for small pours where ready-mix is not practical.
Formula
Volume ÷ the volume one bag produces, rounded up to a whole bag.
- Volume (m³) = L × W × T × (1 + wastage % ÷ 100)
- Bags = ceiling( Volume ÷ Bag yield )
Worked example
A 2 m × 1 m pad, 100 mm thick, using 25 kg bags that yield 0.0125 m³ each, at 10% wastage.
- Net volume = 2 × 1 × 0.1 = 0.20 m³
- With wastage = 0.20 × 1.10 = 0.22 m³
- Bags = 0.22 ÷ 0.0125 = 17.6
- Round up = 18 bags
18 bags, which is 450 kg of pre-mix.
Assumptions and limitations
- The bag yield you enter is the finished concrete volume from one bag, not the dry weight. The two are different numbers and mixing them up is the most common error here.
- Yield depends on the product and how much water is added. The figure printed on the bag is the one to use.
- No allowance is made for bags that split, get wet, or are part-used and go off.
What this calculator does not do
- Does not choose a concrete grade or check strength. Bagged products state their own strength class; this tool only counts bags.
- Does not compare bagging against ready-mix. Above roughly one cubic metre, ready-mix is normally cheaper and more consistent, and that judgement is yours to make.
- Does not account for mixing losses, spillage or the water you add. The yield figure on the bag already assumes correct water addition.
Practical tips
- Buy one or two spare bags. Running out mid-pour on a small slab leaves a cold joint you cannot undo.
- Above roughly 0.5–1 m³, hand-mixing stops being sensible. Price a ready-mix load or a mini-mix truck before committing to 60 bags.
- Store spare bags off the ground and covered. Cement picks up moisture from the air and hardens in the bag.
Frequently asked questions
How many bags of concrete do I need per cubic metre?
Divide 1 m³ by the yield printed on the bag. At a typical 0.0125 m³ per 25 kg bag, that is 80 bags per cubic metre — which is why ready-mix wins quickly on larger pours.
Is bag yield the same as bag weight?
No, and confusing them is the usual mistake. A 25 kg bag weighs 25 kg dry but produces only about 12.5 litres of finished concrete. Always use the yield figure.
When should I switch from bags to ready-mix?
Around 0.5–1 m³ for most people. Beyond that the bag count, the mixing time and the risk of a cold joint all work against you.
Why does the calculator round up?
Because bags are sold whole. Rounding down would leave you short at the worst possible moment — part-way through a pour.
Related calculators
- Concrete Volume Calculator Work out the volume of concrete needed for a rectangular pour, with an allowance for wastage.
- Concrete Slab Calculator Size a floor or ground slab: volume, ready-mix load, weight and area, including an edge thickening if there is one.
- Concrete Footing Calculator Calculate concrete for strip footings or a set of identical pad footings.
- Mortar Calculator Convert a mortar volume into cement, sand and water quantities for a given mix ratio.
- Plaster Calculator Calculate plaster or render volume for a wall, and the cement and sand it needs.
Estimating tool, not a design. This calculator works out quantities from the dimensions you enter. It does not size any structural member, verify a design, or confirm compliance with any building code. Check every result against your project specification and applicable local standards, and involve a qualified engineer where the work requires one.