Concrete Bags Calculator
Estimates how many premix concrete bags are needed for a slab or footing volume.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the length, width and depth.
- Optionally set the yield per bag, then Calculate.
How it works
The volume is length × width × depth. Divide by the yield per bag (about 0.01 m³ for a 20 kg pre-mix) to get the number of bags.
Worked example
2 m × 2 m × 0.1 m. Volume 0.4 m³ ÷ 0.01 = 40 bags.
Common mistakes
- Forgetting that a bag yields less volume than its weight suggests — check the litres-per-bag figure.
- Not adding waste; order about 5–10% extra for spillage and uneven ground.
Frequently asked questions
How many bags of concrete do I need?
Divide the volume needed by one bag's set yield (often around 0.01–0.017 m³ per 20 kg bag), round up and add waste.
When should I use bags instead of ready-mix?
Bags suit small jobs like post holes and small pads; over roughly a cubic metre, ready-mix is usually cheaper and easier.
Bags or pre-mixed delivery?
Bags suit small jobs; for anything over about 0.5 m³ a concrete delivery is usually cheaper and easier.
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Cite this tool
Zerdly. (2026). Concrete Bags Calculator. Retrieved from https://www.zerdly.com/tools/concrete-bags-calculator/
Tip: Enter any known values to calculate the remaining results.
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