Concrete Bag Calculator — How Many Bags Do I Need?
Calculate how many bags of concrete mix you need for a slab, footing or post holes. Supports 25 kg and 40 kg bags in metric and 60 lb and 80 lb bags in imperial, with waste factor and a ready-mix comparison.
How the bag count is calculated
- 1Volume = length × width × depth (m³ or ft³)
- 2Add 5–10% for waste, uneven formwork and spillage
- 3Bags needed: volume ÷ yield per bag (0.012 m³ for a 25 kg bag, 0.019 m³ for 40 kg)
- 4Ready-mix alternative: order in m³, rounded up to the nearest 0.25 m³
- 5Mix strength: C20/25 for most domestic slabs, C25/30 for driveways, C30/37 for structural footings
- 6Round bags up to whole bags — partial bags can't be poured later
Worked example
Concrete slab: 4 m × 3 m × 0.1 m depth = 1.2 m³. With 8% waste: 1.3 m³.
You need 109 bags of 25 kg premix, or approximately 5–6 bags of 40 kg premix per m² at 100 mm depth. For ready-mix, order 1.5 m³.
Frequently asked questions
How many bags of concrete for a 10×10 slab?
A 10×10 ft slab at 4 inches deep = 0.93 m³. With 8% waste: ~1 m³. That is approximately 83 bags of 25 kg premix or 53 bags of 40 kg premix (each 40 kg bag yields approx. 0.019 m³).
How much concrete does one bag make?
A 25 kg bag yields approximately 0.012 m³; a 40 kg bag approximately 0.019 m³. Check the bag — yields vary slightly between products.
When is ready-mix cheaper than bags?
Ready-mix is usually more economical above approximately 1 m³ and gives more consistent quality. Below 1 m³, bagged premix is more practical. Check truck access before ordering ready-mix.
How long does bagged concrete take to cure?
Concrete reaches approximately 70% of its design strength after 7 days and 99% after 28 days. Keep it moist for at least 7 days and don't load it during that time.