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Kg to Bags Converter — How Many Bags Do I Need?
Convert a total weight in kilograms to the number of bags you need to buy. Covers cement, plaster, sand, aggregate, postcrete, and more with standard UK bag sizes.
How many kg of material do you need in total?
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Set to override the standard bag size (0 = use standard size)
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How We Calculate This
This converter divides your total weight requirement by the selected bag size, then rounds up to the nearest whole bag. You cannot buy partial bags, so we always round up to ensure you have enough material.
The formula
Bags needed = ROUNDUP(Total weight ÷ Bag size)
For example, if you need 120kg of cement in 25kg bags: 120 ÷ 25 = 4.8, rounded up = 5 bags. You'll have 5kg spare (5 × 25 = 125kg total purchased).
Standard UK bag sizes
- Cement: 25kg bags
- Plaster: 25kg bags
- Sand / Aggregate: 25kg bags or 850kg bulk bags
- Postcrete: 20kg bags
- Self-Levelling Compound: 20kg bags (most brands; some come in 25kg)
- Tile Adhesive: 20kg bags
- Grout: 5kg bags
Frequently Asked Questions
Standard UK cement bags are 25kg. This applies to most brands including Blue Circle, Hanson, and Cemex. Some specialist products come in different sizes, but 25kg is the industry standard available at all builder's merchants, Screwfix, B&Q, and Wickes.
A standard bulk bag (also called a jumbo bag or ton bag) of sand or aggregate typically weighs around 850kg, not a full tonne as many people assume. This is because the bags are filled by volume, and the weight varies by material density. Always check with your supplier for exact weights.
A tonne (1,000 kg) requires 40 × 25kg bags. This is useful to know when comparing the price of buying individual bags versus ordering bulk. Bulk delivery is almost always cheaper for quantities above 20-30 bags.
Yes, always round up to the next whole bag. You cannot buy partial bags, and it is far better to have a small amount left over than to run short mid-job. The remainder shown in the calculator tells you how much spare material you will have.
Postcrete (post-mix concrete) comes in 20kg bags, not 25kg like standard cement. Bag count is driven by the hole VOLUME (diameter × depth), not the fence height. As a guide for a 600mm-deep hole: a 200mm-wide hole (light, sheltered) takes about 1 bag, a 250mm-wide hole (standard 6ft fence) about 2 bags, and a 300mm-wide hole (heavy or exposed/coastal site) about 3 bags. Tarmac's own data sheet rates 1 bag for a 100mm post in a 200mm-wide × 750mm-deep hole. These are typical-conditions guides — measure your hole and add a spare bag per run so you don't run short mid-job.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.