🧱 Cement Bags Calculator UK

For a 3m × 3m concrete slab at 100mm depth using a standard 1:2:4 mix, you need approximately 9m² × 0.1m = 0.9m³ of concrete. At roughly 6–7 bags of 25kg cement per m³, that's 6–7 bags before waste. Adding a 10% waste factor gives you 7–8 bags of 25kg cement. You'll also need approximately 1,100kg of sand and 1,800kg of aggregate (20mm) for the same volume.

The number of 25kg cement bags per m³ depends on the mix ratio:

• 1:2:4 (General Purpose): 6–7 bags per m³
• 1:1.5:3 (Strong / C25): 9–10 bags per m³
• 1:3:6 (Foundation / Lean): 4–5 bags per m³
• 1:3 (Mortar / Pointing): 11–12 bags per m³
• 1:4 (Mortar / Screed): 8–9 bags per m³

Always add a minimum 10% waste factor to these figures.

These three terms are often confused but describe different materials:

• Cement — the dry powder binder (e.g. Ordinary Portland Cement) that reacts with water to set and harden
• Concrete — cement mixed with sand AND coarse aggregate (gravel/stone) and water — used for structural work, slabs, and foundations
• Mortar — cement mixed with sand and water only (no aggregate) — used for laying bricks, blocks, pointing, and render

Our cement bags calculator works for all three types based on your selected mix ratio.

For volumes under approximately 1m³, mixing your own from bagged cement is usually more cost effective and practical. For volumes over 1m³ — such as large slabs, driveways, or extensions — ready-mix concrete delivered by truck is often cheaper per m³, more consistent in quality, and saves significant labour. Ready-mix typically costs £90–£130 per m³ delivered in the UK, compared to £120–£160+ to mix 1m³ from bags including labour. Use a tool like The Concrete Centre to explore specification options.

An unopened cement bag stored correctly in a cool, dry place off the ground lasts 3–6 months. Once opened, exposure to air moisture begins the hydration process — use within 24 hours of opening for best results. Signs of deterioration include lumps, hardened clumps, or a greyish discolouration. Never use cement that has gone hard or lumpy — it will produce a significantly weaker and unreliable mix. Store bags on wooden pallets, never directly on concrete floors.

The type of sand matters significantly for the finished result:

• Sharp sand (grit sand): Use for concrete, screed, and render. Angular grains provide strength and drainage
• Soft sand (building sand): Use for mortar when laying bricks and blocks. Fine grains improve workability and adhesion
• Silver sand: Very fine — used for pointing, jointing, and specialist decorative work only
• Plastering sand: Pre-graded for internal plaster and finishing coats

Never use beach sand or unwashed sand in cement mixes — salt contamination causes long-term corrosion and strength loss.