Cement is easy to buy too little of and expensive to buy too much of. The right amount is a function of volume and mix strength — two numbers you can nail down before the hardware store.
Volume and mix strength
Concrete isn't pure cement — it's a blend of cement, sand, aggregate and water. The cement share is set by the mix design. Multiply your volume by the cement-per-cubic-metre figure for that mix, and you have the cementneeded in weight.
Mixing strength to the job
A strong 1:1.5:3 mix carries structural loads; a lean 1:3:6 mix suits paths and footings where economy matters. Choosing by application — not by habit — gets the right balance of cost and performance.
Bags, yield and storage
Bags come in standard sizes, so the weight converts to a neat bag count. The biggest on-site enemy is moisture: cement that has set or partially reacted loses strength, which is why dry storage is non-negotiable.
Frequently asked questions
Compute your concrete volume, multiply by the cement per cubic metre for your mix (about 300–420 kg), then divide by the bag size. A 4×3×0.1 m slab at a general mix needs about 17 × 25 kg bags.
A general-purpose 1:2:4 mix (one part cement, two sand, four aggregate) suits most jobs. Use 1:1.5:3 for stronger structural work and 1:3:6 for leaner economy mixes.
Yes — stronger mixes carry more load but cost more and are harder to finish. Match the strength to the job and, for anything structural, to the engineered specification.