Cement Calculator

Nine bags for a cubic metre

One cubic metre of M20 concrete needs about 9 bags of cement, 0.42 m³ of sand and 0.84 m³ of coarse aggregate.

That per-cubic-metre figure is the useful one to carry around, because it scales directly. A 12 m³ pour needs roughly 108 bags; a 0.5 m³ footing needs about 5.

How the bag count is derived

M20 uses a nominal mix of 1 : 1.5 : 3 by volume. The dry material required is about 1.54 times the wet concrete volume, so 1 m³ of finished concrete needs roughly 1.54 m³ of dry ingredients.

Cement's share is 1 part of 5.5 total: 1.54 × (1 ÷ 5.5) = 0.28 m³.

Converting to bags at a bulk density of 1,440 kg/m³ and 50 kg per bag: 0.28 × 1,440 ÷ 50 ≈ 8.06 bags, rounded up to 9 once wastage and partial bags are accounted for, since cement is ordered whole.

For M15 at 1 : 2 : 4, cement's share is 1 of 7 parts, giving roughly 6.3 bags per cubic metre — noticeably less cement for correspondingly less strength.

Choosing a grade, and a cement type

M15 suits plain concrete: levelling courses, mass fill, footpaths, non-structural bedding.

M20 is the general-purpose residential grade for reinforced work — slabs, beams, columns and foundations in ordinary construction.

M25 and above should be design mixes, proportioned by an engineer against actual loads rather than by nominal ratio.

Cement type matters too. OPC 43 and OPC 53 grades differ in strength gain — 53 grade develops strength faster and is used where early loading matters. PPC, blended with fly ash, gains strength more slowly but produces lower heat of hydration, better long-term durability and improved resistance to sulphates. For most residential work PPC is an entirely reasonable and often preferable choice, provided curing is extended accordingly.

Storage, which spoils more cement than anything else

Cement is hygroscopic — it absorbs moisture from the air and begins hydrating in the bag. Practical rules:

Store off the floor on timber battens or a plastic sheet, away from external walls, in a covered dry space. Stack no more than 10 bags high to avoid compaction. Use within three months of manufacture; cement stored longer loses strength measurably, and the date is printed on the bag.

A bag with hard lumps that do not crumble under finger pressure has already partly hydrated and should not be used in structural work.

Reading the result

The quantities are nominal-mix estimates for ordering purposes. Add 5–10% for wastage on any real order.

Note that the ratio is by volume in the nominal system, while the design mixes used for structural work specify by weight, which is more precise because it removes the variability of how loosely sand is measured. Site batching by volume — the ubiquitous head-pan or box measure — carries real error, and damp sand in particular bulks by up to 25–30%, meaning a volume measure of wet sand contains substantially less actual sand than the same measure dry.

That bulking effect is the most common reason site concrete comes out sand-rich and weaker than intended.

Common mistakes

Measuring damp sand by volume without allowing for bulking. It can add 25–30% apparent volume.

Using old or lumpy cement. Strength falls with storage age.

Applying nominal ratios above M20. Higher grades need designed mixes.

Stacking bags against a wall or on bare ground. Moisture ingress ruins the bottom layers.

Ordering exactly. Round up; a pour interrupted for want of a bag is a cold joint.