Cement Quantity Calculator Guide: Bags per Square Foot

The three numbers behind every estimate

Every cement estimate reduces to three things: the wet volume of the work, a dry volume factor that accounts for voids and wastage, and the mix ratio that tells you how much cement is in the dry volume. A 50 kg bag of cement occupies about 0.0347 cubic metres of volume, which is the conversion that turns bags into cubic metres.

Dry volume factors used in practice are around 1.33 for concrete and around 1.54 for mortar used in plaster and brickwork. These are planning figures, not laboratory constants: bulking of sand, moisture content and site wastage all move the final number.

Plaster

For 12 mm thick internal plaster at a 1:4 mix over 100 square feet, the wet volume is about 0.111 cubic metres. Multiply by 1.54 for dry volume to get roughly 0.171 cubic metres. In a 1:4 mix the cement share is one fifth of that, about 0.034 cubic metres, which is very close to one 50 kg bag. The 1:6 mix used on external or rougher work drops the cement share, and the same area then needs roughly three quarters of a bag.

As a working rule of thumb, internal plaster at 12 mm at 1:4 consumes around one bag per 100 square feet, and 6 mm finishing coats use about half of that. Thicker coats scale up almost linearly.

Brickwork

A 9 inch thick masonry wall at a 1:6 mortar mix uses roughly 0.03 cubic metres of mortar per square metre of wall. That works out to around 0.15 bags per square metre of wall at 9 inch thickness, or about one bag for every seven square metres. Half brick walls consume roughly half the mortar.

RCC concrete

For RCC, the mix is specified either by ratio or by grade. An M20 mix at 1:1.5:3 with a dry factor of 1.33 needs roughly 0.22 cubic metres of cement per cubic metre of concrete, which is about 6.3 bags. An M25 mix at 1:1:2 comes to roughly 0.24 cubic metres per cubic metre of concrete, about 7 bags. These are hand-mix figures; ready-mix supply is ordered in cubic metres and the plant handles the proportioning.

Where estimates go wrong

• Not separating wet and dry volume, which understates cement by about a third.
• Ignoring site wastage, which on bagged cement is realistically 5-8% between spillage, hardened part-bags and stacking loss.
• Forgetting the items that quietly consume cement: coping, plinth protection, mortar for block joints, and setting out work.
• Assuming one mix fits everything — structural concrete and finishing work rarely share a ratio.

Treat any estimate as a starting quantity with a sensible margin, then confirm with the actual drawing. Site conditions decide the final figure, and mix proportions for structural work should come from the structural consultant rather than a rule of thumb.

Send us the quantities and we will confirm availability and the delivery window on WhatsApp. Cement options are listed on the cement page, and the sand and coarse aggregate that the same estimate calls for are on the sand and aggregate page.