There is a standard question every bricklayer gets asked by the person supplying materials: how many bags of cement do I need? And the standard answer is: how many square metres of wall?
The calculation is not complicated but it does require a few numbers that are not immediately obvious - the mortar volume per m2, the mix ratio, and the bulking factor for dry materials. Most people estimating mortar for the first time underorder, then have to make an emergency trip to the merchant mid-job. A few over-order by so much they have half a pallet of cement curing in the shed.
Here is what the numbers actually are.
Mortar volume per m2
The amount of mortar in a brick wall depends on the brick size, joint thickness and bond pattern. For standard UK brick construction with 10mm joints:
| Wall type | Bricks per m2 | Mortar per m2 (approx) |
|---|---|---|
| Half-brick wall (102.5mm thick) | ~60 | 0.015 m3 (15 litres) |
| Full-brick wall (215mm thick) | ~120 | 0.030 m3 (30 litres) |
| 100mm blockwork | ~10 blocks | 0.007 m3 (7 litres) |
| 140mm blockwork | ~10 blocks | 0.009 m3 (9 litres) |
| 215mm blockwork | ~10 blocks | 0.012 m3 (12 litres) |
These are net figures before wastage. Add 15% wastage for mortar - it is a genuinely messy process and significant material is lost.
Converting mortar volume to cement and sand
Once you have your mortar volume, you need to apply the mix ratio to work out cement and sand separately.
For a 1:4 mix (1 cement : 4 sand by volume):
- Cement fraction = 1 / (1+4) = 0.2 (20%)
- Sand fraction = 4 / (1+4) = 0.8 (80%)
Apply a 1.25 bulking factor - dry materials take up more volume than their contribution to the wet mix. This is a practical correction for real-world site mixing.
Formula:
- Cement volume = mortar volume x 0.2 x 1.25
- Sand volume = mortar volume x 0.8 x 1.25
Then convert to weight:
- Cement: volume x 1440 kg/m3 (loose density)
- Sand: volume x 1600 kg/m3 (loose sharp sand)
Worked example - 20m2 half-brick wall, 1:4 mix
- Net mortar volume = 20 x 0.015 = 0.30 m3
-
- 15% wastage = 0.345 m3
- Cement volume = 0.345 x 0.2 x 1.25 = 0.086 m3
- Cement weight = 0.086 x 1440 = 124 kg = 5 x 25kg bags
- Sand volume = 0.345 x 0.8 x 1.25 = 0.345 m3
- Sand weight = 0.345 x 1600 = 552 kg = approximately 0.55 tonnes
Or if using pre-mixed mortar (25kg bags, yield ~0.012 m3 each):
- 0.345 / 0.012 = 29 bags of pre-mixed mortar
Quantities by wall area at 1:4 mix
For a standard half-brick wall with 10mm joints and 15% wastage:
| Wall area | Mortar volume | Cement (25kg bags) | Sharp sand | Pre-mixed bags |
|---|---|---|---|---|
| 5m2 | 0.086 m3 | 2 bags | ~0.14t | ~8 bags |
| 10m2 | 0.173 m3 | 3 bags | ~0.28t | ~15 bags |
| 20m2 | 0.345 m3 | 5 bags | ~0.55t | ~29 bags |
| 30m2 | 0.518 m3 | 8 bags | ~0.83t | ~44 bags |
| 50m2 | 0.863 m3 | 13 bags | ~1.38t | ~72 bags |
These are estimates. Actual quantities depend on joint width, brick size variation and mixing method.
Mix ratio effect on cement quantity
The mix ratio significantly affects cement consumption - and therefore cost, since cement is the expensive component.
| Mix ratio | Cement per m3 of mortar | Cement for 10m2 wall (inc 15% wastage) |
|---|---|---|
| 1:3 | 360 kg | ~6.2 bags |
| 1:4 | 288 kg | ~5.0 bags |
| 1:5 | 240 kg | ~4.1 bags |
| 1:6 | 206 kg | ~3.5 bags |
Moving from 1:3 to 1:5 nearly halves your cement requirement. This matters on large jobs - it is also why using the right mix (not the strongest one available) saves money without compromising the structure.
My tips on mortar quantities
Order a bit more sand than you think. Sand is cheap and bulky. Running short of sand means stopping work. Most builders merchants will take back unused bulk bags or deliver additional quantities, but there is lead time. Start with slightly more than the calculation suggests.
Keep a bag count as you go. On any job of more than 10-15 bags, note how many bags you have opened vs how many you expected to use at this stage. This lets you spot quickly if the mix is inconsistent or the joint width has crept up.
Bag the cement the day before in a dry shed. On a large job, pre-counting your bags and staging them near the mixer saves time and makes it easier to keep track.
Do not mix more than you can lay in 30-45 minutes. Fresh mortar stiffens as the cement hydrates. Working time varies with temperature - longer in winter, shorter in hot weather. Mix in quantities you can use within the working window.
Use the Mortar Calculator for accurate cement and sand quantities based on your wall area and mix ratio.