Every trade on a building site runs on the same hidden arithmetic: turning a shape someone drew into a quantity someone can buy. The carpenter converts board feet, the tiler converts square metres, and the concrete foreman converts buckets. This guide is about the third of those, and it is the one where an error is hardest to spot, because a wall that is slightly too thin still stands up.
Start with volume, because volume is what you can actually measure
Concrete is specified as a ratio — 1:2:4, 1:1.5:3, 1:1:2 — and those numbers are volume ratios of cement, sand and coarse aggregate, not weights. A ratio like 1:2:4 means one bucket of cement to two of sand to four of stone, measured by volume in the same bucket. That matters more than it sounds, and the reason is moisture.
Sand arrives wet. A delivery described as "damp" carries perhaps 5–8% water by mass, and a rainy delivery carries more. If you mix by weight — 1 kg of cement to 6 kg of sand — the water already in the sand adds to the mixing water, and the ratio of water to cement silently rises. More water means easier pumping but weaker concrete, because strength is governed by the water-to-cement ratio rather than by the cement-to-aggregate one. A volume ratio measured on site does not have this failure mode: you scoop the same bucket of damp sand either way, and the water it carries is inside the number you already measured.
That is not a small detail. A drop of 0.1 in the water-to-cement ratio can cut compressive strength by a large fraction, and nothing about the finished surface will tell you. This is the same asymmetry described in the percent change guide, applied to materials: small changes in the part you are not measuring produce large changes in what you care about.
The method: divide by the sum of the parts
The field method is arithmetic you can do while standing next to the mixer. Write the ratio as three numbers and add them, because that sum is the total number of parts. Then take the required volume and divide by that sum to get the cement share.
Cement volume = required volume ÷ sum of ratio parts
For 1:2:4 the sum is 1 + 2 + 4 = 7. For 1:1.5:3 it is 5.5. For 1:1:2 it is 4. The lower the sum, the richer the mix and the more cement per cubic metre.
Next convert the cement volume into bags. A 40 kg bag of cement occupies about 0.0157 m³, which is 40 kg divided by an apparent density of roughly 2,550 kg/m³. So bags = cement volume ÷ 0.0157.
Worked check on a single cubic metre of 1:2:4: 1 ÷ 7 = 0.1429 m³ of cement, and 0.1429 ÷ 0.0157 = 9.1 bags. Slightly higher than the rule of thumb below, because this method counts the water volume as if it were aggregate — which is exactly why it is safe. Divide the same way for a 1:1:2 mix: 1 ÷ 4 = 0.25 m³, then 0.25 ÷ 0.0157 = 15.9 bags, against a computed 12.6 by the stricter method. The shortcut always rounds up.
The concrete calculator runs the stricter version and reports bags, aggregate volume and water together, which makes the difference easy to see.
How much concrete does one bag actually make?
Here is a figure that circulates and should not be trusted: that one 40 kg bag makes 0.30 m³ of finished concrete. It is off by more than a factor of two. Take it from the ratio instead.
One bag is 0.0157 m³ of cement. At 1:2:4 that is 1 part out of 7, so it contributes 7 × 0.0157 = 0.110 m³ of dry mix. Add the mixing water, about 19 litres for that bag at a water-to-cement ratio near 0.5, and one bag yields roughly 0.13 m³ of concrete.
Check that against the other direction: 1 ÷ 0.13 = 7.7 bags per cubic metre, which matches the accepted figure of about 8. Now apply the bad figure: 1 ÷ 0.30 = 3.3 bags per cubic metre. That would put roughly 130 kg of cement in a cubic metre of concrete, which is a lean screed, not a structural mix. If a rule of thumb hands you a number more than double the real one, it is not a rule of thumb, it is an error.
What each strength grade needs per cubic metre
Because the ratio sum drives everything, the table below follows directly from the formula. Values are for 40 kg bags at the stated water-to-cement ratio, which is the other thing that sets strength.
- 1:2:4, water-to-cement 0.50 — about 8 bags per m³. Suitable for non-structural work: floor screeds, small pads, blinding.
- 1:1.5:3, water-to-cement 0.50 — about 10 bags per m³. The usual domestic structural grade for slabs, footings and walls.
- 1:1:2, water-to-cement 0.45 — about 12.6 bags per m³. Richer and stronger; a common choice for columns and suspended slabs.
- 1:1:2, water-to-cement 0.38 — about 13 bags per m³. High-strength work where durability matters as much as strength.
The interesting part is the gap between the last two lines: dropping the water-to-cement ratio from 0.45 to 0.38 buys a large strength increase for only about 0.4 extra bags. Strength responds far better to lowering the water ratio than to adding cement, which is why the specification always quotes both numbers. The concrete calculator takes the grade and the water ratio rather than the ratio alone, for exactly this reason.
Worked example: a 4.2 × 3.6 m floor slab
This is the worked example to keep, because every step can be checked by hand with a tape and a calculator. Pour a ground-floor slab 4.2 m by 3.6 m at 100 mm thick in 1:2:4, and add 8% for waste.
Step 1 — volume. 4.2 × 3.6 × 0.100 = 1.512 m³.
Step 2 — cement volume. Ratio sum is 7, so 1.512 ÷ 7 = 0.216 m³ of cement.
Step 3 — bags. 0.216 ÷ 0.0157 = 13.76, so 14 bags before any allowance. The other ingredients follow from the same division, scaled by their parts:
- Cement: 1 part → 0.216 m³ → 14 bags of 40 kg
- Sand: 2 parts → 0.432 m³, and allow about 15% for bulking and spillage → about 0.50 m³
- Coarse aggregate: 4 parts → 0.864 m³, allowing about 15% → about 1.00 m³
- Water: roughly half the cement mass, and cement is 14 × 40 = 560 kg → about 280 litres
Step 4 — waste. 14 bags × 1.08 = 15.1, so order 16 bags. On sand and aggregate, 8% takes 0.50 m³ to 0.54 m³ and 1.00 m³ to 1.08 m³.
Step 5 — sanity check on weight. One cubic metre of plain concrete weighs about 2,400 kg. The mix above totals 560 kg of cement, about 690 kg of sand and about 1,590 kg of stone with 280 kg of water, which is roughly 3,120 kg over 1.512 m³ — about 2,060 kg/m³ before the aggregate bulking is added back. A result in the 2,000–2,500 kg/m³ band is right; if you get 3,500 you have double-counted something. That single check catches most arithmetic slips.
Per square metre of column: where the grade really shows
Because quantities are easier to compare per unit of structure, take a 300 × 300 mm column and ask what one metre of its length consumes. That is 0.09 m³ of concrete.
- At 1:1.5:3 (about 382 kg/m³) → 34 kg of cement per metre of column.
- At 1:1:2 with a 0.45 ratio (about 503 kg/m³) → 45 kg per metre.
- At 1:1:2 with a 0.38 ratio (about 521 kg/m³) → 47 kg per metre.
The step from a standard structural grade to a richer one costs roughly 11 kg of cement per metre of column — about a quarter more. The step from a 0.45 to a 0.38 water ratio costs about 2 kg. Reading those two numbers side by side is the practical argument for specifying by water-to-cement ratio rather than by mix ratio alone.
Mortar for walls
Mortar has no coarse aggregate, so the ratio sums are much smaller. A 1:3 mortar is 1 cement to 3 sand, summing to 4; a 1:4 mortar sums to 5. At 0.0157 m³ per bag, one bag of cement produces roughly 0.08 m³ of 1:3 mortar or 0.10 m³ of 1:4 mortar, including the mixing water.
Per cubic metre, that works out to about 12 bags for 1:3 and about 10 for 1:4 — noticeably more cement per cubic metre than in 1:2:4 concrete, which is why mortar is expensive to buy and is frequently pre-mixed. A 10 mm joint on standard brick consumes roughly 0.02 m³ of mortar per square metre of wall face.
Counting bricks with a module, not a brick
Brick quantities are usually wrong on the first attempt because people count bricks when they should count modules. A brick does not occupy 240 × 115 × 53 mm in a wall; it occupies its own dimensions plus a joint on every side.
With standard 10 mm joints the module is 250 × 125 × 63 mm = 0.001969 m³. So one cubic metre of brickwork holds 1 ÷ 0.001969 = 508 bricks.
Per square metre this is easier to use on site. A half-brick wall, which is 115 mm of brick plus joints, holds 63.5 bricks per m². A full 240 mm wall holds about 127. The general rule is: divide the wall thickness in millimetres by the module height of 63 mm to get bricks per square metre.
Mortar falls out of the same arithmetic. The bricks themselves occupy 508 × 0.001463 = 0.743 m³, so the remaining 0.257 m³ — about a quarter of the wall volume — is mortar. That single figure also checks any brick estimate you have done: a brick quantity implying 8% mortar is wrong, and a quantity implying 45% is equally wrong.
A trap worth naming. If you take the brick's own size and add only 5 mm to the width and 2 mm to the height, you get 240 × 120 × 55 = 0.001584 m³ and a count of 631 per m³. That looks plausible and is too high, because it ignores the joint along the length and understates it everywhere else. It implies mortar is only 7.7% of the wall, when the real figure is nearer a quarter. When a brick count and a mortar estimate disagree by a factor of three, the brick count is wrong.
Blinding, lean concrete, and structural concrete
Three different materials get called "concrete" on site, and they are not interchangeable.
Blinding is a weak, cheap layer — typically 1:4:8 or even 1:5:10 — poured 50 to 75 mm thick under foundations to level the ground and stop the structural concrete losing water into soil. It is measured by area and thickness like everything else, but it needs far less cement: roughly 4 to 5 bags per cubic metre.
Lean concrete sits between blinding and structural work, around 1:3:6, and is used for floor slabs below the main structure or for a working surface.
Structural concrete is the designed mix, typically 1:1.5:3 or richer, and it carries load. Using lean concrete where structural concrete was specified is a serious error that no visual check will catch. Before ordering, confirm which of the three the drawing actually calls for.
Waste: one percentage, applied once
Use 5% for simple work with few cuts, and 10% where there is pumping, complex formwork, many openings or awkward access. The important discipline is applying it once, to the total. Adding 5% to the cement, then 5% to the sand, then 5% to the aggregate does not give a 5% allowance — it gives about 16%, and the extra cement is pure cost.
Record the volume, the ratio, the water-to-cement ratio and the waste assumption together. When the order arrives, the four numbers let anyone check the delivery, and that is the difference between a concrete order and a guess. Estimating is easier when the dimensions are already measured properly — the room measurements guide covers getting those first, and lumber and framing covers the timber side of the same trade.
Frequently asked questions
How many bags of cement do I need per cubic metre of concrete?
For a 1:2:4 mix, roughly 8 bags of 40 kg per m³. A 1:1.5:3 mix needs about 10, a 1:1:2 mix about 12.6. The figure follows from dividing the total mix volume by the sum of the ratio parts, then converting the cement share using a 40 kg bag at about 0.0157 m³.
Why is a volume ratio better than a weight ratio?
Because aggregate moisture varies. Sand absorbs different amounts of water between deliveries, and a weight recipe quietly changes the water content and therefore the strength. A volume ratio measured on site stays fixed even when the sand is damp, which is why it is the standard way to specify a mix.
How many bricks are in one cubic metre of brickwork?
About 500 for standard 240×115×53 bricks laid with 10 mm joints. That figure comes from the module size of 250×125×63 mm, which allows for the joint. Multiplying by 508 bricks and subtracting the brick volume gives roughly 0.26 m³ of mortar per m³ of wall.
What waste allowance should I add to concrete and bricks?
Use 5% for straightforward work with few cuts and 10% for anything with awkward shapes, pumping losses or many openings. Apply the percentage to the total quantity, not to each ingredient separately, because a 5% loss on every component compounds into more material than the job actually needs.