Key takeaways
- One cubic metre of M20 concrete needs about 403 kg of cement (8 bags), 0.42 m³ of sand and 0.84 m³ of aggregate, plus roughly 200 litres of water.
- The whole calculation rests on one number: 1.54, the dry volume factor that converts wet concrete back into loose materials.
- M20's nominal ratio is 1 : 1.5 : 3 by volume, which is why gauge boxes exist and headpans do not work.
- Damp sand bulks by 20–30%, so measuring it by loose volume without allowing for that quietly under-sands the mix.
This is probably the most-searched calculation in construction, and most of the answers online give you the numbers without the method. The numbers are only useful for exactly 1 m³ of exactly M20. The method works for any grade and any volume, which is what you actually need on site.
Start with the ratio
M20's nominal mix under IS 456 Table 9 is 1 : 1.5 : 3 — one part cement, one and a half parts fine aggregate (sand), three parts coarse aggregate (stone). Add those up and you have 5.5 parts in total. Everything else follows from that number and one correction factor.
Note that this is a ratio by volume, not weight. Nominal mixes are volumetric by definition, which is why properly run sites batch with gauge boxes sized to one bag of cement rather than counting headpans.
Why you multiply by 1.54
Here is the step that trips people up. If you measure out 1 m³ of dry ingredients and mix them with water, you do not get 1 m³ of concrete. You get noticeably less.
Loose dry sand and aggregate are full of air voids between the particles. Cement paste fills those voids, and compaction — vibration, tamping — drives the remaining air out. The material densifies. Working backwards, producing 1 m³ of compacted wet concrete takes roughly 1.54 m³ of dry material.
Dry volume = Wet volume × 1.54
Figures between 1.52 and 1.57 are all in common use. 1.54 is the usual middle ground; use 1.57 if your aggregate is poorly graded.
That single factor is the difference between an order that covers the pour and one that leaves you a bag short at four in the afternoon.
The calculation, step by step
For 1 m³ of finished M20 concrete:
- Dry volume. 1 × 1.54 = 1.54 m³
- Total parts. 1 + 1.5 + 3 = 5.5
- Cement volume. 1.54 × (1 ÷ 5.5) = 0.28 m³
- Cement mass. 0.28 × 1440 kg/m³ = 403 kg → 403 ÷ 50 = 8.06 bags
- Sand volume. 1.54 × (1.5 ÷ 5.5) = 0.42 m³ → about 672 kg at 1600 kg/m³
- Aggregate volume. 1.54 × (3 ÷ 5.5) = 0.84 m³ → about 1260 kg at 1500 kg/m³
- Water. At a water–cement ratio of 0.5: 403 × 0.5 = 202 litres
The 1440 kg/m³ in step 4 is the bulk density of loose cement, which is why a 50 kg bag occupies 0.0347 m³ — a useful number to remember, because it is what gauge boxes are sized against.
| Material | Volume (m³) | Cubic feet | Mass (kg) | Per cubic yard |
|---|---|---|---|---|
| Cement | 0.28 | 9.89 | 403 | 680 lb |
| Sand (fine aggregate) | 0.42 | 14.83 | 672 | 1133 lb |
| Aggregate (coarse) | 0.84 | 29.66 | 1260 | 2124 lb |
| Water | 0.20 | 7.13 | 202 | 40.7 US gal |
Per 1 m³ of M20 concrete, nominal 1:1.5:3. Cubic feet at 35.315 cft per m³.
Worked example: a real slab
A roof slab measuring 6.0 m × 4.2 m, 150 mm thick.
- Wet volume. 6.0 × 4.2 × 0.15 = 3.78 m³
- Cement. 3.78 × 403 = 1523 kg → 1523 ÷ 50 = 30.5 bags → order 31 bags
- Sand. 3.78 × 0.42 = 1.59 m³ (56.1 cft)
- Aggregate. 3.78 × 0.84 = 3.18 m³ (112.1 cft)
- Water. 3.78 × 202 = 764 litres
Round the cement up, never down — a part-used bag is cheaper than a cold joint. And this figure is for the slab alone; add the beams and any drop, and allow 3–5% for spillage and formwork leakage on a site pour.
Scaling to other grades
The method does not change. Only the number of parts does:
| Grade | Nominal ratio | Total parts | Cement (kg/m³) | Bags/m³ |
|---|---|---|---|---|
| M10 | 1 : 3 : 6 | 10 | 222 | 4.4 |
| M15 | 1 : 2 : 4 | 7 | 317 | 6.3 |
| M20 | 1 : 1.5 : 3 | 5.5 | 403 | 8.1 |
IS 456 Table 9 stops at M20 — above that, use a design mix. See M20 vs M25 for why that matters.
Three things that quietly ruin the mix
- Bulking of sand. Damp sand occupies 20–30% more loose volume than dry sand of the same mass. Measured by volume without correction, the mix ends up sand-poor and harsh. Test for bulking when the sand is delivered, not after the pour.
- Water added by eye. Every extra litre past the design water-cement ratio costs strength permanently. Workability problems are solved with a plasticiser or better grading, not a hose.
- Headpan batching. Volumetric mixes are only as accurate as the container. Gauge boxes sized to one 50 kg bag — 0.0347 m³, typically 300 × 300 × 385 mm — keep the ratio honest.
Frequently asked questions
How many cement bags for 1 m³ of M20?
About 8 — precisely 8.06. The 1:1.5:3 mix gives 0.28 m³ of cement per cubic metre, which at 1440 kg/m³ is 403 kg, or 8.06 bags of 50 kg.
Why 1.54 and not some other factor?
Dry materials contain air voids between the particles. Adding water and compacting drives that air out, so the mix occupies less space than its dry ingredients did. It takes roughly 1.54 m³ of dry material to make 1 m³ of compacted concrete. Anything from 1.52 to 1.57 is defensible.
How much water for 1 m³ of M20?
About 200 litres, from a water-cement ratio of 0.5 on 403 kg of cement. IS 456 caps the ratio at 0.55 for reinforced concrete in mild exposure. Deduct any free moisture already in the sand.
Is 1:1.5:3 by volume or by weight?
By volume — that is what makes it a nominal mix. Design mixes are proportioned by weight, which is more accurate because weight is unaffected by the bulking of damp sand.