On a house site, the M20 concrete ratio is usually spoken as “ek, dedh, teen”: one part cement, one and a half parts sand, three parts aggregate. Behind that simple ratio sits a calculation that decides how many bags of cement to buy. This guide works it out per cubic metre, explains where the 1.54 factor comes from, and covers when M20 is not enough.
M20 concrete ratio in one line
M20 means concrete with a characteristic compressive strength of 20 N/mm² at 28 days. As a nominal mix it is proportioned 1 : 1.5 : 3 (cement : sand : 20 mm aggregate) by volume. Above M20, IS 456 expects design mixes proportioned by weight.
Material per cubic metre
Wet concrete is denser than the loose dry materials that go into it, because sand fills the gaps between stones and cement fills the gaps in sand. Estimators therefore multiply the wet volume by about 1.54 to get the dry volume of materials.
Dry volume = 1 m³ × 1.54 = 1.54 m³ Sum of ratio = 1 + 1.5 + 3 = 5.5 Cement = 1.54 × 1 ÷ 5.5 = 0.28 m³ × 1,440 kg/m³ ≈ 403 kg ≈ 8.06 bags Sand = 1.54 × 1.5 ÷ 5.5 = 0.42 m³ Aggregate 20 mm = 1.54 × 3 ÷ 5.5 = 0.84 m³
| Material | Quantity | In site units |
|---|---|---|
| Cement | ≈ 403 kg | ≈ 8 bags of 50 kg |
| Sand | ≈ 0.42 m³ | ≈ 14.8 CFT (≈ 0.15 brass) |
| 20 mm aggregate | ≈ 0.84 m³ | ≈ 29.7 CFT (≈ 0.3 brass) |
Why 1.54 and not 1.52 or 1.57?
It is a rule-of-thumb allowance for voids and shrinkage that varies slightly with materials. Some textbooks use 1.52 or 1.57. The difference is a few kilograms of cement per cubic metre.
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Water: the hidden strength killer
IS 456 limits the water-cement ratio for reinforced concrete to 0.55 in mild exposure and 0.50 in moderate exposure. For 403 kg of cement at 0.50, that is about 200 litres of water per cubic metre. Adding extra water to make concrete easier to pour is the most common reason site concrete falls short of its grade; plants use admixtures to keep workability without extra water.
Worked example: a small slab
A roof slab of 12 ft × 15 ft at 5 inches thick: 12 × 15 × 0.417 = 75 CFT = 2.12 m³. Add 5% for wastage: 2.23 m³.
- Cement: 2.23 × 8.06 ≈ 18 bags
- Sand: 2.23 × 0.42 ≈ 0.94 m³ ≈ 0.33 brass
- Aggregate: 2.23 × 0.84 ≈ 1.87 m³ ≈ 0.66 brass
When M20 is not enough
IS 456 allows M20 for reinforced concrete only in mild exposure. For moderate exposure (a humid climate, or concrete buried in non-aggressive soil) the minimum is M25, and coastal or severe conditions need more. That is why many structural engineers now specify M25 for columns and footings even in ordinary houses. The structural drawing is always the final word.
For slabs above a few cubic metres, ready-mix concrete (RMC) gives a consistent mix by weight and cube test results. The RMC calculator works out the order volume, number of trucks and cost with GST.
Curing matters as much as the ratio
Concrete gains strength by hydration, which needs water over time. Keep slabs wet (ponding or wet hessian) for at least 7 days with ordinary Portland cement, and longer with blended cements, especially in hot, dry weather.
Frequently asked questions
What is the M20 concrete ratio?
1 : 1.5 : 3 (cement : sand : aggregate) by volume as a nominal mix.
How many cement bags are needed for 1 m³ of M20?
About 8 bags of 50 kg using the 1.54 dry-volume factor.
How much sand and aggregate for 1 m³ of M20?
About 0.42 m³ of sand and 0.84 m³ of 20 mm aggregate.
What is the water-cement ratio for M20?
IS 456 allows a maximum of 0.55 in mild exposure and 0.50 in moderate exposure for reinforced concrete.
Is M20 enough for house columns?
Only in mild exposure. In humid or coastal areas IS 456 requires at least M25; follow the structural drawing.
Keep reading
- RMC Calculator: the free tool this guide explains.
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Sources: IS 456:2000 (concrete grades, exposure conditions, nominal mixes, water-cement ratio limits); cement bulk density 1,440 kg/m³, 50 kg bag.