Ask three contractors which concrete to use for a house and you will hear three different “ratios”. The code answers the question differently: grade is a strength, not a recipe, and the minimum grade is set by exposure and structural design — not by habit.
What a grade actually means
M25 means a characteristic compressive strength of 25 N/mm² — the strength that at least 95% of 150 mm cubes must reach at 28 days. The number is a performance target, verified by cube tests, not a promise made by a volumetric ratio.
Nominal mix vs design mix
- Nominal mix: fixed proportions (1:1.5:3 is conventionally associated with M20). IS 456 permits nominal mixes only up to M20, and they inherently over-use cement to cover variability.
- Design mix: proportions engineered (IS 10262) for the target strength and workability with the actual site materials. All grades M25 and above must be design mixes.
- Ready-mix concrete (RMC) is the practical way most projects get a controlled design mix, with batching records and slump control.
When M20 is enough — and when the code forces M25+
Per IS 456 Table 5, M20 is the minimum for reinforced concrete only in mild exposure — interiors protected from weather. Members exposed to rain and weather (external columns, chajjas, parapets, plinth region) fall under moderate exposure, where the code minimum is M25. Severe conditions (coastal spray, wet zones) push the minimum to M30.
| Member / situation | Sensible grade |
|---|---|
| Footings and plinth (in contact with soil/moisture) | M25 |
| Columns and beams, protected interior, low-rise | M20 minimum — M25 preferred |
| External members, coastal towns | M30 |
| Slabs, typical residential | M20–M25 |
| Anything for a building of 4+ storeys | M25 and above, by design |
The two things that ruin any grade
- Water: added on site “for easy pouring”, it silently converts M25 into M15. Strength is controlled by the water–cement ratio more than by cement content. Use slump as the control, never a hosepipe.
- Curing: strength quoted at 28 days assumes continuous moist curing. Two days of wetting followed by neglect can forfeit a large fraction of the strength you paid for.
Verify, don’t assume
Cast test cubes (a set of 3 per sample, sampled per IS 456 frequency rules), cure them in water, and test at 28 days. For RMC, keep the delivery/batch slips. These records cost almost nothing and settle every future argument about what was actually built.
Frequently asked questions
Is 1:1.5:3 the same as M20?
Only loosely. 1:1.5:3 is the conventional nominal mix associated with M20, but the strength actually achieved depends on the water added, aggregate quality and curing. M20 is a tested strength, not a ratio.
Can I use M20 for footings?
Footings are in contact with soil and moisture, which is generally moderate (or worse) exposure — for which IS 456 sets M25 as the minimum for reinforced concrete. M20 footings are common historically, but M25 is the code-aligned choice.
Is ready-mix concrete better than site-mixed?
For controlled quality, usually yes: weigh-batched materials, controlled water, documented grade. Site mixing can achieve good results but needs measured water, competent supervision and cube testing to prove it.