"What grade should we use?" often gets the answer "M20, like always". Under IS 456, the right answer depends on where each member is and what it is exposed to.
Grade by Part of the House
| Member | Typical grade | Why |
|---|---|---|
| PCC bed under footings | M10 – M15 | Plain concrete, level base |
| Footings | M25 | Buried in soil: moderate exposure |
| Plinth beam | M20 – M25 | Near ground; follow drawing |
| Columns, beams, internal slabs | M20 – M25 | Mild exposure minimum M20 |
| Roof slab, sunshades, exterior members | M25 | Exposed to rain: moderate exposure |
| Sump, water tanks | M25 – M30 | Watertightness, IS 3370 practice |
| Floor base (sub-floor) | M15 | Plain concrete |
IS 456 Minimums for RCC
| Exposure | Minimum grade | Max w/c | Min cement |
|---|---|---|---|
| Mild | M20 | 0.55 | 300 kg/m³ |
| Moderate | M25 | 0.50 | 300 kg/m³ |
| Severe | M30 | 0.45 | 320 kg/m³ |
Nominal or Design Mix?
IS 456 permits nominal mixes — fixed volume ratios such as 1:1.5:3 for M20 — only for M20 and lower. For M25 and above, the mix should be designed from trial batches for the actual materials. On house sites that usually means ready-mix concrete to a stated grade, or a mix design from a lab.
How Grade Changes the Order
| Grade | Cement per m³ |
|---|---|
| M10 (1:3:6) | about 4.4 bags |
| M15 (1:2:4) | about 6.3 bags |
| M20 (1:1.5:3) | about 8 bags |
| M25 (design mix) | commonly about 7–8 bags with OPC 53 and a plasticiser; about 11 if volume-batched at 1:1:2 |
The Short Version
M15 for plain concrete, M20 for protected RCC, M25 for footings and anything exposed to weather — with M25 and above as design mixes. Follow the grade on your drawing for each member.
