Concrete Grades and Mix Ratios: M5, M7.5, M10, M15, M20 and M25

Short answer: the usual nominal mix ratios (cement : sand : aggregate, by volume) are M5 = 1:5:10, M7.5 = 1:4:8, M10 = 1:3:6, M15 = 1:2:4 and M20 = 1:1.5:3. M25 is often quoted as 1:1:2, but IS 456:2000 allows nominal mixes only up to M20, so M25 and above must be design mix. For reinforced concrete the minimum grade is M20 in mild exposure and M25 in moderate exposure.

The rest of this page explains what the "M" number means, how IS 456 groups the grades, where each one is used on an Indian site and how much cement each needs per cubic metre.

What the grade number means

In "M20", M stands for mix and 20 is the characteristic compressive strength in N/mm² of 150 mm cubes tested at 28 days. "Characteristic" means the strength below which not more than 5% of test results are expected to fall, so the average strength the mix must actually achieve on site is higher than 20 N/mm².

Grade groups in IS 456

IS 456:2000 groups concrete into three classes:

M5 and M7.5 do not appear in this grouping, but they are listed in IS 456 among the nominal mix proportions and are widely used as lean concrete (PCC) below foundations and floors.

Nominal mix ratios and material per m³

The table gives the conventional volume ratios and the approximate material for 1 m³ of finished concrete. Quantities are calculated with a dry volume factor of 1.54, cement at 1440 kg/m³ and 50 kg bags; your figures will shift by about 2% if your office uses 1.52 or 1.57.

How IS 456 actually states nominal mixes

IS 456 does not write nominal mixes as 1:2:4. It gives the total mass of fine plus coarse aggregate per 50 kg of cement: 800 kg for M5, 625 kg for M7.5, 480 kg for M10, 330 kg for M15 and 250 kg for M20. The sand-to-aggregate split is adjusted for the grading of your materials (generally 1:2, varying between about 1:1.5 and 1:2.5).

Worked check for M20: one bag of cement occupies 50 / 1440 = 0.0347 m³. In 1:1.5:3 the aggregate is 1.5 + 3 = 4.5 parts, i.e. 4.5 × 0.0347 = 0.156 m³. At a loose bulk density of about 1600 kg/m³ that is 0.156 × 1600 = 250 kg, exactly the IS 456 figure. Doing the same for M15 gives 333 kg (code: 330) and for M10 about 500 kg (code: 480), so the familiar volume ratios and the code's mass values are consistent.

Where each grade is used

• M5 and M7.5 (1:5:10, 1:4:8): lean concrete, levelling course and PCC bed under footings, mass fill, base under floors on firm ground.
• M10 (1:3:6): PCC below footings and grade slabs where a better bed is wanted, non-structural fill, sub-base of ground floors, bed for brick foundations.
• M15 (1:2:4): plain concrete floors, pathways, drain beds, coping and plain concrete in moderate exposure. It is no longer acceptable for reinforced concrete under IS 456:2000.
• M20 (1:1.5:3): the minimum grade for RCC in mild exposure; slabs, beams, columns, footings, lintels and staircases in ordinary houses in inland, sheltered conditions.
• M25 and above (design mix): RCC in moderate or harsher exposure (which covers most work exposed to rain, buried in soil or in coastal areas), columns of multi-storey buildings, and most work done with ready-mixed concrete. Water-retaining structures are governed by IS 3370, which calls for higher grades.

Minimum grade for RCC by exposure (IS 456)

Durability, not only strength, decides the grade. IS 456 sets minimum cement content, maximum water-cement ratio and minimum grade for each exposure condition. For reinforced concrete with 20 mm nominal maximum aggregate:

"Mild" covers concrete protected against weather or aggressive conditions, except in coastal areas. Concrete exposed to rain and condensation, buried in non-aggressive soil or sheltered from salt air near the coast is "moderate". Many house slabs are still cast in M20, but an external beam or a footing in damp soil strictly falls under moderate exposure. The water-cement ratio limits above are explained in water-cement ratio.

When is design mix required?

• Always for M25 and above, because IS 456 allows nominal mix only up to M20.
• Whenever IS 456 prefers it: the code says design mix concrete should be used in preference to nominal mix, even for M20.
• Large or important works (multi-storey frames, bridges, water tanks, industrial floors) where saving cement and controlling quality pays back the cost of trial mixes.
• Ready-mixed concrete is always supplied to a mix design.
• When using admixtures, fly ash or GGBS, because proportions must be established by trial.

Worked example: choosing PCC grade under footings

A house has 2.0 m³ of PCC under its footings. Compare 1:4:8 with 1:3:6 (dry factor 1.54):

1. Dry volume = 2.0 × 1.54 = 3.08 m³
2. 1:4:8 → total 13 parts → cement = 3.08/13 × 1440 = 341 kg = 6.8 bags
3. 1:3:6 → total 10 parts → cement = 3.08/10 × 1440 = 444 kg = 8.9 bags

Going one grade richer costs about 2 extra bags here. For a levelling bed 1:4:8 is normally enough; follow the structural drawings where they specify otherwise. For whole-building quantities, try the RCC estimator.

Site tips

• Take the grade from the structural drawing, not from habit. If the drawing says M25, a 1:1:2 volume mix does not satisfy it.
• Batch by volume using a box equal to one bag (about 1.23 cft), not head pans.
• Correct for sand bulking; damp sand in a 1:1.5:3 box gives a different mix.
• Make cube samples for every important pour; the grade is proved by 28-day cube results, not by the ratio.
• Revise grades and IS 456 limits for exams with our PSC mock tests.