A concrete mix ratio gives the proportions of cement, sand and coarse aggregate in a concrete grade. In nominal mixes, M20 is 1 : 1.5 : 3, M15 is 1 : 2 : 4 and M10 is 1 : 3 : 6 by volume.
“M” stands for mix, and the number is the characteristic compressive strength in N/mm² after 28 days of curing — M20 concrete should reach 20 N/mm².
IS 456:2000 allows nominal mixes only up to M20. M25 and stronger grades should be design mixes, proportioned by testing the actual materials as described in IS 10262 (M25 is often quoted as 1 : 1 : 2 for rough estimates).
The dry volume of materials is about 1.54 times the finished wet volume, which is why calculators multiply the volume by roughly 1.54 before splitting it by the ratio.
1 m³ of M20: dry volume ≈ 1.54 m³; cement = 1.54 × 1 ÷ 5.5 ≈ 0.28 m³ ≈ 403 kg ≈ 8 bags of 50 kg.
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