Nominal mixes
A nominal mix states the proportions of cement, sand and coarse aggregate by volume: M10 is 1:3:6, M15 is 1:2:4, M20 is 1:1.5:3 and M25 is 1:1:2. These are the mixes in IS 456, which the Nepal National Building Code adopts for ordinary work. M20 is the usual minimum for reinforced concrete, and M15 or M10 is used for plain concrete and blinding under foundations.
Enter the concrete volume either as length × breadth × depth of one member, in metres or feet, or as a volume in cubic metres or cubic feet, then the number of identical items. A wastage allowance of 2–5 % covers spillage and uneven formwork.
How the quantities are worked out
Loose dry materials shrink when mixed with water and compacted, so 1 m³ of finished concrete needs about 1.54 m³ of dry materials. That dry volume is split in the mix proportions: for M20 the total of 1 + 1.5 + 3 is 5.5 parts, so cement is 1.54 × 1 ÷ 5.5 = 0.28 m³, sand 0.42 m³ and aggregate 0.84 m³. Cement weighs 1,440 kg/m³, so 0.28 m³ is 403 kg, or 8.07 bags of 50 kg. The tool shows the exact bag count and the whole bags to buy.
Sand and aggregate are shown in cubic metres and cubic feet because Nepali suppliers sell them by the cft or by the tipper. Water is shown for a water–cement ratio of 0.5, about 200 litres per m³ of M20; adjust for the moisture already in the sand.
Limits
This is an ordering estimate. Columns, beams and slabs in a designed building need the mix the engineer specifies, which may be a design mix with admixtures and a controlled water–cement ratio. The bulking of moist sand, the grading of aggregate and the shape of the member all change the real yield by a few percent.