Concrete mix calculator (M20, M15, M10): cement bags, sand and aggregate

How does the concrete mix calculator work?

It turns a volume of finished (wet) concrete into the cement, sand and aggregate that go into the mixer, the way Indian site estimates do it:

• Wet volume: length × width × thickness, times how many. In feet, cubic feet (cft) ÷ 35.31 gives cubic metres.
• Dry volume: wet volume × 1.54. Loose cement, sand and aggregate shrink when water goes in and the fine particles fill the gaps between the stones, so 1 m³ of concrete takes about 1.54 m³ of dry material.
• Parts: add up the ratio. M20 at 1 : 1.5 : 3 is 1 + 1.5 + 3 = 5.5 parts.
• Cement: dry volume ÷ parts gives cubic metres of cement; × 1440 kg/m³ gives kilograms; ÷ 50 gives bags.
• Sand and aggregate: dry volume × 1.5 ÷ 5.5 and × 3 ÷ 5.5 for M20, in m³; × 35.31 for cft; ÷ 100 for brass.
• Water: bags × the most IS 456 Table 9 allows per 50 kg bag (30 litres for M20). Use no more than the mix needs.

The ratios are by volume, the way they are measured on site with a box or a head pan: one bag of cement, then sand and aggregate by the same box. The 1440 kg/m³ for cement and the 1.54 are the usual estimating figures; the sources are at the end of the page.

How much cement, sand and aggregate for 1 m³ of M20?

8.06 bags of cement, 0.42 m³ (14.8 cft) of sand and 0.84 m³ (29.7 cft) of aggregate, at 1 : 1.5 : 3:

1. Dry volume: 1 × 1.54 = 1.54 m³.
2. Parts: 1 + 1.5 + 3 = 5.5.
3. Cement: 1.54 ÷ 5.5 = 0.28 m³, and 0.28 × 1440 = 403.2 kg, which is 403.2 ÷ 50 = 8.06 bags.
4. Sand: 1.54 × 1.5 ÷ 5.5 = 0.42 m³, and 0.42 × 35.31 = 14.8 cft.
5. Aggregate: 1.54 × 3 ÷ 5.5 = 0.84 m³, or 29.7 cft.
6. Water: at most 8.06 bags × 30 litres = about 242 litres.

Buy 9 bags for that cubic metre: cement comes in whole bags, and a part bag left open goes off. Put The volume, 1 m³ and M20 into the calculator above to see the same figures.

Worked example: a 12 × 10 ft roof slab, 5 in thick

These are the numbers the calculator opens with: a room 12 × 10 ft, a slab 5 in (125 mm) thick, M20.

1. Wet volume: 12 × 10 × 5 ÷ 12 = 50 cft. In metres: 3.6576 × 3.048 × 0.127 = 1.416 m³ (50 ÷ 35.31 gives the same).
2. Dry volume: 1.416 × 1.54 = 2.180 m³.
3. Cement: 2.180 ÷ 5.5 = 0.396 m³ × 1440 = 571 kg, so 11.42 bags; buy 12.
4. Sand: 2.180 × 1.5 ÷ 5.5 = 0.595 m³ = 21.0 cft (0.21 brass).
5. Aggregate: 2.180 × 3 ÷ 5.5 = 1.189 m³ = 42.0 cft (0.42 brass).
6. Water: at most 11.42 × 30 = about 343 litres.

The slab size is the room's inside size here. A real roof slab runs over the walls and has beams under it: add their volume before you work out the materials, and take the thickness and grade from the structural drawing.

Cement, sand and aggregate per m³ for M20, M15 and M10

Per cubic metre of wet concrete, with the 1.54 dry factor, cement at 1440 kg/m³ and 50 kg bags:

For 100 cft of concrete (2.83 m³), multiply the M20 row by 2.83: 22.8 bags, 42 cft of sand and 84 cft of aggregate. Water is IS 456 Table 9's most per 50 kg bag (30, 32 and 34 litres) times the bags; the lean mix has no grade, so no Table 9 figure; the mix needs only enough for the slump the job calls for, so add it a little at a time.

What does IS 456 say about nominal mix?

IS 456:2000 prefers design mix, and allows nominal mix only up to M20. Clause 9.3: “Nominal mix concrete may be used for concrete of M 20 or lower.” Clause 9.1.1 adds that it needs the permission of the engineer-in-charge and “is likely to involve a higher cement content”. Its Table 9 sets the proportions by mass, per 50 kg bag of cement:

Fine to coarse aggregate is “generally 1:2” by mass, between 1:1½ and 1:2½. The volume ratios people use on site come from BIS's handbook on the older code, SP 24:1983, clause 8.3: “A rough guide for the nominal mix proportions by volume will be 1:3:6, 1:2:4 and 1:1½:3 for M 10, M 15 and M 20 concretes respectively”, with 20 mm aggregate. The two agree: 4.5 parts of sand and aggregate per part of cement is 4.5 × 0.0347 = 0.156 m³ a bag, which is 250 kg at a loose weight of 1600 kg/m³.

Three more rules from IS 456 worth knowing before you mix by the box:

• Batching: cement and aggregate are to be measured by mass (clause 10.2). “Volume batching may be allowed only where weigh-batching is not practical”, with an allowance for the bulking of damp sand (clause 10.2.4).
• Reinforced concrete: Table 5 sets M20 as the lowest grade for RCC, with at least 300 kg of cement per m³ and a water-cement ratio of 0.55 or less for mild exposure. M20 at 1 : 1.5 : 3 gives 403 kg per m³ by this calculator.
• Plain concrete (PCC, such as a bed under a footing) needs at least 220 kg of cement per m³ in mild exposure, also Table 5. M10 at 1 : 3 : 6 gives 222 kg.

CPWD's Delhi Analysis of Rates 2014 uses nearly the same quantities. Per cubic metre, item 4.1:

CPWD's aggregate is 20 mm and 10 mm graded stone together (for 1 : 1.5 : 3, 0.570 + 0.280 m³). Its totals work out to about 1.56 times the wet volume, a little more than 1.54.

Why multiply by 1.54?

Because dry ingredients take up more room than the concrete they make. Sand and aggregate are loose, with air between the grains and stones; once water goes in, the cement and sand fill those gaps and the mix settles. Estimates in India allow for that, and a little loss, by taking 1.52 to 1.57 times the wet volume; 1.54 is the figure most often used, and it is the one this calculator uses. UltraTech and JK Cement both give 1.54 for M20 and explain it the same way; CPWD’s analysis of rates comes to about 1.56. No Indian Standard fixes the figure, so treat it as an estimating allowance.

The result is an estimate for ordering, not a mix design. Moist sand also bulks (takes up more room than dry sand), so a box of damp sand holds less sand than it seems: if you batch by volume, allow for it, as IS 456 asks.

Common mistakes

• Working from the wet volume. 1 m³ of M20 split by 1 : 1.5 : 3 without the 1.54 gives 5.2 bags of cement instead of 8.06.
• Mixing up cft and m³. 1 m³ is 35.31 cft. Sand quoted at 14.8 cft per m³ of concrete is 0.42 m³, not 14.8 m³.
• Treating a brass as a cubic metre. 1 brass is 100 cft, which is 2.83 m³.
• Using 1 : 2 : 4 for a roof slab or beam. That ratio is the usual M15 proportion; IS 456 sets M20 as the lowest grade for reinforced concrete, and your drawing may ask for more.
• Forgetting the beams and the bearing on the walls. A room's floor area is less concrete than the roof slab over it.
• Buying the exact bags. Round up, and keep a bag in hand for spillage and the last batch.

Questions people ask

What this calculator doesn't do

• No design mix. M25 and above, and any job where the engineer asks for it, need a mix designed to IS 10262 from tests on your own materials.
• No ready-mix concrete (RMC): order RMC by the cubic metre and grade, and the plant sets the mix.
• No steel, shuttering or admixtures, and no allowance for spillage beyond what the 1.54 carries: round up the bags.
• It doesn't decide the grade, thickness or reinforcement. Those come from the structural drawing and IS 456, and your local body's building rules decide what needs an engineer.

For bricks, cement bags and sand for the walls, see the brick calculator for India. For the volume alone, in cubic metres or cubic yards with an order allowance, see the concrete calculator, and for Indian units in one place, construction calculations in India.

Sources

Checked on 1 November 2026.

• IS 456:2000, Plain and reinforced concrete, code of practice (PDF, Public.Resource.Org copy): clauses 9.1.1, 9.3 and 10.2, Table 2 note 1, Table 5 and Table 9.
• SP 24:1983, Explanatory handbook on IS 456:1978 (PDF): clause 8.3, the 1:3:6, 1:2:4 and 1:1½:3 rough guide.
• CPWD Delhi Analysis of Rates 2014, Volume 1 (PDF): items 4.1.2, 4.1.3 and 4.1.5, materials per m³.
• UltraTech Cement, what is M20 concrete ratio: the 1.54 dry volume and cement at 1440 kg/m³.
• JK Cement, what is M20 concrete ratio: wet volume “increased by 54% to derive the dry volume”.
• CPWD Specifications 2009, Volume 1 (PDF): Table 3.3, bulking of sand (15 to 30% at 2 to 5% moisture).

Related guides

• Brick calculator for a wall (India): bricks, cement and sand
• Construction calculations in India: feet, mm, cft, brass and bags
• Concrete for slabs, footings and post holes, worked out in m³ or cubic yards
• How much concrete do I need for a slab?
• How to calculate cubic metres from millimetres