Kumar IronUpdated
In this guide
What M20 means
M20 is concrete with a 28-day characteristic compressive strength of 20 MPa (N/mm²). IS 456 gives a nominal mix proportion for M20 of 1 part cement, 1.5 parts sand and 3 parts coarse aggregate (gitti) by volume. This is the usual mix for small house slabs, beams and columns.
IS 456 allows nominal mixes only up to M20. For M25 and above, the concrete should be a design mix, worked out for the actual materials under IS 10262, with cube tests. Many sources quote clause numbers differently, so ask your engineer rather than depending on a clause number from a website.
The math, step by step
Step 1. One m³ of finished (wet) concrete needs more than 1 m³ of loose dry material, because the fine particles fill the gaps between the larger ones. The common factor is 1.54, which is 54 per cent extra. Some designers use 1.52 or 1.57. This is the main reason sources differ.
Step 2. Add up the mix parts: 1 + 1.5 + 3 = 5.5.
Step 3. Cement volume = 1.54 x 1 / 5.5 = 0.28 m³. Sand = 1.54 x 1.5 / 5.5 = 0.42 m³. Gitti = 1.54 x 3 / 5.5 = 0.84 m³.
Step 4. Cement bags. Cement weighs about 1440 kg per m³, so 0.28 x 1440 = 403 kg. At 50 kg a bag that is 8.06 bags. So M20 needs about 8 bags of cement per m³. That matches the number you most commonly see.
| Material | Volume | Approx. quantity |
|---|---|---|
| Cement | 0.28 m³ | about 8 bags of 50 kg |
| Sand | 0.42 m³ | about 15 cubic feet |
| Gitti (20 mm) | 0.84 m³ | about 30 cubic feet |
1 m³ is about 35.3 cubic feet. Order a little extra for spillage and uneven shuttering.
Bags for a 100 sq ft slab
Try it: Cement calculator
- Cement: 8.1 bags (50 kg)
- Sand: 0.42 m³ (14.8 cu ft)
- Aggregate: 0.84 m³ (29.7 cu ft)
100 sq ft is 9.29 m². A 5 inch (127 mm) slab has volume 9.29 x 0.127 = 1.18 m³. Cement = 1.18 x 8.06 = 9.5 bags. Sand = 1.18 x 0.42 = 0.5 m³ (about 17.5 cubic feet) and gitti = 1.18 x 0.84 = 0.99 m³ (about 35 cubic feet).
| Slab thickness | Concrete volume | Cement bags |
|---|---|---|
| 4 inch (100 mm) | 0.93 m³ | about 7.5 |
| 5 inch (125 mm) | 1.16 m³ | about 9.4 |
| 6 inch (150 mm) | 1.39 m³ | about 11.2 |
Add 3 to 5 per cent for wastage. Beams and columns in a house frame use much more cement per sq ft than the slab alone, which is why the existing slab and house guide gives larger totals.
Why you may see 7.5 or 9 instead of 8
- A dry volume factor of 1.52 gives 7.9 bags, and 1.57 gives 8.2 bags.
- Some sources use 1440 kg per m³ for cement, others 1500. A heavier assumption means fewer bags.
- Some sources mix up volume and weight proportions, which gives a completely different answer. The IS 456 nominal mix is by volume.
- Real sites waste 3 to 7 per cent. Mixing on the ground, leaking shuttering and over-filled forms all add to it.
Take 8 bags per m³ as a clean working number, then add wastage.
Practical tips for the mix
- Measure by a fixed box, not by shovels. A common box holds 1 bag volume (about 35 litres) for cement.
- Keep water low. A wetter mix flows easily but loses strength.
- Vibrate or rod the concrete to remove air pockets.
- Cure for the number of days in our curing guide.
- For ready mix or machine mixing at scale, ask for a design mix with a test certificate.
Frequently asked questions
- How many bags of cement per m³ for M20?
- About 8 bags of 50 kg using the 1:1.5:3 mix and a dry volume factor of 1.54. The working is shown above.
- Is M20 a nominal mix or design mix?
- IS 456 gives a nominal mix for M20 (1:1.5:3). M25 and above should be design mixes under IS 10262.
- What is the dry volume factor of 1.54?
- Dry materials occupy about 54 per cent more volume than the wet concrete they form, so you must buy that much extra material per m³.
- How many bags of cement for a 100 sq ft 5 inch slab?
- About 9.5 bags of M20 cement for the slab concrete, before wastage.
- Is M20 enough for a house?
- Yes for most small residential RCC work, but follow the grade written in the structural drawing.