Cement, Sand and Aggregate for 1 m³ of M20 Concrete (1:1.5:3)

Short answer: for 1 m³ of M20 nominal mix concrete (1 : 1.5 : 3) you need about 8.06 bags of cement (403 kg), 0.42 m³ of sand (14.8 cft) and 0.84 m³ of 20 mm aggregate (29.7 cft), using a dry volume factor of 1.54. Water is roughly 180–200 litres at a water-cement ratio of 0.45–0.50.

These are the figures most Indian estimates use. Below you will see where each number comes from, how the answer changes if your office uses 1.52 or 1.57 instead of 1.54, and a worked example for a real pour.

What 1 : 1.5 : 3 means

M20 nominal mix is proportioned by volume: 1 part cement, 1.5 parts fine aggregate (sand) and 3 parts coarse aggregate (usually 20 mm down). The total is 1 + 1.5 + 3 = 5.5 parts. "M20" means the characteristic compressive strength of 150 mm cubes at 28 days should be 20 N/mm².

Why we multiply by 1.54 (dry volume factor)

Loose, dry cement, sand and aggregate contain a lot of air voids. When water is added and the mix is compacted, the fine particles and cement paste fill those voids, so the finished concrete occupies less volume than the dry materials you heaped up. To get 1 m³ of compacted wet concrete you therefore need more than 1 m³ of dry material.

The multiplier used for this is not fixed by any code; it is an estimating convention. Different departments and textbooks use 1.52, 1.54 or 1.57, and some add a little for wastage on top. This article uses 1.54. The table shows how little the final answer changes:

Step-by-step calculation for 1 m³

Assumptions: dry volume factor 1.54, bulk density of cement 1440 kg/m³, one bag = 50 kg, 1 m³ = 35.31 cft.

1. Dry volume = 1 × 1.54 = 1.54 m³
2. Cement volume = 1.54 × 1/5.5 = 0.280 m³
3. Cement mass = 0.280 × 1440 = 403.2 kg → 403.2 / 50 = 8.06 bags
4. Sand = 1.54 × 1.5/5.5 = 0.420 m³ = 0.420 × 35.31 = 14.8 cft
5. Coarse aggregate = 1.54 × 3/5.5 = 0.840 m³ = 0.840 × 35.31 = 29.7 cft
6. Water (w/c 0.50) = 0.50 × 403.2 = about 202 litres; at w/c 0.45 it is about 181 litres

So the rounded site answer is 8 bags cement, 15 cft sand and 30 cft aggregate per cubic metre. If your supplier sells aggregate by weight, 0.84 m³ of loose 20 mm aggregate is roughly 1.25–1.35 tonnes, depending on its bulk density (typically about 1.5–1.6 t/m³ loose; weigh or measure a delivery to be sure).

Thinking per bag of cement

On a mixer you load one bag at a time, so it helps to know the batch per bag:

• One 50 kg bag = 50/1440 = 0.0347 m³ = 1.23 cft of cement
• Sand per bag = 1.5 × 1.23 = 1.84 cft
• Aggregate per bag = 3 × 1.23 = 3.68 cft
• Concrete produced per bag ≈ 1/8.06 = 0.124 m³

A practical way to batch is a measuring box (farma) of 1.23 cft, the volume of one bag: fill it 1.5 times with sand and 3 times with aggregate for each bag of cement. Do not measure with a head pan (tasla), whose filled volume varies a lot from worker to worker.

Worked example: 6 m³ of M20 for footings and column stubs

Suppose your footings and column stubs total 6.0 m³ of M20 (net volume from drawings). Add 2% for wastage and spillage (practice varies, usually 2–5%).

1. Volume with wastage = 6.0 × 1.02 = 6.12 m³
2. Dry volume = 6.12 × 1.54 = 9.42 m³
3. Cement = 9.42 × 1/5.5 = 1.714 m³ × 1440 = 2468 kg = 49.4 bags → order 50 bags
4. Sand = 9.42 × 1.5/5.5 = 2.57 m³ (90.8 cft) → order about 95–100 cft to allow for bulking and handling loss
5. Aggregate = 9.42 × 3/5.5 = 5.14 m³ (181.5 cft) → order about 185–190 cft
6. Mixing water at w/c 0.50 = 0.50 × 2468 = about 1230 litres (curing water is extra)

If you want these numbers for an entire building (footings, columns, beams, slabs, plus steel and cost), the RCC estimator and the Nirmaan house estimator do the same arithmetic for every member.

Sand bulking: why your sand runs short

The 1 : 1.5 : 3 ratio assumes dry sand. Damp sand swells (bulks) because thin water films push the grains apart; at common site moisture contents the increase in volume is often in the range of 15–30%. If you fill the farma with damp sand without correcting for this, you put in less sand than intended and the mix becomes richer in cement and harsher to work. Do a quick bulking test (fill a measuring jar with damp sand, then flood it with water and see how much it settles) and increase the sand volume accordingly. Fully saturated sand does not bulk in the same way, so test the sand you are actually using that day.

Nominal mix vs design mix for M20

IS 456:2000 allows nominal mix only for M20 and lower grades, and it states that design mix concrete should be preferred. A nominal mix is a fixed recipe that does not account for your particular cement, sand grading or aggregate shape, so it usually carries more cement than needed.

• Nominal mix (1 : 1.5 : 3): about 400 kg cement per m³ by the volume method above. In IS 456, nominal mixes are expressed by mass as total aggregate per 50 kg of cement; for M20 that figure is 250 kg, which works out to the same 1 : 1.5 : 3 by volume at a loose aggregate density of about 1600 kg/m³.
• Design mix (IS 10262): proportions are worked out by trial for your actual materials, by mass. A well-designed M20 often needs noticeably less cement than the nominal mix, particularly when a plasticiser is used, but the exact figure depends entirely on the mix design report. It must still meet the minimum cement content and maximum water-cement ratio of IS 456 for the exposure condition (for reinforced concrete in mild exposure: at least 300 kg/m³ cement and w/c not more than 0.55, for 20 mm aggregate).

Ready-mixed concrete is always a design mix, so you do not calculate cement, sand and aggregate separately; you order cubic metres of the specified grade. Read more about the role of water in water-cement ratio.

Common mistakes

• Dividing 1 m³ by 5.5 without the dry factor. That gives only about 5.2 bags of cement per m³, and every material runs short by roughly a third.
• Mixing units. Do the whole calculation in m³ and convert to cft only at the end (× 35.31).
• Ignoring wastage. Add 2–5% on concrete depending on the method of placing, and keep sand and aggregate stock a little above the calculated figure.
• Adding water to "make it work". Every extra bucket of water raises the w/c ratio and lowers strength. Use a plasticiser instead if workability is a problem.
• Forgetting curing. Correct proportions do not help if the concrete dries out early; see curing of concrete.