The short answer
20 mm is the general-purpose size for reinforced concrete and bituminous macadam. 10 mm goes into thin or heavily reinforced sections and the road's wearing course. 40 mm suits mass concrete, lean concrete and drainage. 6 mm is used in bituminous mixes and paver blocks. The size named is the nominal maximum: nearly all of the stone passes that sieve and is held on the next one down.
Aggregate is sold by a number that describes a sieve, not a stone. A load of 20 mm contains pieces from about 10 mm up to 20 mm, with a small allowance above and below. Knowing which sieve pair defines each product is the first step to ordering the right one.
01The sizes and what they are for
| Nominal size | Passes / held on | Main uses | Poor choice for |
|---|---|---|---|
| 40 mm | 40 mm / 20 mm | Mass concrete, foundations, dry lean concrete, drainage layers, soling | Thin slabs, congested reinforcement |
| 20 mm | 20 mm / 10 mm | Reinforced concrete, pavement quality concrete, DBM, precast | Sections under about 80 mm thick |
| 10 mm | 12.5 mm / 4.75 mm | Slabs and beams with close steel, bituminous concrete, paver blocks | Mass concrete, where it wastes cement |
| 6 mm | 6.3 mm / 2.36 mm | Bituminous mixes, surface dressing, paver and block making | Structural concrete on its own |
| Dust | Passes 4.75 mm | Fine aggregate, bedding, block making, filler in bituminous mixes | Any use that needs free drainage |
Sieve pairs are the usual trade definition. The exact limits for each product are in IS 383 for concrete and in the MoRTH tables for road layers.
02Choosing a size for concrete
IS 456 gives the rule in two parts. The nominal maximum size should be as large as the job allows, and it must not exceed one quarter of the thinnest dimension of the member. In reinforced work it must also be at least 5 mm smaller than the clear gap between main bars, and 5 mm smaller than the cover.
Run the numbers on an ordinary 125 mm slab and a quarter is 31 mm, so 20 mm is the right stone. A 75 mm precast panel gives 19 mm, and 10 mm is the safer choice. A raft foundation a metre thick could take 40 mm and save cement by doing so, as long as the steel spacing lets it through.
Larger stone needs less cement paste to coat it, which is why the code says as large as possible. Smaller stone flows more easily round steel. Most structural concrete in India settles on 20 mm blended with 10 mm, commonly near 60 to 40, because two sizes pack more tightly than one.
03Single size or graded
A single-size aggregate is one fraction, as in the table. A graded aggregate is a deliberate blend of fractions down to 4.75 mm, supplied as one material. Batching plants generally buy single sizes and do the blending themselves, since that lets them correct the mix when a delivery varies. Small sites buying one material for hand-mixed concrete are better served by a graded 20 mm.
04Size is not the whole specification
Two loads of 20 mm that pass the same sieves can behave very differently. One is cubical and clean. The other is flat, coated in dust and will not hold bitumen. Shape, strength and cleanliness are separate tests with separate limits, covered in flakiness and elongation index and tests on aggregate for road work.
05How to order
- The nominal size, and whether single size or graded.
- The specification it must meet: IS 383 for concrete, or the MoRTH table for the road layer.
- The quantity in tonnes. Volumes are converted in brass, cft, cubic metre and tonne.
- Any limit your consultant has set on flakiness, impact value or water absorption.
- Where it is going and by when.
A supplier who has those five can quote a delivered rate and tell you honestly whether the rock at the plant will meet the limits. Our standing sizes are listed on the materials page, and delivery terms under aggregate supply.