The short answer
If you remember one thing from this page, remember the 20mm rule: every reinforced concrete member in an ordinary building, from footing to roof slab, is poured with 20mm graded jalli. Everything else is a special case. 40mm goes into plain concrete beds and mass pours, 12mm goes where steel is congested or the section is thin, 6mm goes into blocks, pavers and precast, and 53mm is a road-base and soling stone that never sees a concrete mixer.
In the Tamil trade the sizes carry inch names: 20mm is முக்கால் ஜல்லி (mukkal jalli, three-quarter inch), 12mm is அரை ஜல்லி (arai jalli, half inch), and the fines below 4.75 mm are கிரஷர் டஸ்ட் (crusher dust). The tables below use millimetre names, because that is what the IS codes and your drawing use.
Size-to-use matrix
This is the matrix we work from when a customer calls without a specification. It follows IS 383:2016, IS 456:2000 and MoRTH Section 400, plus what block and paver plants around Gummidipoondi and Ponneri actually order.
| Size (trade name) | What it is | Use it for | Not for |
|---|---|---|---|
| 6mm chips | 4.75–10 mm; tested as 10 mm single-size | Paver and hollow-block manufacture, precast, kerbs, screeds, chip-sealing, 5–10% correction in pumped RMC | Structural concrete on its own |
| 12mm (arai jalli) | 12.5 mm nominal, 8–16 mm fraction | Lintels, sunshades, stairs, precast, thin walls, congested columns and beams, 60:40 blend with 20mm for RMC, bituminous surface course | Whole-house RCC by default |
| 20mm (mukkal jalli) | 20 mm graded, 16–24 mm fraction | All RCC: slabs, beams, columns, footings (M20–M40); PCC where 40mm is not specified; RMC; DBM and WMM fractions; drainage | Sections under about 80 mm or bars closer than 25 mm |
| 40mm | 40 mm nominal, 32–45 mm fraction | PCC and lean concrete (M7.5–M15), mass concrete and rafts where IS 456 permits, GSB/WMM fraction, French drains, yard filling | Reinforced members with normal cover and spacing |
| 53mm road metal | 45–63 mm road-metal fraction (MoRTH sieve size; also called 60mm or 63mm), on request | WBM Grading 2 and 3, GSB top size, WMM (53 mm down), soling and hard-core, gabions | Concrete of any grade |
| Crusher dust | Fines below 4.75 mm | Paver bedding and jointing, block making with 6mm chips, trench backfill, plinth filling, GSB/WMM fines | Plastering, or replacing sand in RCC without a mix design |
The IS 456 rule that decides the size
Aggregate size is not a matter of taste. IS 456:2000 gives two limits for the maximum nominal size in reinforced concrete: not more than one-quarter of the minimum thickness of the member, and at least 5 mm less than the minimum clear spacing between bars and the clear cover.
Work it through for a house. A 125 mm roof slab gives a quarter-thickness limit of about 31 mm, so 20mm passes easily. A beam with two layers of 20 mm bars at 25 mm clear spacing is exactly where 20mm starts to hang up and 12mm earns its keep. A 75 mm chajja fails the quarter-thickness test for 20mm outright (75 ÷ 4 is under 19 mm); that is 12mm work.
The same code allows 40mm in mass concrete and plain footings, where there is no steel to bridge and the larger stone cuts cement demand and heat of hydration.
20mm vs 40mm: the foundation question
"Which is better for foundation, 20mm or 40mm?" is the question we hear most often. The answer is both, in different layers.
- PCC bed under the footing (1:4:8 or 1:3:6, M7.5–M10): 40mm. It is unreinforced and only has to give a clean, level working surface. A cubic metre of 1:4:8 takes roughly 1,400 kg of 40mm and about 3.4 bags of cement.
- RCC footing, plinth beam and column (M20 and above): 20mm graded. Steel is present, cover is 40–50 mm, and the stone must pass the bars.
- Raft or mass foundation: the engineer may specify 40mm for the body of the raft if bar spacing allows; follow the drawing.
On our list 40mm is priced below 20mm ex-works (see pricing), but that is no reason to push 40mm into a reinforced member: the saving in PCC comes from needing less cement paste, and a honeycombed column costs far more than the stone. Both weigh out at roughly 4.1–4.7 MT per unit (100 cft, 2.832 m³); we sell by weighbridge tonne, so the conversion only matters against a per-unit quote (see 1 unit of jalli = how many tons?).
Concrete grade vs aggregate size
IS 456 Table 9 gives the nominal mixes with 20mm assumed as the coarse aggregate; design mixes under IS 10262 pick the size from the section first and then adjust water and cement.
| Grade / application | Typical mix | Aggregate size |
|---|---|---|
| M7.5, M10, M15 (PCC, lean concrete) | 1:4:8, 1:3:6, 1:2:4 nominal | 40mm, or 20mm graded where 40mm is not available |
| M20 RCC (house slabs, beams, columns) | 1:1.5:3 nominal | 20mm graded; 12.5 mm where the section is under about 60 mm or bars are congested |
| M25 and above, design mix, RMC | IS 10262 design | 20mm + 12.5 mm blend, typically 60:40 |
| Paver blocks / hollow blocks | Dry-cast | 6mm (or 12mm) chips + crusher dust or M-Sand |
| Road base and sub-base | MoRTH 400 gradings | GSB 53–75 mm down; WMM 53 mm down; WBM 63–45 or 53–22.4 mm |
As a planning figure, M20 needs about 0.84 m³ (roughly 1.26–1.33 MT) of 20mm per cubic metre of concrete, with about 0.42 m³ of M-Sand (எம் சாண்ட்) and 8 bags of cement; see our 1,500 sq ft house estimate.
Why RMC plants blend 20mm and 12mm 60:40
Ready-mix plants in the north Chennai belt around Red Hills and Madhavaram rarely run a single stone. They take 20mm and 12mm separately and blend them, most commonly 60:40, because a 20mm single-size has a gap between 20 and 10 mm; adding 12mm fills it, so the mix pumps without segregating and the batcher can hold the IS 383 graded band (10 mm sieve 25–55% passing) even when a stockpile drifts.
For an RMC buyer that means two products, not one, and the ratio belongs to the mix design. We dispatch 20mm and 12mm as separate screened fractions on separate weighbridge slips, so the plant batches its own ratio rather than trusting a pre-mixed pile.
6mm and 53mm: the two sizes IS 383 does not name
Buyers search for "6mm blue metal" and "53mm aggregate" every day, yet neither is a nominal size in IS 383:2016. The single-size list is 63, 40, 20, 16, 12.5 and 10 mm; the graded list is 40, 20 and 12.5 mm.
6mm chips
What the trade calls 6mm is the 4.75–10 mm fraction (our screen deck cuts it at about 3.5–8 mm), tested against the IS 383 10 mm single-size grading: 12.5 mm 100% passing, 10 mm 85–100, 4.75 mm 0–20, 2.36 mm 0–5. Its buyers are block and paver plants, precast yards, screed contractors and road crews doing surface dressing. In concrete it is a corrective, not a coarse aggregate.
53mm road metal
53 mm is a MoRTH sieve size: the top size for WMM and for GSB Gradings II, III, IV and VI, and the coarse aggregate for WBM Grading 3 (53–22.4 mm; 53 mm 95–100% passing, 45 mm 65–90, 22.4 mm 0–10). Buyers often ask for it as 60mm, 63mm or 65mm jalli, which is neighbouring WBM Grading 2 territory (63–45 mm). If your drawing says WBM, WMM or GSB, ask for the grading number rather than a stone size; see our WMM/WBM page and the GSB vs WBM vs WMM explainer.
IS 383 grading tables for 20mm and 40mm
A size name only means something with a grading behind it. These are the IS 383:2016 Table 7 envelopes for the two sizes that go into concrete.
| IS sieve | 20 mm graded (% passing) | 20 mm single-size (% passing) |
|---|---|---|
| 40 mm | 100 | 100 |
| 20 mm | 90–100 | 85–100 |
| 10 mm | 25–55 | 0–20 |
| 4.75 mm | 0–10 | 0–5 |
| IS sieve | 40 mm graded (% passing) | 40 mm single-size (% passing) |
|---|---|---|
| 63 mm | 100 | 100 |
| 40 mm | 90–100 | 85–100 |
| 20 mm | 30–70 | 0–20 |
| 10 mm | 10–35 | 0–5 |
| 4.75 mm | 0–5 | — |
The difference is what sits below the top sieve. Graded 20mm carries 25–55% passing 10 mm, which is what makes it self-packing in a slab; single-size 20mm is nearly all 20 mm stone and suits filter media, drainage and blending. For RCC you want the graded column. Our 20mm and 40mm are produced to the graded envelopes; gradation, silt, crushing value and flakiness are tested through accredited third-party laboratories and certificates accompany every bulk contract. For checks you can do yourself, see how to check blue metal and M-Sand at site.
Ordering tips from the weighbridge
- Order by the drawing, not the habit. If the structural drawing says "20 mm down graded", that is the product. If a mason says 40mm is stronger, he is thinking of PCC.
- Split the foundation order. 40mm for the PCC bed, 20mm for the RCC. Two smaller loads beat one 40mm load that ends up in the columns.
- Count in tonnes. A 10-wheeler tipper carries a legal 16–20 MT (18 MT is typical); one load of 20mm covers roughly a 1,000 sq ft, 5-inch slab, with about 8 MT of M-Sand alongside. A unit (யூனிட், 100 cft) of 20mm is about 4.1–4.5 MT.
- Keep the papers. Every load leaves with a weighbridge slip, GST tax invoice at 5% under HSN 2517, AP e-transit pass and an e-way bill where the consignment exceeds the inter-state threshold. Royalty is itemised separately on the invoice.
- Check the lead. Approximate road distance from our Perindesam works is about 82 km to Gummidipoondi, 85 km to Ponneri, 67 km to Tiruvallur, 89 km to Avadi and 90–100 km into Chennai, subject to route and heavy-vehicle timing rules on some city roads.
Still unsure? Send us the drawing or a photo of the reinforcement and we will tell you what fits. Indicative ex-works rates and a delivered-cost estimator are on the pricing page; request a quote or WhatsApp / call +91 90039 03003 with your site pin and the sizes you need.
Quick answers
Which jalli size is used for a roof slab?
20mm graded jalli (mukkal jalli) is the standard for RCC slabs, beams and columns. IS 456 assumes 20mm for the M20 nominal mix, and it fits a 100 to 150 mm slab with normal bar spacing.
What is the difference between 20mm and 40mm jalli?
20mm is for reinforced concrete, where it must pass between bars and under cover. 40mm is for plain (unreinforced) PCC beds, lean concrete, mass concrete and rafts, and for road layers such as GSB and WMM. 40mm in an RCC column tends to bridge on the steel and leave honeycombs.
What is 6mm blue metal used for?
Paver and hollow-block manufacturing, precast, screeds and thin toppings, chip-sealing on roads and as a 5 to 10 percent correction in pumped RMC mixes. It is the 4.75 to 10 mm fraction and is tested as 10 mm single-size under IS 383.
Where is 53mm aggregate used?
53mm is a MoRTH road sieve size, not a concrete aggregate. It is the top size for WMM and most GSB gradings, the coarse aggregate for WBM Grading 3 (53 to 22.4 mm), and is used for soling and hard-core under floors and pavements.
Can I use 12mm jalli instead of 20mm for a slab?
You can, and IS 383 recommends 12.5 mm where 20mm cannot pass congested steel or sections under about 60 mm. For an ordinary slab it is not needed and costs more in cement; most engineers keep 20mm and use 12mm only in lintels, chajjas, stairs and precast.