| Primary value | 20 mm (slab / beam) (10 mm thin slab · 40 mm mass / raft) |
| Applies to | Selection of coarse aggregate at the batching plant · RCC and PCC concrete in residential / commercial buildings · Mass concrete and mat foundations |
| Exceptions | Thin slabs (< 100 mm) / heavily reinforced sections → ≤ 10 mm |
| Standard slab / beam / column → ≤ 20 mm | |
| Mass concrete / raft → ≤ 40 mm | |
| PCC under footing → ≤ 40 mm | |
| IS 456 absolute limits — least of: → 1/4 of least dimension · 5 mm less than min bar spacing · cover | |
| Measured as | Largest sieve size that retains less than 5% of the aggregate sample. Standard nominal sizes in India: 10, 12.5, 20, 40 mm. |
| Source | IS 456 — Clause 5.3.3 ✓ Verified |
67 related items across IS codes, knowledge articles, design rules, maps and tools
Aggregate size has three constraints: 1) it must fit between bars without bridging, 2) cannot exceed 1/4 the least section dimension, 3) cannot interfere with cover. 20 mm is the workhorse for slabs/beams/columns because it satisfies all three for typical 230 × 450 sections with 100 mm bar spacing. Going to 40 mm helps on raft pours by reducing cement demand.
Indian batching plants stock 10, 20 and 40 mm aggregate. Most residential projects order 20 mm for everything; switch to 10 mm for thin column zones with congested rebar (especially seismic zones IV/V where IS 13920 confining hoops are tighter).