What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Same word, different thing20 mm aggregate
also 4/20, single size, graded aggregate
Stated as a size range — the lower and upper sieve sizes — so 4/20 means the material passes 20 mm and is retained on 4 mm. The numbers ARE dimensions.
Australia
Close, not identical20 mm aggregate
also nominal size, graded aggregate
Metric nominal sizes as the UK, with Australian standard gradings — so the convention matches even where the specific envelopes do not.
United Statesyours
Same word, different thing#57 stone
also No. 57, #8, #4, size number
The number indexes a GRADING TABLE, not a size. A #57 is not 57 of anything — it names an envelope of sieve percentages.
Canada
Same word, different thingsize number
also #57 stone, clear stone, 20 mm clear
Both conventions in use — American size numbers alongside metric descriptions — so a Canadian specification may carry either and they are not interchangeable.
The governing standard, by market
United States
ASTM C33 / AASHTO M43
Standard sizes of coarse aggregate, defined as grading envelopes by size number.
United Kingdom
BS EN 12620
Aggregates for concrete, designated by lower and upper sieve size.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- What does the number in #57 stone mean?
- It is an index, not a measurement. American and Canadian coarse aggregate is specified by a SIZE NUMBER that refers to a row in a standard grading table, and that row gives the percentage passing each of a series of sieves. So a #57 is defined by a distribution of particle sizes rather than by a single dimension, and the number itself carries no length. That is the opposite of the European convention, where a designation such as 4/20 states the lower and upper sieve sizes directly and the numbers are millimetres. Reading a size number as though it were a dimension, or converting it as though it had units, produces nonsense — the route across is through the grading table.
- What is the difference between single-size and graded aggregate?
- Whether the material spans a range of particle sizes or is concentrated near one. Graded aggregate contains a spread from fine to coarse, so the smaller particles pack into the spaces between the larger ones — which gives a dense, well-interlocked material that compacts to a stable layer, and is what a sub-base wants. Single-size aggregate is concentrated near one dimension, so it has large voids between its particles and drains freely; that is what a drainage layer, a soakaway or a filter wants, and it is why the same job can call for both. Using one where the other belongs gives either a sub-base that will not compact or a drain that silts up.
- Why does maximum aggregate size matter in concrete?
- Because it has to fit, and because it changes the mix. The stone must pass between the reinforcement and into the corners of the formwork, so the maximum size is limited by the bar spacing, the cover and the section's dimensions — which is why a thin, heavily reinforced element is specified with a smaller aggregate than a mass pour. It also affects the mix economics: larger aggregate reduces the surface area the paste has to coat, so a mix with bigger stone generally needs less cement and less water for the same workability. Reducing the maximum size to get concrete into a congested section is therefore a real change to the mix rather than a substitution.