RL means reduced level: the height of a point measured from the vertical datum the drawing nominates as zero, written in metres as RL 24.350.
What RL means
RL stands for reduced level. It is the height of a point measured vertically from whatever surface the drawing nominates as zero.
The word “reduced” is a leftover from levelling fieldwork. A surveyor books staff readings through a run of instrument setups, then reduces those raw readings – by rise and fall, or by height of collimation – into one height per point on a single common datum. What comes out of that arithmetic is the reduced level.
Three things about an RL trip people up.
- An RL is a height, not a dimension. It says where a point sits, not how far it is from anything else. Subtract two RLs and you get the fall or rise between them.
- An RL means nothing without its datum. RL 24.350 to AHD and RL 24.350 to a site assumed datum are different points in the real world, potentially by tens of metres.
- An RL does not say which surface it belongs to. The prefix or suffix does that: FFL, SSL, TOC, TOS, IL, NGL.
How RL appears on a drawing
RL sits in front of the number, in metres, normally to three decimal places (millimetres) on set-out and structural drawings. Some civil and landscape sheets drop to two. You will see it as RL, R.L. or R.L: – all the same thing.
A typical run of level callouts on an Australian residential set:
LEVELS TO AHD. SITE BENCH MARK: SSM 84213, RL 22.146
GROUND FLOOR FFL RL 24.350
SLAB SSL RL 24.290 (ALLOW 60 FLOOR BUILD-UP)
EX. NGL RL 23.950 AT SE CORNER
BASEMENT FFL RL 21.350
Every relationship on that job falls out of subtraction, because none of it is dimensioned anywhere. The ground floor sits 2.204 m above the bench mark (24.350 – 22.146). The slab surface is 60 mm below its own finished floor (24.350 – 24.290), which is exactly the floor build-up allowed for. The ground floor stands 400 mm above natural ground at the south-east corner (24.350 – 23.950). The basement floor is 3.000 m below the ground floor (24.350 – 21.350).
On a survey plan you will also see bare spot levels – a small cross or dot with 23.95 beside it – and contours labelled with the value only, on the assumption that the datum note in the title block covers the whole sheet.
Levels below the datum carry a minus sign, which is why a basement level can be negative and still be correct. That is a property of the site, not of the basement: in the run above the basement is 3 m down and still comfortably positive, because the whole site sits 22 m up. On a low-lying coastal site the same convention produces negative values outright – a basement slab on a foreshore job might read:
BASEMENT SSL RL -0.450 (AHD)
On sections and elevations, level markers are usually a filled or half-filled triangle sitting on the level line with the RL text alongside.
Terms RL gets confused with
- AHD is the datum; RL is the height measured from it. “RL 24.350 (AHD)” reads as 24.350 m above the Australian Height Datum.
- Relative level. Some offices expand RL that way. In practice it means the same thing, and it is a fair description when the datum is assumed rather than national.
- RL mesh. This one causes real errors on Australian concrete drawings. RL followed by four digits with no decimal point is rectangular reinforcing mesh to AS/NZS 4671 – RL818, RL1218 – not a level at all. “SLAB: 150 THK, RL1218 TOP” is a mesh specification.
- Depth. Excavation and pier depths are dimensions from an existing surface. An RL is absolute. Digging “to RL 19.200” and digging “3.0 m deep” only agree if the ground is exactly where the survey said it was.
AU and UK versus US
RL is standard in Australia, New Zealand, Britain and much of Asia and the Middle East. Heights are metres to a national datum – AHD in Australia, NZVD2016 in New Zealand, Ordnance Datum Newlyn in Great Britain, where levels are often written with AOD after the number rather than RL in front of it. Northern Ireland works to a different datum again, so “UK levels” is not one thing.
American drawings mostly do not use RL. The same idea is called an elevation, written EL or ELEV, in feet and inches, referred to NAVD 88 or to a project datum where the ground floor is set at EL 100′-0″ so nothing goes negative. Detailing to a US set, expect EL and expect an assumed datum note.
Where to check on your own drawing set
- Cover sheet or general notes: the abbreviations legend, and the datum statement.
- Survey plan title block: the datum and the mark it was picked up from. If it says ASSUMED DATUM, those RLs cannot be compared with flood planning levels or council height limits.
- Structural general notes: whether slab levels are quoted as SSL or FFL.
- The site bench mark itself, physically, before you set anything out.
If the architectural and civil sets quote different RLs for the same point, raise an RFI. Do not split the difference. More level and datum abbreviations are collected in the engineering drawing abbreviations list.
Other meanings of RL
Outside drafting, RL is best known as reinforcement learning in machine learning, R-sub-L for load resistance in electronics, and Rocket League. The collision that actually matters is inside drafting: in Australia and New Zealand, RL818 and RL1218 are rectangular reinforcing mesh designations to AS/NZS 4671, not reduced levels.