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How many backstroke flags does your pool need?
In short
A backstroke flag line carries one flag every 0.45 m — 0.20 m of flag plus a 0.25 m gap. A 25 m wide pool takes 56 flags per line and 112 in total, because every pool needs a line at each end, set 5.0 m from the wall and 1.8 m above the water.
- 0.45 m
- Per flag, including the gap
- 5.0 m
- From each end wall
- 1.8 m
- Above the water
- 2
- Lines per pool
SourcesSwimming Canada, Facility Rules & GuidelinesWorld Aquatics, Pool Certification — Swimming
Calculator
Wall to wall, across the lanes. Between 1 and 60 m.
Or estimate from lanes
An estimate at the standard 2.5 m lane. Measuring your pool beats deriving it.
- 56
- Flags per line
- 112
- Flags in total, both ends
- 24.95 m
- Span covered
- 0.05 m
- Left over at the ends
Where the lines go
| From each end wall | 5.0 m |
| Above the water surface | 1.8 m |
| Lines required | 2 |
Each flag
| Width on the rope | 0.20 m |
| Triangle sides | 0.40 m |
| Gap between flags | 0.25 m |
These figures cover the span across the water. They do not include the extra line needed to reach and tie off at the stanchions, which depends on how your pool is rigged — we work that out with you when we quote.
How the arithmetic works
World Aquatics sets each flag at 0.20 m along the rope, with 0.25 m between one flag and the next. So every flag after the first adds 0.45 m to the line.
For a pool of width W, the number of flags that fit is the largest whole number n where n × 0.20 + (n − 1) × 0.25 ≤ W. Rearranged, that is n = ⌊(W + 0.25) ÷ 0.45⌋.
A 25 m pool gives ⌊25.25 ÷ 0.45⌋ = 56 flags, which cover 24.95 m and leave 5 cm spare. A 20 m pool takes 45 flags and lands exactly on the width with nothing left over.
These figures describe the span across the water. They do not include the extra rope needed to reach the stanchions and tie off, which depends on how the pool is rigged.
Common questions
- How many backstroke flags do I need?
- One flag every 0.45 m across the width of the pool — 0.20 m of flag plus a 0.25 m gap between them. A 25 m wide pool takes 56 flags per line. Every pool needs two lines, one at each end, so that is 112 flags in total.
- How far from the wall do backstroke flags go?
- 5.0 metres from each end wall, suspended 1.8 metres above the water surface. Both Swimming Canada and World Aquatics specify these distances for sanctioned competition.
- What size is a backstroke flag?
- World Aquatics specifies flags measuring 0.20 m along the rope, forming a triangle with 0.40 m sides, spaced 0.25 m apart.
- Do I need flags at both ends of the pool?
- Yes. A flagged rope is suspended across the pool at each end, both set 5.0 metres from their end wall, so a backstroke swimmer gets the same warning approaching either wall.