Short answer, you don't. You should have a general sense of where you are, but Recruit someone to count laps for you. There will be lap boards available. For the 1500 they'll start at 59 and go down by 2's, (scm). Last lap (25 to go) is an orange or red square. You'll also get a bell with 50 to go.

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2020 National Masters Champion - M50-54 - 50m Butterfly