1 + 2 + 3 + 4 + 5 + 4 + 3 + 2 + 1 = 25 laps = 2,500m
You will therefore only need to count a maximum (in a 50-metre pool) of 5 laps, which avoids having to keep track of a silly amount of numbers and giving yourself a headache!
Simply insert it in a plastic sleeve and bring it with you, to the edge of the water, next to your water bottle. Take a quick look with every sip and hurray!
So what do you think is the best solution?
And how about you, how do you count your lengths?