Bucket test
Also called: evaporation test, pool leak bucket test.
The bucket test separates evaporation from a leak. Fill a 5-gallon bucket with pool water to about an inch from the top, set it on a pool step immersed a few inches, mark the level inside and outside, and compare the drops. Equal means evaporation; a bigger drop outside means a leak.
Why it matters
Pool water and immersion both matter because they keep the water inside the bucket at the same temperature as the water outside, and temperature is what drives evaporation. Leave the pool at its normal level with the pump on its usual schedule. Run it longer than 24 hours: an ambiguous result is not a pass. Then stop topping up and note where the level stabilises, because that level is the height of the leak. On an above-ground pool the test is the same, on a step or ladder tread, but treat a leak as urgent, because escaping water washes the base out from under the wall.
In numbers
About a quarter inch a day is most likely evaporation; half an inch a day is worth investigating. Owners repeatedly report the two marks looking essentially identical at 24 hours and clearly separated by day three.