Every bottle of pool chemical has a dosing chart on the back, and every one of those charts assumes you know how much water you have. Most people guess, and a guess of 10,000 gallons on a pool that holds 16,000 means every dose is wrong from the start.
Work out the volume once
For a rectangular pool, multiply length by width by average depth in feet, then by 7.5 for gallons. For a round pool, multiply the diameter squared by the average depth by 5.9. Do it once, write it down, and every calculation after that gets easier.
Test before you add anything
Free chlorine, pH, total alkalinity, calcium hardness and stabiliser all interact. Adding chlorine to water with the pH out of range does very little, and chasing pH while the alkalinity is wrong just makes it bounce. Get the readings first, then fix them in order.
Alkalinity, then pH, then chlorine
Alkalinity steadies the pH, so it goes first. Once the pH holds in range, chlorine can do its job properly. Doing it the other way round is how people end up adding chemicals every day and never getting clear water.
Hot tubs are the same maths, smaller
A hot tub holds a few hundred gallons, so dosing errors show up much faster. Overdosing a small volume of hot water is easy, and it is unpleasant for whoever gets in next.
ClearPool is an Android app where you type the readings off your test kit and get the dose for your pool’s actual volume, covering chlorine, acid, baking soda, calcium, stabiliser and salt, for pools and hot tubs.