If you cannot find the calculator, the rule of thumb is:
1 gallon of bleach will add the same number of its concentration to 10,000 gallons. IOW, 1 gallon of 6% adds 6ppm to 10,000 gallons. 1 gallon of 12.5% adds 12.5ppm to 10,000 gallons. So for 20,000 gallons you need twice as much.

The underlying formula used in all bleach calculators is:
(1,000,000/Pool Vol in Gallons) * (Bleach Concentration) * (# of gallons of bleach) = FC added in PPM.

Bleach concentration is a decimal, so 6% is 0.06. 5.25% is 0.0525, 12.5% is 0.125.

For people who measure their pool in liters and buy bleach in liters, simply replace liters wherever I have gallons.

Carl