calculate pH of phosphoric acid

 1/4 teaspoon of 10% phosphoric acid  for 19 litres. calculated from bate: pH 4.16

For 1 drop = 0.05ml, it means roughly 25 drops

Step 1. Convert mass ( or volume) and concentration ( % w/w) to mole per litre ( mol/L)

Step 2. Use BATE or other app to calculate pH

If aiming for pH 5.6, conc will be 2.5 e-6, or 1/100 tsp , 0.044 ml, roughly 1 drop

From homebrewtalk, AHA, summer rye, hefe Gordon Strong suggested to use 1/4 teaspoon of 10% phosphoric acid. pH would be 4.16....

my excel

For 2L, 

for pH4.16, then it means ~2.6 drop

for pH5.6, then it means ~0.1 drop

Better still, 

using pH value to calculate mol/L 

Using the calculated mol/L to find required volume ( or mass) or acid required 

For pH4, or  more diluted acids, molarity=10^(-1*pH)

Molarity= moles/ L

using Henderson-Hasselbalch equation, as explained in BATE pH calculator cheatsheet:

pH 3 to 4, 10% phosphoric acid:

pH 4 to 5, 10% phosphoric acid:

pH 4 to 5, 1% phosphoric acid:

So for 19L volume, 10% phosphoric acid as starting point is practical. But for 2L volume, 1% would be better