What is the original concentration of your 100 ml?
To dilute 100ml of something to 2.5% by volume, first calculate 100 divided by the desired % 100/2.5 = 40.
So you would want to end up with 40 times the volume, or 4000 ml (4L). Add 3900 ml water (3.9L) because 100ml +3900ml = 4000 ml
https://en.wikipedia.org/wiki/Dilution_(equation)
But that assumes that your 100 ml is pure, or 100% concentration of your active ingredient. If your 100ml is already diluted to a certain percent, you have to reduce the added water in proportion.
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