Calculator
Brine by weight of the finished solution
The percentage a salinity meter reads. Salt over salt plus water, not salt over water.
Result
52.6g salt
Working
Percent by weight of the finished brine. salt / (salt + water) = percent / 100 salt = water x percent / (100 - percent) = 1.000 kg x 5.00% / (100 - 5) = 52.6 g Finished brine: 1.053 kg, of which 5.00% is salt.
Why this needs a calculator at all
A fifteen percent brine in this sense holds fifteen grams of salt in every hundred grams of solution, which needs about 17.6 grams of salt per hundred grams of water. This is the reading a refractometer or a salinometer gives, so it is the number to work in if you intend to check the brine later, which the FAO bulletin says you should: it warns that a cucumber brine falling below twelve percent will spoil.
What this is not
Arithmetic, not advice. It converts the number you type; it does not know what you are fermenting, how warm your kitchen is or whether your salt is pure. Nothing on this page certifies that a preparation is safe, and no result here should be read as a recommendation to ferment at any particular strength. The published parameters, with the sentence each came from, are on the tradition pages, and what can go wrong is on safety.