Priming sugar is a small, precisely measured dose of fermentable sugar added at bottling time so the beer's own residual yeast has exactly enough new fuel to produce the target amount of CO2 in a sealed bottle — too little and the beer stays flat; too much and it risks over-carbonation.
Not every sugar behaves the same, gram for gram
Corn sugar (dextrose), table sugar (sucrose), and dry malt extract all ferment differently and yield different CO2 per gram, which is why a priming calculation needs to know which sugar you're actually using rather than treating "priming sugar" as one interchangeable ingredient — run your specific sugar type through the priming sugar calculator alongside your batch volume, temperature, and target carbonation, since substituting one sugar for another at the same weight without recalculating changes the outcome.
Residual CO2 already in the beer is part of the real math
A beer isn't starting from zero dissolved CO2 at bottling — some CO2 remains in solution from fermentation itself, more if the beer was cold at packaging, and a proper priming calculation accounts for that residual amount rather than calculating purely from the target volume alone, which is exactly why the same target carbonation calls for a different sugar dose depending on the beer's temperature at bottling.