more than likely it was pre-measured for 5 ga at a midway point of atmospheric pressure for that style and optimal temperature range for the yeast, which gives it some leeway, so you should be OK using the entire amount
OR
if you want to be absolutely sure to use the correct amount, you'll need to consult one of the many online priming sugar calculators, input your batch size, the temperature of the beer and what carbonation level you want. then you'll need to break out a scale and WEIGH out the proper amount.
my beers were severely under-carbed until I started going by weight instead of volume