A hip roof's square footage is equal to that of a gable roof with the same dimensions. The difference is in the amount of ridge capping you need and the amount you will waste. The top ridge will be shorter for a hip roof, but the 4 hips will add to the amount of ridge capping needed and because of the angles the amount of waste will be higher than for a similar sized gable roof.
With all the different types of shingles available, not all packs are equal in the amount of pieces they contain, the length of the individual shingle and the square metre coverage per pack. Not all types of shingles can be used for the starter strip and the ridge capping needed or you could be using ridge caps to cover the top ridge and the hips. That is why this calculator requires "the square metres one pack will cover" and the results for starter strip and ridge capping is in lineal metres needed for those two items and is not included in the "Total Packs Needed".