Set the field size and the format. Every figure is derived arithmetically, not estimated, and the derivations are written out underneath so you can check them.
The formulas, so you can check them
Single elimination. Matches = N − 1. Not an approximation: each match eliminates exactly one entrant, and exactly N − 1 entrants must be eliminated. Rounds = ceil(log2 N). Bracket size = the next power of two at or above N; byes = bracket size − N.
Double elimination. Matches = 2N − 2, or 2N − 1 with a bracket reset. The same argument run on losses instead of entrants: the event ends when all but one entrant hold two losses, which is 2(N − 1) losses, and each match yields one. The reset adds a single loss and therefore a single match. Upper-bracket rounds = ceil(log2 N); lower-bracket rounds = 2·ceil(log2 N) − 2.
Round robin. Matches = N(N − 1)/2 — every unordered pair, once. Rounds = N − 1 for even N and N for odd N, using the circle method, where one entrant sits out each round when the field is odd.
Swiss. Matches = R × floor(N/2), because every round pairs as much of the field as it can. Minimum rounds for a single undefeated entrant = ceil(log2 N), since each round at most halves the undefeated set.
GSL groups. Five matches per group of four, two qualifiers. It is a four-entrant double-elimination bracket truncated the moment two entrants hold two losses: 2×4 − 2 = 6 for the full thing, minus the grand final you do not need, because you are not ranking the two qualifiers against each other.
Groups into a playoff. Group matches + (qualifiers − 1). With sixteen entrants that is four groups (20 matches) feeding an eight-entrant bracket (7 matches) for 27 in total, across three group rounds and three playoff rounds. This is the standard shape of a real event, and the calculator will size it for any field.
Two things the calculator deliberately does not do
It counts series, not games. If your matches are best-of-three, multiply by somewhere between two and three for a game count, and note that the variance in that multiplier is what makes tournament days overrun. Series length is a separate lever from bracket shape, and often the better one — see the note at the end of tournament formats.
It says nothing about wall-clock time. Match count is one constraint on your schedule; round count is a completely independent one, because rounds cannot be parallelised. There is a tool for that in running a tournament.