How the calculator works
The bonus is your deposit times the match percentage, limited by any cap. The wagering base is either the bonus alone or the deposit plus the bonus, depending on the offer, and the total you must bet is that base times the multiplier. If the game you play counts for less than 100%, you need to bet proportionally more: at a 10% contribution, a $4,000 requirement means $40,000 of bets.
Each bet loses the house edge on average, so the expected cost of clearing the bonus is the bets needed times the house edge. Subtract that from the bonus for the expected value. The break-even multiplier is the highest wagering multiplier at which the bonus would still be worth its expected cost.
What the result does and does not tell you
The calculator gives an average over a very large number of bets, not a forecast of your session. Real results swing widely: you can finish far ahead or lose the whole bonus. It also ignores maximum-bet rules, cash-out caps, time limits and any games excluded from the offer, all of which can change what a bonus is worth in practice - check the offer terms for those. The house edge of a game is 100% minus its RTP; the game info screen or provider shows the RTP, and a typical slot is somewhere around 94-98%.