What is the Yahtzee bonus?
Yahtzee actually has two bonuses, and the big one - 100 points per extra Yahtzee - is what turns a good game into a monster score.
The first Yahtzee: 50 points
Roll five of a kind and the Yahtzee category pays a flat 50, the single biggest box on the card. It doesn't matter whether it's five ones or five sixes - the category scores 50 either way. The catch is that you only have one Yahtzee box, which is where the bonus rule comes in.
Extra Yahtzees: +100 each
Roll a second (or third, or fourth) Yahtzee in the same game and each one adds a 100-point bonus - but only if your Yahtzee box holds a 50 rather than a zero. The dice from the extra Yahtzee then also fill another open category under the joker rules, so the roll pays twice. Stacked bonuses are how the theoretical maximum reaches 1,575, as multiple Yahtzees explains.
Don't confuse it with the upper bonus
The other bonus lives in the upper section: score 63 or more across Ones through Sixes and you earn 35 extra points. That one is the reliable, plan-for-it bonus, while the 100-point Yahtzee bonus is the jackpot - the odds are covered in what are the odds of a Yahtzee, and the upper-section math in the upper bonus guide.
Related questions
Can you get more than one Yahtzee in a game?
Yes. Your first Yahtzee scores 50 in the Yahtzee box, and every additional one earns a 100-point bonus - as long as that box holds a 50 rather than a zero. The extra Yahtzee's dice then also fill another open category under the joker rules, so a repeat five-of-a-kind pays twice.
What is the upper section bonus?
Score 63 or more points across the six upper categories - Ones through Sixes - and you earn a 35-point bonus. The threshold equals exactly three of each number: 3x(1+2+3+4+5+6) = 63. Chasing this bonus early is the single most reliable habit in good Yahtzee play.
What are the odds of rolling a Yahtzee?
On a single throw of five dice, the odds of all five matching are 6 in 7,776 - that's 1 in 1,296, or about 0.08%. Across a full turn of three rolls with optimal keeping, the chance rises to roughly 4.6%. In practice that works out to about one Yahtzee every couple of games.