Riichi Mahjong Odds Calculator

Instant real-time win rates for Riichi Mahjong, backed by solid math.

Visualize the "win rate" you can't see at the table.

"Should I push with this hand, or fold now?" In mahjong, push-or-fold decisions have always relied on vague factors like experience, intuition, and luck. As a result, players often charge ahead on momentum when their odds are actually low, or fold when their odds are actually high — costly misjudgments that happen all too often. This app displays your final probability of completing a winning hand as a concrete number (%) in real time, calculated from your current situation such as the number of discards and winning tiles. It gives you a precise, data-driven standard for push-or-fold decisions — not gut feeling. Use it as a tactical assistant that calculates the "true win rate" of your hand in real time, based on rigorous mathematical principles (such as the hypergeometric distribution) that strip away vague factors like luck.

■ Main Features This app is a tool for learning and analyzing mahjong probabilities. Its main features are as follows.

・Visualize your winning probability When you reach tenpai, see your probability of winning by self-draw (tsumo) and by claiming a discard (ron), each as a concrete number (%). It even factors in the risk of an opponent winning first, making your "net final win rate" clear at a glance.

・Predict the future with the probability trend table See a table predicting how your win rate changes as discards pile up. Read the future trend — "how many more turns can I hold out?" and "when should I fold?"

・Tenpai odds and tile-acquisition odds, too Beyond your winning probability, you can also calculate your odds of reaching tenpai from one-away (iishanten), and your odds of obtaining the tiles you're aiming for.

・Automatic correction for turn shifts from calls Even when pon, chi, or kan change the turn count, just enter the call details with a few simple taps, and the remaining draws and probability calculations are corrected automatically.

・Subjectively assess opponents' behavior Reflect on-the-ground realities — like "that player has folded completely" or "someone declared riichi" — in your calculations through a simple coefficient selection. Translate your reads into a more realistic win rate.

■ Built for real games We adopted a battle-ready UI you can operate smoothly with one thumb while holding your phone in one hand. Just enter the number of discards, and your probability appears with a single tap. It's thoroughly designed to be used stress-free even during fast-paced online mahjong.

■ Accuracy grounded in mathematics This app's probability calculations are based on the fact that the chance of a winning self-draw in mahjong follows a probability mass function known as the "hypergeometric distribution." The probability that an opponent wins first, or that you deal into their hand, cannot truly be quantified; but by combining a statistical model with correction coefficients, we calculate a win rate closer to real play. Specifically, we ran Monte Carlo simulations of roughly 500,000 games under each condition, varying the probabilities of opponents winning first and of dealing in. As a result, we confirmed that the win rate derived approximately through our original correction coefficients closely matches the measured simulation results — achieving a high level of accuracy that is more than sufficient for practical use. These probabilities can also be adjusted according to your own subjective assessment of each opponent's situation, enabling win-rate calculations that faithfully account for the realities of the table.

■ Recommended for ・Anyone who wants an objective standard for push-or-fold decisions ・Anyone who wants to review whether their discards were statistically correct ・Anyone who wants to learn mahjong theoretically, one step deeper ・Anyone who plays online mahjong often

■ Highlights ・No ads, no in-app purchases ・Works fully offline (no connection required) ・Comfortable to use even in fast-paced games like online mahjong