Expected value: how to tell whether a choice is worth it
Expected value (EV) is the thread that runs through every skill you have — it answers one question: “should I push, or should I fold?” Not by feel, but by arithmetic.
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Expected value for mahjong: how to compute tile-draw probability, how to read the EV of a hand, and the maths behind push/fold — replace feel with arithmetic.
What you will learn on this page
- What expected value is, with one worked example
- The three EVs mahjong players compute most often
- How EV changes the way you play
- Three common misconceptions
Lesson 1 below is free to read · Lesson 2 onward is member content
EV = Σ (outcome × probability)
Every example on this page is computed by
mahjong-score.js and mahjong-furi-ten.js, not estimated.
Lesson 1 · What expected value is, in one example
Suppose you face a choice with two outcomes:
| Option | Outcome | Probability | Contribution to EV |
|---|---|---|---|
| A: push | Win the hand, +2,600 points | 40% | 2600 × 40% = 1040 |
| Deal in, −3,900 points | 10% | −3900 × 10% = −390 | |
| EV of pushing = 1040 − 390 = 650 points | |||
| B: fold | Probably an exhaustive draw, lose 0 (you are not tenpai) | 100% | 0 |
Conclusion: pushing has positive EV, so you should push
But note — that conclusion only holds when you are last and folding loses anyway. If you are ahead, the 10% chance of paying 3,900 has to be weighed against the value of staying ahead, and the conclusion can flip. That is what push/fold really is: EV gives you the direction, the score gives you the threshold.
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What you can do afterwards
- What expected value is, with one worked example
- The three EVs mahjong players compute most often
- How EV changes the way you play
- Three common misconceptions
Lesson by lesson · 5 lessons on this page
Each lesson is one executable step: the principle, a worked hand, and a checklist you can follow. Non-members can read lesson 1; the rest unlocks with membership.
What expected value is, in one example
Available nowThe three EVs mahjong players compute most
How expected value changes the way you play
Three common misconceptions
From expected value to action
Sources
Rules follow the World Mahjong Organization (MCR) competition rules and mainstream Japanese riichi rules; methods and examples are compiled by this site. Terms and hand values: glossary, yaku encyclopedia. Regional variants (Sichuan, Guangdong, Changsha and others) follow the conventions of the table you actually play at.