In no-limit or pot-limit poker, a player's M-ratio (also called "M number", "M factor" or just "M") is a measure of the health of a player's chip stack as a function of the cost to play each round. In simple terms, a player can sit passively in the game, making only compulsory bets, for M laps of the dealer button before running out of chips. A high M means the player can afford to wait a high number of rounds before making a move. The concept applies primarily in tournament poker; in a cash game, a player can in principle manipulate his M at will, simply by purchasing more chips.
A player with a low M must act soon or be weakened by the inability to force other players to fold with aggressive raises.
The term was named after Paul Magriel.
The M-ratio is calculated by the formula:
For example, a player in an eight-player game with blinds of $50/$100, an ante of $10, and a stack of $2,300 has an M-ratio of 10:
That is, if the player only makes the compulsory bets, he will be "blinded out" of the game in 10 rounds, or 80 hands.
|Zone name||M-ratio||"Optimal" strategy|
|Green zone||M ≥ 20||Most desirable situation, freedom to play conservatively or aggressively as you choose|
|Yellow zone||10 ≤ M < 20||Must take more risks, hands containing small pairs and small suited connectors lose value|
|Orange zone||6 ≤ M < 10||Main focus is to be first-in whatever you decide to play, important to preserve chips|
|Red zone||1 ≤ M < 6||Your only move is to move all-in or fold|
|Dead zone||M < 1||You are completely dependent on luck to survive. The only move is to push all-in into an empty pot|
Harrington further develops the concept to account for shortening tables, as is seen at the closing stages of multi-table tournaments. The M-ratio is simply multiplied by the percentage of players remaining at the table, assuming a ten-player table to be "full". 
Therefore, for a player with a "simple M ratio" of 9 at a five player table, the effective M is 4.5:
This means that although the player's simple M value places him in the orange zone, his effective M dictates a shift in playing style appropriate for the red zone. In essence, ten times the effective M denotes the expected number of hands a player can let pass before running out of chips.
- PokerGreed.com: Understand what hands to play in a tournament . Accessed 2008-03-25
- Harrington on Holdem: The Endgame - Poker Book Reviews Archived 2006-12-07 at the Wayback Machine
- Tournament No-Limit Hold'em: Harrington's Zone System - Poker Strategy - PokerListings.com
- Harrington on Holdem, Volume II, p129
- Poker Tracker 4 Review