What is leverage index in baseball?

Leverage index (LI) answers a different question than win probability. Win probability tells you who’s likely to win; leverage tells you how much the next play could change that answer. It’s a measure of how much is riding on the current moment — how far win expectancy might swing depending on what happens next.

The stat was created by analyst Tom Tango, and it’s built to be dead simple to read because it’s normalized around a single anchor: 1.0 is an average, perfectly neutral situation. Anything above 1.0 is more important than average; anything below is less. A leverage of 2.0 means the moment is twice as pivotal as a typical one.

That’s it — one number, centered on 1, telling you how much the game hangs on the next pitch. It’s the difference between a meaningless at-bat and one where the whole season can turn.

Because leverage is centered on 1.0, the scale is easy to internalize:

• Below 1.0 — low tension. A blowout in the fifth, or the leadoff at-bat to start a game. Whatever happens barely moves the needle.
• Around 1.0 — an average, ordinary situation.
• Above 1.0 — rising tension, climbing all the way to roughly 10 in the most extreme spots a game can produce.

The distribution is lopsided in a telling way: about 60% of all game situations have a leverage index below 1.0, and only around 10% climb above 2.0. In other words, most of a baseball game is fairly low-stakes, and the truly decisive moments are rare.

That’s why they’re worth catching — the handful of plays above LI 2.0 are where games are actually won and lost, even though they’re a small slice of the pitches thrown.

Three ingredients stack up to create a high-leverage moment, and they’re the same ones that drive win probability:

• Late innings. The fewer outs remain, the less time there is to recover, so each play matters more.
• A close score. Tie games and one-run margins are where a single swing flips the outcome; leverage collapses in a blowout.
• Runners on base. The more runners aboard — especially in scoring position — the bigger the potential swing on the next pitch.

The textbook example: bases loaded, bottom of the ninth, one-run game. A single wins it; a double play ends it. Win expectancy could lurch by 50 points or more on one pitch, so leverage skyrockets.

Compare that to the very first batter of the game, where nothing that happens can meaningfully move the odds — that’s about as low-leverage as baseball gets.

Leverage isn’t just a fan’s lens — it’s a management tool. One of its most useful applications is deciding which players to deploy in the moments that matter most. A manager who saves his best reliever for a low-leverage blowout and burns lesser arms in the tightest spot is, in leverage terms, using his roster backwards.

That’s why you increasingly see elite relievers brought in for the highest-leverage inning rather than mechanically in the ninth, and why pinch-hitting decisions cluster around big spots.

Leverage also lets analysts grade usage after the fact: add up the leverage of every situation a reliever entered, and you can see whether a manager trusted him with real pressure or mop-up duty. The stat turns a fuzzy notion — “that was a big moment” — into a number you can plan around and review.

It’s tempting to assume some players simply rise in high-leverage moments — that they’re clutch. Leverage lets you test that, and the honest answer from the data is deflating: performance in high-leverage spots isn’t a reliably repeatable skill.

A hitter who looks ice-cold clutch one season is no more likely than anyone else to repeat it the next, and most clutch reputations turn out to be small-sample noise.

That doesn’t make leverage less useful — it makes it more honest. Leverage tells you which moments mattered and which players were on the spot when the game was on the line, without pretending that showing up in those moments is a mystical talent. It measures the situation, not the soul. Knowing that keeps you from overrating a hot October and helps you appreciate the moments themselves for what they are.

The problem with leverage is that the highest-leverage plays are rare and easy to miss — the very moments you most want to catch. TwentySeven is built to surface them. Open any game and its Top Moments panel pulls out the five highest-leverage plays — the pitches where win probability swung the most — so you can see exactly where the game turned without rewatching all nine innings.

It works live, too. Smart notifications fire only for moments that actually carry leverage: high-leverage spots, late-and-close games, walk-off opportunities, and no-hitters in progress — never routine runs in a blowout.

To go deeper, start with how win probability works, the foundation leverage is built on, then see how win probability added credits the players who came through in those big spots.