What is DEF (Defensive Value)? Definition, Formula, and Example
DEF is FanGraphs' defensive value metric, combining a player's fielding runs relative to average with a positional adjustment that credits the difficulty of their position.
What is DEF?
DEF is FanGraphs' all-in-one defensive value stat. It measures how many runs a player contributes on defense compared to an average player, combining two things: their actual fielding performance and the inherent difficulty of the position they play. A league-average shortstop posts a strongly positive DEF because simply holding down shortstop — a position most players can't handle — is itself valuable.
How DEF is Calculated
The formula is straightforward:
DEF = Fielding Runs + Positional Adjustment
- Fielding Runs comes from UZR for most seasons (DRS-based for recent years), measuring range, arm, double-play, and error runs versus average at the position.
- Positional Adjustment is a fixed run value per 1,450 innings (roughly a full season) that scales positions by scarcity and difficulty: catcher (+12.5), shortstop (+7.5), second base (+2.5), center field (+2.5), third base (+2.5), right field (-7.5), left field (-7.5), first base (-12.5), and designated hitter (-17.5).
The logic: moving an average shortstop to first base would make him a great first baseman, so the shortstop deserves credit for playing the harder spot.
A Real Example
Take a shortstop who is exactly average with the glove over a full season. His fielding runs are 0, but his positional adjustment is +7.5, so his DEF is +7.5 — he's worth 7.5 runs on defense before making a single above-average play. Contrast that with a first baseman with genuinely good glovework, say +3 fielding runs: his DEF is 3 − 12.5 = -9.5. Even a good-fielding first baseman is a defensive liability in value terms, which is why first basemen must hit to stay in the lineup. This is the math behind why WAR loves catchers and shortstops.
Why DEF Matters
DEF is the defensive engine inside FanGraphs WAR. It explains roster construction decisions that look strange on the surface — why teams carry light-hitting shortstops, why bat-first prospects get moved off catcher early, and why a "good defensive first baseman" is a backhanded compliment. For fantasy and card evaluation, DEF tells you which players have a defensive floor that keeps their bat in the lineup through slumps.
Limitations and Misconceptions
DEF is not a pure measure of glove skill — a player can have a high DEF while being a mediocre defender, purely because of position. For fielding skill alone, look at OAA or DRS. The positional adjustments are also estimates based on historical position-switch data, and they can lag behind modern usage (e.g., the rising athleticism of second basemen). And because fielding metrics need large samples to stabilize, one season of DEF carries real uncertainty.
In Legends Deck: DEF-style positional scarcity is baked into card values — premium-position defenders carry hidden worth beyond their raw fielding grade, just like in real WAR calculations.
Related: WAR · UZR · DRS · OAA · Range Factor