What is Vega in Options Trading?
Vega is the change in option price for a one-percentage-point change in implied volatility. It is the Greek that defines a volatility book.
Definition
Vega measures sensitivity to implied volatility. A position with vega +$10,000 will make $10,000 if implied vol rises by one point (e.g. 18% → 19%) and lose $10,000 if it falls one point. Vega is positive for long options (calls and puts both) and negative for short options. It is the primary Greek a vol trader manages — directional traders care about delta, vol traders care about vega. Vega peaks at-the-money and increases with time to expiry: long-dated options are vega-heavy, short-dated options are gamma-heavy. That asymmetry is the entire architecture of a vol book — you separate "front-month vega" (small, noisy) from "back-month vega" (large, structural) and trade them differently.
Why it matters & how it's calculated
Vega is ∂V/∂σ. Under standard models, vega = S · φ(d₁) · √T, where φ is the normal PDF and T is time to expiry in years. Three properties matter on a desk. First, vega is not constant — it varies with spot (vanna) and with vol itself (volga). Second, "1 vol point" of a 1-month option is structurally different from 1 vol point of a 2-year — you cannot net them naively. The fix is "vega buckets" (front, belly, back) or "weighted vega" using the square-root-of-time rule. Third, vega is not the same across the surface — implied vol of a 25-delta put and 25-delta call move differently from ATM vol, which is why vol books are managed by ATM-vega plus skew vega plus convexity vega rather than a single number. A book that is "vega-flat" on ATM can still be massively long skew and lose violently in a rally that compresses puts.
Formula
Vega = S · φ(d₁) · √T | Weighted vega ≈ vega × √(T_ref / T)
Worked example
You buy 100 SPX 6-month 5,000 straddles when implied vol is 18%. Each straddle has vega around $14 per vol point. Total vega = 100 × 100 × 14 = $140,000 per vol point. If vol drops to 16% over the next week (and spot is unchanged), you lose about $280,000 just on the vega move — before any time decay.
Related concepts
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