Options Greeks Explained Simply — Delta, Theta, Gamma, Vega

Your option price moves for reasons beyond the stock price. It can lose value on a flat day (theta). It can drop when the stock goes up (IV crush). It can accelerate in your favor as the stock moves deeper (gamma). The Greeks explain all of it. Here they are in plain English.

The Four Greeks That Matter

Greek	What It Measures	Analogy	Range
Delta	Option price change per $1 stock move	Your speedometer — how fast you are moving	0 to 1.0 (calls) or 0 to -1.0 (puts)
Theta	Daily time decay (value lost per day)	A melting ice cube — shrinks every day	Always negative for buyers
Gamma	Rate of change of delta	Your acceleration — how fast your speed changes	Highest at-the-money, near expiration
Vega	Option price change per 1% IV change	Weather sensitivity — how much the forecast affects value	Higher for longer-dated options

Delta — The Direction Greek

Delta tells you how much the option price moves when the stock moves $1.

Delta	Option Type	Stock Moves +$1	Stock Moves -$1	Probability of Expiring ITM
0.80	Deep ITM call	+$0.80	-$0.80	~80%
0.50	ATM call	+$0.50	-$0.50	~50%
0.20	OTM call	+$0.20	-$0.20	~20%
0.05	Far OTM call	+$0.05	-$0.05	~5%
-0.50	ATM put	-$0.50	+$0.50	~50%
-0.80	ITM put	-$0.80	+$0.80	~80%

Delta also approximates the probability of the option finishing in-the-money. A 0.30 delta call has roughly a 30% chance of being profitable at expiration.

Theta — The Time Decay Greek

Theta is the daily cost of holding an option. It is always working against option buyers.

Days to Expiry	Theta (ATM $100 Call)	Daily Cost per Contract	Impact
60 days	-$0.04	-$4/day	Slow — manageable
30 days	-$0.07	-$7/day	Noticeable
14 days	-$0.12	-$12/day	Accelerating quickly
7 days	-$0.20	-$20/day	Rapid decay
3 days	-$0.35	-$35/day	Extreme — option melting
1 day	-$0.50	-$50/day	Nearly worthless if OTM

This is why buying options with less than 14 days to expiration is risky. Theta accelerates and you need a large, fast move just to overcome the daily decay. Option sellers love theta — it puts money in their pocket every day.

Gamma — The Acceleration Greek

Gamma measures how much delta changes when the stock moves $1. High gamma means your delta (and therefore your P&L) changes rapidly with stock movement.

• At-the-money options: Highest gamma. Small stock moves create big delta changes. This is why ATM options feel "jumpy"
• Deep ITM or far OTM: Low gamma. Delta is already near 1.0 or 0.0 and does not change much
• Near expiration: Gamma explodes for ATM options. A $0.50 move can swing delta from 0.30 to 0.70, making the option either very valuable or worthless in hours

Vega — The Volatility Greek

Vega measures how much the option price changes when implied volatility (IV) moves 1%.

• High vega (long-dated options): A 5% IV increase could add $2-$3 to the option price. Conversely, a 5% IV decrease removes $2-$3
• Low vega (near-expiry options): IV changes have minimal effect because there is little time value left to inflate or deflate
• The IV crush trap: Before earnings, IV is high (50-80%). After earnings, IV drops to normal (25-35%). A long-dated call with high vega can lose $3+ of value from the IV crush alone — even if the stock moves in your direction

Putting It All Together

Your $5.00 call option price changed to $4.20 overnight. The stock went up $0.50. Why did you lose money?

• Delta (+$0.50 stock move x 0.40 delta): +$0.20 from stock movement
• Theta (-$0.08 per day): -$0.08 from time decay
• Vega (IV dropped 3% x $0.30 vega): -$0.90 from volatility decrease
• Net change: +$0.20 - $0.08 - $0.90 = -$0.78

The stock went up but IV dropped (maybe an event passed). The vega loss overwhelmed the delta gain. This is exactly the kind of situation the Greeks help you anticipate.

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