Time Decay
Time decay is the daily erosion of an option's time value as expiration approaches. Learn why it speeds up, who it helps, and how it relates to theta.
Time decay is the steady loss of an option's time value as expiration approaches. Every day that passes, an option gives up a little of the value it held purely because of the time left on the clock.
If time value is the price of possibility, time decay is that possibility running out, one day at a time. It is the single most relentless force acting on an option. Let me show you the shape of it.
The Melting Ice Cube
An option is like an ice cube on a warm counter. From the moment you own it, it starts to shrink, not because anything went wrong, but simply because time is passing.
And it does not melt evenly. A big, fresh cube loses only a thin layer at first, then melts faster and faster as it gets small, until the last sliver vanishes in a rush. Time decay does the same thing. It is gentle when there is lots of time left, then accelerates hard in the final weeks. That speed-up has its own name: theta acceleration.
Watch It Bite
Apple is at $200 and stays there all week. You own a $200 call worth $5 a share, made entirely of time value.
One day passes, stock unchanged. The call slips to about $4.92. You lost roughly $0.08 a share, about $8 on the contract, and Apple never moved. That is time decay, plain and simple.
A full week passes, stock still $200. The call has bled to around $4.40. Nothing happened to the stock, yet the option quietly lost value every single day. This is the trap that catches new buyers: you can be right that a stock will not fall and still lose money, because the option needed a move that never came while decay kept nibbling.
Time Decay: Measure Versus Process
It helps to separate the process from its measurement.
Time decay is the process: time value eroding away as the days tick by. Theta is the measurement: the Greek that tells you how much value the option loses per day right now. When someone says an option has a theta of negative 0.08, they mean time decay is currently costing about $0.08 a share a day.
And time decay is a two-sided coin. It hurts the buyer, who watches their option melt, and helps the seller, who collects the premium up front and profits as it fades. That single fact is the engine behind income strategies like the covered call and cash-secured put.
- Time decay is the daily erosion of an option's time value.
- Like a melting ice cube, it speeds up near expiration.
- It is the process; theta is the number that measures it.
- It hurts buyers and helps sellers.
Pop Quiz
Three quick questions to see what stuck. Pick an answer and the explanation shows up right away.
What is time decay?
Time decay is time value eroding day by day, purely from the clock running down.
You own a call, the stock sits flat all week, and the call still loses value. Why?
Even with a flat stock, each day drains a little time value. That is time decay at work.
How do time decay and theta relate?
Time decay is the erosion itself, and theta is the number that tells you how fast it is happening.
Bottom Line
Time decay is the clock working against every option buyer. Time value melts away a little each day, slowly at first and then rapidly near the end, until nothing is left at expiration. Theta puts a number on that melt.
Which side of it you are on decides everything. Buy an option and you are racing the decay. Sell one and the decay is quietly paying you.
Keep going: the value that is eroding is time value, the Greek that measures the melt is theta, and the way it speeds up is theta acceleration.