Theta is the rate at which an options contract loses value each day purely due to the passage of time — independent of whether the underlying stock or index moves at all. Theta decay is the accumulated loss of that time value as the contract approaches its expiry date.
Every option has two components of value: intrinsic value (how far in-the-money the option currently is) and time value (the additional premium reflecting the possibility that price could move further in the buyer's direction before expiry). Theta measures how much of that time value disappears each calendar day.
How Theta Accelerates Near Expiry
Theta decay is not linear. An option with 30 days to expiry loses time value slowly. An option with 5 days to expiry loses time value quickly. An option in the final session before expiry can lose most of its remaining time value in hours.
This acceleration follows a curve: the same option that loses ₹2 per day three weeks before expiry may lose ₹15–20 per day in the final week, and ₹50–100 per day on expiry day itself — all without any movement in the underlying.
For NSE weekly options (BankNifty expiry Thursday, Nifty 50 weekly expiry), this decay accelerates through Wednesday and becomes extremely rapid on Thursday morning.
What This Means for Option Buyers
Option buyers pay a premium at entry. Theta works against them every day the trade is open. If an option buyer holds a BankNifty call that cost ₹200 at Monday's open, and BankNifty is exactly flat by Thursday's close, that call is worth approximately ₹0 at expiry — the entire ₹200 premium has decayed to zero through theta alone.
An option buyer's position is profitable only if the underlying moves enough in the right direction, fast enough, to outpace theta decay. On expiry day, this threshold is high: a significant intraday move is required just to offset the accelerating time decay, and the window for that move is shrinking by the minute.
What This Means for Option Sellers
Option sellers collect premium at entry and profit from theta decay. Every day that passes without the underlying moving against their position is a day that works in their favour. Option sellers on NSE — particularly those selling weekly BankNifty or Nifty options — are structurally positioned to benefit from theta, provided the market does not make a large move against their strikes.
This structural advantage is why a large proportion of consistent profits in Indian F&O accrue to option sellers rather than option buyers, and why SEBI data shows the majority of retail traders who primarily buy options lose money consistently.
For educational purposes only. Profitma is not a SEBI-registered investment adviser or research analyst. Nothing in this article constitutes investment advice.