What Theta is, why it accelerates near expiry, and how the same number that's a cost for buyers is the income source for sellers. With current NIFTY ATM Theta.
Theta is the daily decay in option price assuming the underlying and IV stay the same. An option with Theta -₹5 loses ₹5 per day just from time passing.
Theta is the most-understood Greek by name and the most-misunderstood in practice. It's not linear — Theta decay accelerates as expiry approaches. An ATM weekly NIFTY option loses very little 7 days from expiry, then bleeds out viciously in the last 2-3 days.
Theta as a percentage of option price is small at 30 days to expiry — typically 1-3% daily decay on ATM options. At 14 days, decay is 3-5% daily. At 7 days, 5-10%. At 2 days, 15-30%. At 1 day, 40-70% in the final hours.
This is the famous 'Theta curve'. It's not linear — the last few days account for most of the total Theta decay over the option's life.
Practical implication for option buyers: don't pay full ATM premium 7+ days before expiry unless you have strong directional conviction. The Theta you pay is much bigger than the directional move usually delivers.
When you sell an option, you're long Theta — you collect that daily decay as profit. As long as the underlying doesn't move enough to overwhelm Theta with Gamma losses, you grind out money every day.
This is why weekly option selling is so popular in India. The accelerating Theta curve means you can sell options 2-3 days before expiry and collect outsized decay in a short window.
Combined with the European-style exercise rule (no early assignment), Indian options selling has cleaner mechanics than US options selling. You harvest Theta with no surprise assignments.
Higher-IV options have higher absolute Theta. BANK NIFTY options bleed more Theta per day than NIFTY options because BANK NIFTY runs higher IV. That's the source of the 'BANK NIFTY pays more for selling' folk wisdom.
But high-IV options also have higher Gamma risk and are more sensitive to surprise IV changes (Vega risk). The 'more income' from BANK NIFTY selling comes with proportionally more variance in outcomes.
Rule of thumb: Theta and Vega are roughly proportional within the same expiry cycle. Doubling your Theta income usually doubles your Vega risk too.
Depends on starting Theta. If Monday's Theta is -₹8, Tuesday's -₹10, Wednesday's -₹15, Thursday's -₹25 (expiry day), the cumulative cost is roughly ₹58 of the ₹100 premium. NIFTY would need to move enough to give the call ₹58 of intrinsic value just for you to break even — even though the spot is wherever it is on Friday.
Because time value collapses non-linearly. An option's price = intrinsic + time value. Time value decays slowly at first, then faster, then crashes to zero at expiry. Mathematically, Theta is proportional to the option's vega and inversely proportional to sqrt(time-to-expiry) — so it grows as expiry approaches.
For deep ITM puts on dividend-paying stocks, yes — the dividend mechanic can make Theta slightly positive. For everything else in Indian markets, Theta is negative for buyers / positive for sellers.
Per day, the weekly has higher Theta (decay concentrated in fewer days). Over its lifetime, the monthly has more total Theta to decay because its starting time value is bigger. Most retail Indian selling concentrates in weeklies because the per-day decay is more aggressive.
Roughly yes for ATM strikes — the put-call parity relationship keeps them close. Slightly different for far-from-ATM strikes due to skew. For practical sizing, treat ATM call Theta ≈ ATM put Theta.