You saw 260 on the screen. You pressed buy. Your fill came back at 260.50. Nothing went wrong, nobody cheated you, and no news broke in that half second.
That fifty paise is the spread, and on a SENSEX weekly option it is one of the most reliably ignored costs in retail F&O. Brokerage you can see on the contract note. This one hides inside the price.
Volume is how many contracts changed hands today. It resets every morning.
Open interest is how many contracts are currently alive and unsettled. It carries over.
Liquidity is neither of those. Liquidity is the answer to one question: if I want to get out right now, how much worse than the screen price will I get?
Volume and open interest are useful because they usually predict liquidity. But the thing that actually costs you money is the spread, and you can read that directly.
Walk into any sabzi market. The shopkeeper sells tomatoes at ₹40 a kilo. If you walked up and offered to sell him tomatoes, he would pay you maybe ₹32. He is not being unfair — that ₹8 gap is his entire business. He takes the inventory risk and he gets paid for it.
An options market has exactly the same two prices. The ask is what somebody is willing to sell to you at. The bid is what somebody is willing to buy from you at. The number your app shows in large font — the LTP — is neither. It is a receipt from a trade that already finished.
Here is the trap that catches most new option buyers. Measured in rupees, spreads look roughly similar across strikes. Measured as a percentage of the premium you paid, they are wildly different.
Illustrative figures for a SENSEX weekly chain with the index near 74,700:
| Strike | Premium | Typical spread | Round trip per lot | As % of premium |
|---|---|---|---|---|
| At the money | ₹260 | ₹0.50 | ₹20 | 0.4% |
| 300 pts away | ₹110 | ₹1.00 | ₹40 | 1.8% |
| 1,000 pts away | ₹18 | ₹1.50 | ₹60 | 17% |
| 2,000 pts away | ₹3 | ₹1.00 | ₹40 | 67% |
Read that last row again. On a ₹3 option, the market can be quoting 2.50 bid and 3.50 ask. You buy at 3.50. If nothing at all happens and you exit immediately, you get 2.50 back. You have lost a third of your capital on a flat market.
This is the real reason the ₹3 lottery ticket disappoints so consistently. It is not only that it expires worthless most weeks. It is that you started the trade already deep underwater, and the option has to travel a long way just to get you back to zero.
SENSEX weeklies expire on Thursday. On expiry day, liquidity does not simply rise or fall — it concentrates.
The strikes near the money get more liquid than any other day of the week. Spreads there can compress to a tick or two. Meanwhile the far strikes get thinner, because nobody wants inventory in a contract with hours left to live. The chain stops being a smooth curve and turns into a spike: dense in the middle, hollow at the wings.
The same thing happens in miniature at the edges of every session. The first few minutes after 9:15 and the last few before the close are when quotes are widest and the book is thinnest.
This is the most common version of the complaint, and the spread explains most of it.
A stop-loss market order does not promise you a price. It promises you an exit. When it triggers, it sweeps whatever bids are sitting in the book. On a thick ATM strike, the bid right below yours is a tick away and you barely notice. On a thin strike during a fast move, the next bid down might be three rupees away — and that is where you get filled.
The stop did its job. The book was simply empty underneath it.
Spreads are not an edge and not a strategy. They are a toll. The point of understanding them is simply to stop paying the toll without knowing you paid it.