You pressed the button at ₹152. The contract note says ₹161. Nobody cheated you. You used a market order on a premium that was moving faster than your thumb.
This is the single most expensive thing a new options trader does not understand, and it has nothing to do with picking the right strike.
A market order says: fill me now, at whatever price is available. You are certain of the fill. You are not certain of the price.
A limit order says: fill me at ₹152 or better, otherwise wait. You are certain of the price. You are not certain of the fill.
That is the entire trade-off. Every argument about order types is just this sentence applied to different situations.
You walk up to a pav bhaji stall at 8 PM on a Saturday. Twenty people are waiting. You can say "one plate, whatever it costs" and eat in five minutes — that is a market order. Or you can say "one plate, but only at ₹60" and stand there while the stall owner serves everyone who did not argue — that is a limit order.
On a quiet Tuesday afternoon with nobody in the queue, both work identically and you pay ₹60 either way. The difference only shows up when there is a crowd. Options premiums have a crowd on expiry day.
SENSEX weekly options trade on BSE, expire on Thursday, come in lots of 20, and have strikes every 100 points. On a calm Monday morning, an at-the-money premium might show a bid of ₹179.50 and an ask of ₹180.50. That ₹1 gap is the spread, and the ladder behind it is thick — plenty of quantity resting at each price.
Now move to Thursday at 2 PM with the index swinging. The same premium might show ₹176 bid and ₹184 ask, and the quantity at each price is thin. Your market order does not get ₹180. It walks up the ladder, taking whatever is there, and your average fill can land several rupees away from the price you saw.
This is where the two order types stop being an academic topic. A stop-loss order comes in two common flavours.
A stop-loss limit order has a trigger price and a limit price. When the premium touches your trigger, the order goes to the exchange as a limit order. If the premium is falling in ₹5 jumps, it can blow straight through both your trigger and your limit in the same instant. Your stop-loss is now a resting order sitting above the market, doing nothing, while your loss keeps growing.
A stop-loss market order (SL-M) has only a trigger. When touched, it becomes a market order and takes whatever the ladder offers. You get out. You may get out several rupees worse than your trigger.
Slippage is invisible because it never appears as a line item. It is buried inside your fill price. With a lot size of 20, here is what each rupee of slippage costs on a round trip — once on entry and once on exit.
| Slippage per side | 1 lot (20 qty) | 4 lots (80 qty) | 13 lots (260 qty) |
|---|---|---|---|
| ₹0.50 | ₹20 | ₹80 | ₹260 |
| ₹2 | ₹80 | ₹320 | ₹1,040 |
| ₹5 | ₹200 | ₹800 | ₹2,600 |
| ₹10 | ₹400 | ₹1,600 | ₹5,200 |
Read the bottom-right cell again. On a 13-lot position, ₹10 of slippage on each side is ₹5,200 — gone before the market has moved in your favour or against you. Twenty such trades in a month and slippage alone is a bigger number than most people's monthly loss.
Three conditions decide whether a market order is cheap or expensive on a given contract.
It is also worth knowing that exchanges apply price-protection bands to market orders in F&O, so a market order cannot fill at an absurd price — but the band is wide enough that a painful fill is still entirely possible inside it.
In our own SENSEX level-break system, the order type is not a preference — it is a consequence of what the order has to do. An exit that must happen has to prioritise certainty of fill. An entry that is optional can afford to prioritise price and be skipped if it does not fill. We also compare our modelled fill price against the actual broker fill, because that gap is the honest measure of whether a strategy survives contact with a real order book.
Nothing here is a recommendation about which order type you should use. It is a description of what each one does, so that the next time your contract note disagrees with the screen you saw, you know exactly which of the two promises you made — and which one the market kept.