A butterfly is a defined-risk option structure that pays most when the underlying finishes near a specific price and loses a small, fixed amount if it finishes anywhere far from it. You buy one option below, sell two in the middle, and buy one above.
That is the whole idea. The reason to care is efficiency: it concentrates your capital on a narrow expectation rather than spreading it across a wide one, which is why the reward-to-risk ratio looks so attractive and also why it is wrong more often than a wider structure.
This post assumes you know what a call and a put do. If credit spreads and iron condors are new, start with income-oriented option spreads, which covers the simpler structures a butterfly is built from. And read the disclaimer at the foot: this is educational, every number below is illustrative, and nothing here is advice.
Why the shape looks like a tent
Think of a butterfly as two spreads stacked back to back. You sell a spread on one side and buy a spread on the other, sharing a common middle strike where you are short two contracts.
The two short contracts at the body are doing the earning. They decay faster than the wings you own, because at-the-money options carry the most time value and lose it quickest. The wings exist for one reason: to cap what happens if you are badly wrong. You give up most of the possible profit to buy certainty about the worst case.
That trade is worth making when you have a view on where something will finish but no appetite for an unbounded loss if you are wrong about it.
The long call butterfly
The construction, with W as the wing width:
- Buy one call at the lower strike,
K1 - Sell two calls at the middle strike,
K2, whereK2 = K1 + W - Buy one call at the upper strike,
K3, whereK3 = K2 + W
You pay a net debit for this, and that debit is the most you can lose.
The arithmetic is worth doing once by hand.
| Quantity | Expression |
|---|---|
| Net debit | Premium of K1 call, minus twice the K2 premium, plus the K3 premium |
| Maximum profit | W minus the net debit, achieved only if the underlying finishes exactly at K2 |
| Maximum loss | The net debit, and nothing more |
| Breakevens | K1 plus the debit, and K3 minus the debit |
An illustrative case, using round numbers rather than live prices. Take strikes 24,000, 24,500 and 25,000, so the wing width is 500, and suppose the structure costs 125 per unit. Maximum profit is 375, maximum loss is 125, and the breakevens are 24,125 and 24,875.
The reward-to-risk ratio there is three to one, which looks excellent until you notice the profitable band is 750 points wide out of a 1,000 point structure, and you only collect the full 375 at a single point. Realistically you are trading a three-to-one payoff for something well under a one-in-three chance. That is the honest framing, and it is the one most descriptions of butterflies leave out.
A long put butterfly built the same way below the money has an identical payoff shape. The choice between them is usually about which strikes are more liquid.
The iron butterfly is the same trade
An iron butterfly sells an at-the-money call and an at-the-money put, then buys a call above and a put below. It arrives as a net credit rather than a net debit, which makes it feel like a different animal.
It is not. Because of put-call parity, an iron butterfly and a long call butterfly at the same strikes have the same payoff at expiry. Credit and debit are two descriptions of the same position, and the choice between them comes down to practical things:
- Liquidity. You want the tightest bid-ask spreads across four legs, and that usually means using out-of-the-money options on both sides, which favours the iron construction.
- Assignment. Short at-the-money options carry more assignment risk than deep out-of-the-money ones. For cash-settled index options this matters less than it does for stock options.
- Margin treatment, which is genuinely different and is covered below.
Traders who believe they are collecting free money by preferring credit structures have simply not drawn the payoff diagram. If the shape is the same, the risk is the same.
The broken-wing butterfly, and its hidden tail
A broken-wing butterfly deliberately makes one wing wider than the other. Widening a wing means buying cheaper protection on that side, which reduces the cost of the structure, often to zero or to a small credit.
This sounds like a free improvement. It is not, and the diagram shows why.
What you have actually done is sell a small amount of tail risk to fund the structure. Most of the time you keep the credit, and occasionally you take a loss several times larger than the balanced version would have suffered. The expected value may still be fine. The distribution of outcomes is not the one your account statement will suggest after a quiet couple of months.
Size a broken wing by its worst case, not by its typical case. That is the discipline the structure demands, and it is the one that gets abandoned after a winning streak.
How the Greeks behave, and why the last week is different
The payoff diagram shows expiry. Everything before expiry is governed by the Greeks, and a butterfly’s are unusually position-dependent.
Theta works for you only near the body. With the underlying sitting at the middle strike, the two short options decay faster than the two you own, and time passing makes you money. Out near a wing, that reverses: the option you own is the one bleeding, and time is now against you.
Vega is negative near the body. You are net short volatility there, so a rise in implied volatility hurts even if price has not moved. This is why entering when volatility is already low is a poor idea. There is little left to fall and plenty of room to rise.
Gamma is the one that bites. Near the body and close to expiry, gamma turns sharply negative, meaning your delta swings violently against you for small moves in the underlying. A position that looks perfectly placed on the Monday of expiry week can flip through its whole profit range in a single session. This is the mechanism behind most stories of a butterfly that was winning and then was not.
The practical reading: a butterfly is comfortable in the middle of its life and genuinely hazardous at the end, which is precisely when its theoretical profit is largest. That tension is the trade.
Choosing strikes
Strike selection is where the view actually gets expressed. Two questions decide it.
Where do you expect the underlying to finish? The body goes there. If you have no real answer, you should not be trading a butterfly, because the entire structure is a bet on a specific level.
How confident are you? That sets the wing width. Narrow wings give a higher reward-to-risk ratio and a smaller target to hit. Wide wings do the reverse. A useful discipline is to set the width from a measure of realised movement rather than from the payoff you would like: something like the recent average daily range, scaled by the days to expiry, gives you a wing width grounded in how the instrument actually moves.
Implied volatility matters more for the timing than the structure. Selling a body when implied volatility is high and expected to fall is a different trade from selling it when volatility is already low. The structure looks identical on the diagram, and the two have quite different odds.
Margin, and why you must check rather than estimate
Indian derivatives margins are computed under a SPAN-based framework with an exposure margin on top, and the exchange publishes updated parameter files through the day. The NSE margin documentation is the authority here.
The practical points that catch people out:
- A defined-risk structure is not always margined as one. Whether your four legs are recognised as a single hedged position depends on how they are entered and held. Legging in can leave you margined on the naked short before the hedge registers.
- Margin moves during the trade. A position that was comfortable at entry can demand more as volatility rises, which is exactly when you least want to fund it.
- Costs across four legs are not negligible. Every leg carries brokerage, exchange fees, GST, stamp duty and securities transaction tax, and a butterfly has four of them at entry and potentially four at exit. Against a maximum profit of a few hundred points, that is a real fraction of the edge. Current rates are on the NSE securities transaction tax page.
Simulate the exact structure in your broker’s margin calculator before assuming it fits. An estimate from a formula is not the number your account will be held to.
Managing the position
Butterflies are unusual in that time is straightforwardly on your side once the underlying is near the body, and straightforwardly against you when it is not.
Most of the value in a butterfly appears very late. A structure sitting perfectly at the body with two weeks to run is worth far less than the same structure at expiry, because there is still time for the underlying to leave. This has a practical consequence: taking a partial profit early usually means giving up a large share of the theoretical maximum, and holding to the end means accepting that a late move can erase everything.
Deciding which of those you are willing to live with before you enter is worth more than any adjustment rule. If the underlying moves decisively through a wing, the original thesis is wrong, and adjusting into a bigger position to rescue it is how a defined-risk trade stops being one.
Practical notes for index traders
| Item | Why it matters |
|---|---|
| Index options are cash-settled | No delivery obligation, which removes a real risk that stock options carry |
| Lot sizes change | Contract notional is revised periodically, and wing widths sized in rupees need recalculating when it moves |
| Order-to-trade limits | Multi-leg entries and adjustments generate many orders; batching them avoids running into limits |
| Volatility regime | A structure sized for a calm market is the wrong size for a volatile one, and India VIX is the quickest check |
Where to go next
Butterflies sit inside a wider question of how much risk any single structure should carry, which is risk management, and of how a book of positions fits together, which is portfolio construction.
If you want to compute the Greeks and price these structures yourself rather than reading them off a broker screen, derivflow-finance is the open-source library I maintain for exactly that, and being able to derive a number yourself is the difference between using a tool and trusting one.
For the discipline of testing whether any of this actually works on historical data, including the cost modelling that decides it, see quantitative analysis.
The butterfly is an elegant structure. It is also a narrow one, and the elegance is easy to mistake for an edge.