Box Spread Calculator
A box spread combines a bull call spread and a bear put spread at the same two strikes. Its payoff at expiration is fixed at the distance between the strikes, no matter where the stock lands — so it behaves like a zero-risk bond or synthetic loan. In practice it is mostly an educational and financing tool, with important real-world caveats.
Interactive calculator
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Key characteristics
- A bull call spread + a bear put spread at the same two strikes.
- Payoff at expiration is fixed (the strike width) regardless of the stock price.
- Acts like a synthetic loan: the "profit" is just the interest-rate discount.
- Defined and flat — but real-world frictions make it far from free money.
How a box spread works
Because the call spread and the put spread cover every outcome, the four legs together always settle at the strike width — say $10 for a $90/$100 box. You pay slightly less than $10 today (the present value), and the small difference is effectively interest, which is why institutions use boxes to borrow or lend cash synthetically.
A "long box" is bought for a debit just under the strike width and pays out the full width at expiration; a "short box" is the reverse, used to raise cash. The position is delta-neutral and price-independent.
The catches — it is not free money
On American-style options (most US equities) any leg can be assigned early, which can break the box and turn a "risk-free" trade into a real loss — the famous risk that has burned retail traders. European-style index options avoid this, which is why boxes are usually done there.
The edge is tiny (just the rate spread), so commissions, bid-ask spreads and margin requirements can wipe it out, and a mispriced box that looks like easy profit usually reflects assignment or liquidity risk you are being paid to take. Treat it as a financing tool, not an arbitrage.
Calculate it live
Use the free OptionProfit Box Spread calculator to load a live option chain, build the trade, and instantly see the payoff chart, breakevens, probability of profit, Greeks and a Monte Carlo simulation of outcomes.
- Bull call spread + bear put spread at the same strikes = a fixed payoff.
- Behaves like a synthetic loan; the return is just the rate discount.
- Early assignment on American options can break it — use European/index options.
- Educational and for financing — frictions mean it is not real arbitrage.
Frequently asked questions
Is a box spread really risk-free?
In theory the payoff is fixed, but in practice no. American-style options can be assigned early and break the box, and commissions, spreads and margin can erase the tiny edge. It is a financing tool, not free money.
Why would anyone trade a box spread?
Mainly to borrow or lend cash synthetically at a rate close to the risk-free rate, using the options market. The fixed payoff makes it behave like a short-term bond.
Why use index options for a box?
European-style, cash-settled index options cannot be assigned early, which removes the main risk that makes equity boxes dangerous.
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