Box spread rate calculator
Pull the four leg prices from your broker's option chain, plug them in below, and compare the implied rate against whatever you're actually being offered on a personal loan, a lease, or a margin loan. Background on how the structure works is in Box Spread as a Low-Interest Loan.
The net credit is what you collect today: the sum of the two legs you sell minus the sum of the two legs you buy, times the contract multiplier. That credit is the loan principal.
The amount owed at expiry is fixed by the strikes alone: (X2 − X1) × multiplier,
regardless of where the underlying ends up. That fixed gap between what you collect
and what you owe is the interest.
The implied rate annualizes that gap as simple interest over the days to expiry:
(owed − credit) / credit × (365 / days). It is the rate you actually get
once you cross the bid-ask spread on all four legs, not the theoretical midpoint rate.
Prefilled with a real quote pulled for the SMI (17 Jun 2027 expiry, weekend market, wide spreads). Replace every field with your own numbers.
Personal record, not investment advice. No amounts, position sizes, or returns disclosed.