Hi,I need help to understand the following.I have been trying to find the atm for different maturity terms 1w, 1m, ..., 1y etc. in the FX option marketIt seems that the ATM strike is found as by the 0-delta straddle, i.e.X(atm) = forward*exp(0.5*vol*vol*time)This formula works fine for FX options with a positive interest rate differential, (r_d-r_f) > 0. If the Interest rate differential is negative, (r_d-r_f) < 0, then the following formula works:X(atm) = forward*exp(-0.5*vol*vol*time)... but why this formula? Does any one know the argument behind this formula?(I have tested the formulas against superDerivatives.com and is able to match all atm strikes)Help would be appreciatedkind regards henrik

>> how to find FX atm?try the airport, many banks have fx atms there

atm = at-the-money but thanks any way.henrik

sorry, I obviously misunderstood

I believe conventions vary. Can be put = call or delta neutral which depends on premium currency.

black scholes: N(d1) = 0.5 => d1 = 0d1 = ln(F/K) + 1/2 sigma^2.t = 0 => K = F exp (1/2 sigma^2.t)or did I miss the point of the question?

Yes, you are right it depends on the currency cross. (and to billyknee: I do know how to solve d1 for the strike, the question was more why the atm strike some times are found by the formula x=F*exp(-0.5*sigma^2*time). It was the minus that I did not understand. I know now that it is dependant on the currency cross). Thanks