Here is the formula for how BTCJam calculates interest.

A posting advertises:
Amount/Monthly Interest Rate: ฿5.0 @ 3.0%
Term: 180 days

r (monthly interest rate) = 3.0%/100 = 0.03
N (number of payments) = Our Term of 180 days/30 days in a month = 6
P (principle/loan amount) = 5.0 BTC

The size of each payment is c, where c = (P*r*(1+r)^N) / ((1+r)^N-1) = 0.9229875 BTC
Total BTC paid back over 180 days = T = N*c = 6*0.9229875 = 5.537925 BTC
Percentage gained over Term = (T/P - 1)*100 = 10.7585%
APR = Percentage gained * A year in days / Term in days = 10.7585% * 365 / 180 = 21.81585%

Here is the formula for how BTCJam calculates interest.

A posting advertises:
Amount/Monthly Interest Rate: ฿5.0 @ 3.0%
Term: 180 days

r (monthly interest rate) = 3.0%/100 = 0.03
N (number of payments) = Our Term of 180 days/30 days in a month = 6
P (principle/loan amount) = 5.0 BTC

The size of each payment is c, where c = (P*r*(1+r)^N) / ((1+r)^N-1) = 0.9229875 BTC
Total BTC paid back over 180 days = T = N*c = 6*0.9229875 = 5.537925 BTC
Percentage gained over Term = (T/P - 1)*100 = 10.7585%
APR = Percentage gained * A year in days / Term in days = 10.7585% * 365 / 180 = 21.81585%