advise formula to calculate % rate knowing the principle, interest and time?

Answer 1

if you're talking about finding the percent rate with the given information. you can use this:

i = Prt

Interest = Principle*Percent Rate*Time

move the formula around.. and you'll get....

r = i/(Pt)

Percent Rate = Interest / (Principal*Time)

Answer 2

The question is kind of vague, but if you are talking about the annual rate of return on a bond, it would be:

(Principle x Rate x number of days) / 365 = Interest in $

So if you are looking for your rate, it would be:

(Interest in $ x 365) / (Principle in $ x number of days).

This is a very simple answer though, there are tons of different formulas for different rates, time periods, floating rates, annual vs compound, etc..

Answer 3

Following is a formula for calculating the interest rate for a compound interest calculation.

T (total) = P (principal) * (1 + i)^n

where i is the periodic interest rate and n is the number of periods.

T = P (1 + i)^n

Divide thru by P

(1 + i)^n = T/P

Take natural log of both sides

(n) ln (1 + i) = ln (T/P)

ln (1 + i) = (ln (T/P))/n

e^(1 + i) = e^((T/P))/n)

1 + i = e^((T/P))/n)

i = e^((T/P))/n) - 1
.