# How do I calculate the Present Value of a future amount

I have a requirement to calculate the present value of a future value taking into consideration an interest rate and tax rate. This is to be used in an insurance calculator so I need the answer to be a formula rather than in excel.

Given the example of a FV worth \$175,000 in 20 years time, how do I calculate the PV of this amount with an annual interest rate of 5% accumulating on the principle and a tax rate of 20% which only applies to the interest earned on the principle?

Inflation does not apply as there are no drawings over the period.

The easiest way is to use the `PV` function in Microsoft Excel or Google Sheets. Due to the 20% tax, the interest rate is effectively 4% instead of 5%. The formula to use (assuming annual compounding of interest) is `PV(0.04,20,0,175000)`: 4% interest, 20 periods (years in this case), no periodic payments, future value of \$175,000. The answer is \$79,867.72.
EDIT: Just saw that you wanted the formula rather than an Excel function, my bad. The formula is `FV = PV x (1 + r)^n`. `FV` is 175000, `r` is 0.04, `n` is 20. Solve for `PV` and you get 79867.72.