Mutual fund investing interest rates are based on the risk factor of the fund - a bond fund has some risk. To make it easier to find a published interest rate maybe you should just pick a money market fund and be done with it. A 401K is not going to typically be all mutual find - there will be a fair proportiomn of fixed-income asset invested. Since it is hypothetical, it doesn't matter if it's correct how is someone going to prove you are wrong?

Go on a site like TD Ameritrade - it may help you find a product you can use for your model.

Well, how long do you have for this project? I use Fidelity.com. And actually, there is a program that you can put in your info and it tells you what could happen in the future. I think you should try that!!!

http://personal.fidelity.com/research/funds/?bar=c

Are you more looking for a way to calculate how the money grows? Or how to get the interest rate in the first place? Sounds like a bit of both.

For the math, you might try the bottom calculator at this site:

http://home.ubalt.edu/ntsbarsh/zero/CompoundCalI.h tm

with whatever values you come up with, for example:

"Present value" = 0
"Payment per period" = -200
"Annual interest rate" = .0525
"Periods per year" = 12
"Number of years" seems to allow fractions if you need partial years

It appears that rather than projecting future gains what is being asked for is the previous month's gain. Thus, on the first day you deposit $200. One month later, before you make your next deposit, you calculate your gain for that month.

Example:
Start of month $200
End of month value $201

Gain (earned interest):
$201-$200 = $1

Rate of Return:

$10/$200 = 0.005 per month
0.005 x 12 = .06 (6%) on a yearly basis.
____________

Next month the starting value is $401 (Last month's ending value ($201) plus your contribution ($200)).

Do this for each month. The result for each month will likely be different and some months may even show a loss.

Realistic results might look like this:


Month #: 1 , 2 , 3 , 4 , 5 , 6
Interest %: 6 , 4.3 , 11 , -9.2 , 0.2 , 4.3