Does the TSP growth calculator have a bug?

[hugehail]

Recently Ive been trying to make graphs in excel showing my TSP account balance even after retirement, assuming various growth rates. The TSP growth calculator allows
one to set the number of years to retirement and then the number of years to stay in TSP total (including after retirement). However, Ive played around with the numbers and
they seem wrong. As a simple first test, lets assume that I am maxing out so that the contributions are 17% of 103K which gives 17500/yr contributions. Then lets set catchup
contributions to zero. Now set the number of years to contribute to 16. Set the account balance to 235K, number of years to keep money in TSP to 23 and the interest rate as 7%.
Now, this gives 1836416. Now instead of 23, set the number of years in TSP to 24. This gives 1921008. Now lets calculate the return for this 1 year period. I will subtract the two
numbers and divide by 1836416. This gives a return of 4.6%, which is a dramatic underestimation. Now, lets go about this another way using the same calculator. Lets grow the money
for 16 years with no investment after retirement (set both to 16). This gives 1384122. Now, lets set all the contributions effectively to zero, then plug in 1384122 into the current balance
box, and grow that for 7 and then 8 years. Using 1% of $1 for contributions and then 0 for catchup, 1 for number of years of contributions, 1384122 as the balance, then 7 as the number
of years to keep the TSP account and 7% interest, you get 2256110. Now try the same for 8 years and you get 2409205. Calculate the 1 year return on this and you get 6.8%. This is much closer to
reality. The final total using 8 years of 2409205 is much greater using this method than using the 1st method which gives 1921008 after 24 years. So can anyone tell me where I am going
wrong or right? Ive called the TSP several times and got nowhere.

 

[Boghie]

Yowser HugeHail,

Plugged your numbers into Quicken's Retirement Planner and came up similar to your baseline. I probably use a slightly smaller inflation value (mine is 3% because I am too lazy to type in 3.1%). I cannot get into EBIS outside of my work environment, so I cannot see the calculator you are using.

The jumpyness is odd, but I think it is based on a couple of things. The first 16 years incorporates the $17,500 into the compounding return, the final years do not. My guess is that the calculator is trying to do something with your retirement date. I think you found a bug. Good luck getting the TSP phone support to fix it. My numbers in Quicken are smoother.