To estimate your monthly TSP annuity payment you can use an online calculator or download a worksheet.
Use the TSP Annuity Calculator on the Web site to estimate what your monthly annuity payments might be. The amount of your monthly annuity payments depends to a large degree on the size of your TSP account balance (or the portion of your account that you intend to use to purchase the annuity). In addition, the type of annuity you choose, your age (and your joint annuitant’s age in the case of a joint life annuity), and the interest rate index at the time that the TSP purchases your annuity, affect the monthly amount that you will receive.
If you prefer to do your own calculations, you can download the TSP Annuity Worksheet (available at: https://www.tsp.gov/PDF/formspubs/worksheet.pdf) with the monthly annuity factor tables and interest adjustment factors to estimate your payment under various annuity options.