The IRS has a whole thing on calculating this number. It's a hassle, but it only happens the one time and you've done it for forever.
From the IRS website-
Single-life annuity.
If you are to get annuity payments for the rest of your life, find your expected return as follows. You must multiply the amount of the annual payment by a multiple based on your life expectancy as of the annuity starting date. These multiples are set out in actuarial Tables I and V near the end of this publication (see How To Use Actuarial Tables , later).
You may need to adjust these multiples if the payments are made quarterly, semiannually, or annually. See Adjustments to Tables I, II, V, VI, and VIA following Table I.
Example.
Henry bought an annuity contract that will give him an annuity of $500 a month for his life. If at the annuity starting date, Henry's nearest birthday is 66, the expected return is figured as follows:
Annual payment ($500 × 12 months)$6,000
Multiple shown in Table V, age 66× 19.2
Expected return$115,200
If the payments were to be made to Henry quarterly and the first payment was made 1 full month after the annuity starting date, Henry would adjust the 19.2 multiple by +.1. His expected return would then be $115,800 ($6,000 × 19.3).
Here's a link to the IRS publication
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