That is required to calculate the non-taxable part. Only that amount will not be taxable on a pro-rated basis.
See IRS Pub 575 page 10
https://www.irs.gov/pub/irs-pdf/p575.pdf
In general, your cost is your net investment in the con-tract as of the annuity starting date (or the date of the dis-tribution if earlier). To find this amount, you must first fig-ure the total premiums, contributions, or other amounts you paid. This includes the amounts your employer con-tributed that were taxable to you when paid. However, see Foreign employment contributions, later. It doesn't include amounts withheld from your pay on a tax-deferred basis (money that was taken out of your gross pay before taxes were deducted). It also doesn't include amounts you con-tributed for health and accident benefits (including any additional premiums paid for double indemnity or disability benefits).
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
