26 CFR § 25.2512-5
In general. Except as otherwise provided in paragraph (b) of this section and 25.7520-3(b), the fair market value of annuities, unitrust interests, life estates, terms of years, remainders, and reversions transferred by gift is the present value of the interests determined under paragraph (d) of this section. Section 20.2031-7 of this chapter (Estate Tax Regulations) and related sections provide tables with standard actuarial factors and examples that illustrate how to use the tables to compute the present value of ordinary annuity, life, and remainder interests in property. These sections also refer to standard and special actuarial factors that may be necessary to compute the present value of similar interests in more unusual fact situations. These factors and examples are also generally applicable for gift tax purposes in computing the values of taxable gifts.