26 CFR § 25.2512-3 - Valuation of interest in businesses.
(a) Care should be taken to arrive at an accurate valuation of any interest in a business which the donor transfers without an adequate and full consideration in money or money's worth. The fair market value of any interest in a business, whether a partnership or a proprietorship, is the net amount which a willing purchaser, whether an individual or a corporation, would pay for the interest to a willing seller, neither being under any compulsion to buy or to sell and both having reasonable knowledge of the relevant facts. The net value is determined on the basis of all relevant factors including -
(1) A fair appraisal as of the date of the gift of all the assets of the business, tangible and intangible, including good will;
(2) The demonstrated earning capacity of the business; and
(3) The other factors set forth in paragraph (f) of § 25.2512-2 relating to the valuation of corporate stock, to the extent applicable.