26 C.F.R. § 1.162-17

(a) Introductory. The purpose of the regulations in this section is to provide rules for the reporting of information on income tax returns by taxpayers who pay or incur ordinary and necessary business expenses in connection with the performance of services as an employee and to furnish guidance as to the type of records which will be useful in compiling such information and in its substantiation, if required. The rules prescribed in this section do not apply to expenses paid or incurred for incidentals, such as office supplies for the employer or local transportation in connection with an errand. Employees incurring such incidental expenses are not required to provide substantiation for such amounts. The term “ordinary and necessary business expenses” means only those expenses which are ordinary and necessary in the conduct of the taxpayer's business and are directly attributable to such business. The term does not include nondeductible personal, living or family expenses.

(b) Expenses for which the employee is required to account to his employer—(1) Reimbursements equal to expenses. The employee need not report on his tax return (either itemized or in total amount) expenses for travel, transportation, entertainment, and similar purposes paid or incurred by him solely for the benefit of his employer for which he is required to account and does account to his employer and which are charged directly or indirectly to the employer (for example, through credit cards) or for which the employee is paid through advances, reimbursements, or otherwise, provided the total amount of such advances, reimbursements, and charges is equal to such expenses. In such a case the taxpayer need only state in his return that the total of amounts charged directly or indirectly to his employer through credit cards or otherwise and received from the employer as advances or reimbursements did not exceed the ordinary and necessary business expenses paid or incurred by the employee.

(2) Reimbursements in excess of expenses. In case the total of amounts charged directly or indirectly to the employer and received from the employer as advances, reimbursements, or otherwise, exceeds the ordinary and necessary business expenses paid or incurred by the employee and the employee is required to and does account to his employer for such expenses, the taxpayer must include such excess in income and state on his return that he has done so.

(3) Expenses in excess of reimbursements. If the employee's ordinary and necessary business expenses exceed the total of the amounts charged directly or indirectly to the employer and received from the employer as advances, reimbursements, or otherwise, and the employee is required to and does account to his employer for such expenses, the taxpayer may make the statement in his return required by subparagraph (1) of this paragraph unless he wishes to claim a deduction for such excess. If, however, he wishes to secure a deduction for such excess, he must submit a statement showing the following information as part of his tax return:

(i) The total of any charges paid or borne by the employer and of any other amounts received from the employer for payment of expenses whether by means of advances, reimbursements or otherwise; and

(ii) The nature of his occupation, the number of days away from home on business, and the total amount of ordinary and necessary business expenses paid or incurred by him (including those charged directly or indirectly to the employer through credit cards or otherwise) broken down into such broad categories as transportation, meals and lodging while away from home overnight, entertainment expenses, and other business expenses.

(4) To “account” to his employer as used in this section means to submit an expense account or other required written statement to the employer showing the business nature and the amount of all the employee's expenses (including those charged directly or indirectly to the employer through credit cards or otherwise) broken down into such broad categories as transportation, meals and lodging while away from home overnight, entertainment expenses, and other business expenses. For this purpose, the Commissioner in his discretion may approve reasonable business practices under which mileage, per diem in lieu of subsistence, and similar allowances providing for ordinary and necessary business expenses in accordance with a fixed scale may be regarded as equivalent to an accounting to the employer.

(c) Expenses for which the employee is not required to account to his employer. If the employee is not required to account to his employer for his ordinary and necessary business expenses, e.g., travel, transportation, entertainment, and similar items, or, though required, fails to account for such expenses, he must submit, as a part of his tax return, a statement showing the following information:

(1) The total of all amounts received as advances or reimbursements from his employer in connection with the ordinary and necessary business expenses of the employee, including amounts charged directly or indirectly to the employer through credit cards or otherwise; and

(2) The nature of his occupation, the number of days away from home on business, and the total amount of ordinary and necessary business expenses paid or incurred by him (including those charged directly or indirectly to the employer through credit cards or otherwise) broken down into such broad categories as transportation, meals and lodging while away from home overnight, entertainment expenses, and other business expenses.

(d) Substantiation of items of expense. (1) Although the Commissioner may require any taxpayer to substantiate such information concerning expense accounts as may appear to be pertinent in determining tax liability, taxpayers ordinarily will not be called upon to substantiate expense account information except those in the following categories:

(i) A taxpayer who is not required to account to his employer, or who does not account;

(ii) A taxpayer whose expenses exceed the total of amounts charged to his employer and amounts received through advances, reimbursements or otherwise and who claims a deduction on his return for such excess;

(iii) A taxpayer who is related to his employer within the meaning of section 267(b); and

(iv) Other taxpayers in cases where it is determined that the accounting procedures used by the employer for the reporting and substantiation of expenses by employees are not adequate.

(2) The Code contemplates that taxpayers keep such records as will be sufficient to enable the Commissioner to correctly determine income tax liability. Accordingly, it is to the advantage of taxpayers who may be called upon to substantiate expense account information to maintain as adequate and detailed records of travel, transportation, entertainment, and similar business expenses as practical since the burden of proof is upon the taxpayer to show that such expenses were not only paid or incurred but also that they constitute ordinary and necessary business expenses. One method for substantiating expenses incurred by an employee in connection with his employment is through the preparation of a daily diary or record of expenditures, maintained in sufficient detail to enable him to readily identify the amount and nature of any expenditure, and the preservation of supporting documents, especially in connection with large or exceptional expenditures. Nevertheless, it is recognized that by reason of the nature of certain expenses or the circumstances under which they are incurred, it is often difficult for an employee to maintain detailed records or to preserve supporting documents for all his expenses. Detailed records of small expenditures incurred in traveling or for transportation, as for example, tips, will not be required.

(3) Where records are incomplete or documentary proof is unavailable, it may be possible to establish the amount of the expenditures by approximations based upon reliable secondary sources of information and collateral evidence. For example, in connection with an item of traveling expense a taxpayer might establish that he was in a travel status a certain number of days but that it was impracticable for him to establish the details of all his various items of travel expense. In such a case rail fares or plane fares can usually be ascertained with exactness and automobile costs approximated on the basis of mileage covered. A reasonable approximation of meals and lodging might be based upon receipted hotel bills or upon average daily rates for such accommodations and meals prevailing in the particular community for comparable accommodations. Since detailed records of incidental items are not required, deductions for these items may be based upon a reasonable approximation. In cases where a taxpayer is called upon to substantiate expense account information, the burden is on the taxpayer to establish that the amounts claimed as a deduction are reasonably accurate and constitute ordinary and necessary business expenses paid or incurred by him in connection with his trade or business. In connection with the determination of factual matters of this type, due consideration will be given to the reasonableness of the stated expenditures for the claimed purposes in relation to the taxpayer's circumstances (such as his income and the nature of his occupation), to the reliability and accuracy of records in connection with other items more readily lending themselves to detailed recordkeeping, and to all of the facts and circumstances in the particular case.

(e) Applicability. (1) Except as provided in subparagraph (2) of this paragraph, the provisions of the regulations in this section are supplemental to existing regulations relating to information required to be submitted with income tax returns, and shall be applicable with respect to taxable years beginning after December 31, 1957, notwithstanding any existing regulation to the contrary.

(2) With respect to taxable years ending after December 31, 1962, but only in respect of periods after such date, the provisions of the regulations in this section are superseded by the regulations under section 274(d) to the extent inconsistent therewith. See § 1.274-5.

(3) For taxable years beginning on or after January 1, 1989, the provisions of this section are superseded by the regulations under section 62(c) to the extent this section is inconsistent with those regulations. See § 1.62-2.