If you do not have complete records to prove an element of an expense, then you must prove the element with:
Your own written or oral statement containing specific information about the element, and
Other supporting evidence that is sufficient to establish the element.
If the element is the description of a gift, or the cost, time, place, or date of an expense, the supporting evidence must be either direct evidence or documentary evidence. Direct evidence can be written statements, or the oral testimony of your guests or other witnesses setting forth detailed information about the element. Documentary evidence can be receipts, paid bills, or similar evidence.
If the element is either the business relationship of your guests or the business purpose of the amount spent, the supporting evidence can be circumstantial, rather than direct. For example, the nature of your work, such as making deliveries, provides circumstantial evidence of the use of your car for business purposes. Invoices of deliveries establish when you used the car for business.
Sampling. You can keep an adequate record for parts of a tax year and use that record to prove the amount of business or investment use for the entire year. You must demonstrate by other evidence that the periods for which an adequate record is kept are representative of the use throughout the tax year.
Example. You use your car to visit the offices of clients, meet with suppliers and other subcontractors, and pick up and deliver items to clients. There is no other business use of the car, but you and your family use the car for personal purposes. You keep adequate records during the first week of each month that show that 75% of the use of the car is for business. Invoices and bills show that your business use continues at the same rate during the later weeks of each month. Your weekly records are representative of the use of the car each month and are sufficient evidence to support the percentage of business use for the year.
Exceptional circumstances. You can satisfy the substantiation requirements with other evidence if, because of the nature of the situation in which an expense is made, you cannot get a receipt. This applies if all the following are true.
You were unable to obtain evidence for an element of the expense or use that completely satisfies the requirements explained earlier under What Are Adequate Records.
You are unable to obtain evidence for an element that completely satisfies the two rules listed earlier under What if I Have Incomplete Records.
You have presented other evidence for the element that is the best proof possible under the circumstances.
Destroyed records. If you cannot produce a receipt because of reasons beyond your control, you can prove a deduction by reconstructing your records or expenses. Reasons beyond your control include fire, flood, and other casualty.
Information courtesy of the Internal Revenue Service.
