Prorating Travel Expenses
Prorate your business and non-business expenses to identify the business expenses that are tax-deductible. If your travel is all business, you can deduct all the travel-related expenses. If your travel is part business and part non-business, you can prorate your expenses between business and non-business to identify business expenses that are tax-deductible. If, for example, you extended your stay at your business destination, made a non-business side trip or had other non-business activities, you can deduct the pro-rata share of your total travel expenses that relate to business only.
You don't have to prorate the travel costs of getting to and from your business destination. Even though your entire travel activities were not business (only), you do not have to prorate the travel costs of getting to and from your business destination. Even though you engaged in non-business activities on your travel, you can deduct the entire cost of getting to and from your business destination--if your travel takes you to your business destination in a reasonably direct fashion. For example, the entire cost of airline tickets to and from your business destination is tax-deductible, even if you also engage in non-business activities while on your business travel.
Source: "Travel-Related Tax Deductions"