Figure in, include as a basic element. For example, In preparing the schedule we factored in vacation and sick days. This term comes from mathematics. [Mid-1900s]
1. To take something into account; take something into consideration; figure something in: We factored sick days and vacations in when we prepared the work schedule. The boss factored in overtime when giving out bonuses.
2. To be taken into account or consideration; to be figured in: These observations are interesting, but they don't really factor in.