Can I count miles when my husband picks up milk on the way home from work?

To claim business miles the primary purpose of the trip must be for your business. So, when your husband drives home his primary purpose is to get home, not to buy milk. However, if he drives extra miles to get the milk, the extra miles do count. So, if his normal commute is 10 miles one way, and he drives 2 miles out the way to get milk on his way home, you can count 2 miles as business miles.