When using words like dad, mom, and grandma, capitalization can be confusing. 

When you refer to your dad in the same way that you would address him by name, Dad should be capitalized.  For example:  “What time will Dad be home?”  This would be as if your dad’s name was Joe, and you would write, “What time will Joe be home?”  They are considered direct address of a specific individual, thus both should be capitalized.

When referring to the concept of a dad, but not addressing dad directly, then it would not be capitalized.  For example, “My dad is taller than my mom.”  You wouldn’t say, “My Joe is taller than my mom.”  If you’re not using dad in place of a name, do not capitalize it.

This works the same with mom, uncle, grandma, and even cute names like sweetie, darling, etc.  Capitalize darling when writing, “It’s about time for Darling to lose those pants.”  Do not capitalize darling when writing, “My darling seems to have lost her pants.”

No need to thank me.