If you add two plus two and get five, you miscalculate. In other words, you've added the numbers incorrectly.
You can use the verb miscalculate when you're talking about actual mathematical calculations, or when you measure something in an inexact way and get it wrong. Another way to use the word is to mean "judge incorrectly," which happens when you guess that your grandmother will laugh after sitting on a whoopie cushion, but you guess wrong. Mis, in this word, means "bad or wrong," and calculate means "to compute or estimate."