The calculator app is one of the most useful apps that come standard in all computers and smart phones. Your students will be able to confidently use the calculator app to perform simple addition and subtraction by the end of this lesson. Included is a lesson plan to guide the session and a printable worksheet for students to follow along on. There is a short True or False plenary activity at the end which will aid the Teacher in gaining immediate feedback about students' understanding of the purpose of the calculator app. The lesson can be completed with or without the use of computers.