Would you use inches or feet to measure the length of a book? What about for the length of a playground? What is the best unit of measurement in that case? See how well your third graders understand the various units of measurement with this free math worksheet for kids. ‘How Do You Measure It?’ covers U.S customary units such as feet, yards, gallons and pounds as well as metric units such as centimeters, meters, liters and kilograms.