I know up to 15², above that I need a calculator. Until today. via lifehacker, john graham-cumming gave us a great method using mental arithmetic. So if I want to find out what is 48².

• round up the number to be squared to the next 10 = 50
• how many to do the round up? 50-48=2
• take 2 from 48 = 46
• multiply 46x50=2300 (it’s easy to do in the head)
• add 2² 2300+4=2304