"ATM" stands for ATMospheres and the number before it, refers to the number of atmospheres of pressure the watch will withstand before leaking. At sea level we are all subjected to one atmosphere of pressure, because we have one atmosphere of air above us pressing down. For every 33 feet (10 metres) underwater you go, the pressure increases by 1 atmosphere. Therefore, at 33 feet underwater the pressure is 2 atmospheres (2 ATM), at 66 feet underwater the pressure is 3 atmospheres (3 ATM), etc. So 5 ATM is equivalent to 132 feet (40 metres) underwater. In conclusion, the ATM rating of a watch is the number of atmospheres of pressure the watch will withstand before breaking/leaking. To convert ATM 's into the depth rating in metres just subtract 1 and multiply by 10. eg: 5 ATM = (5 - 1)*10 = 40 metres.