Microwave Wattage Converter

Microwave instructions on food packages are written for a particular oven power, often 1000 or 1100 watts. If your microwave is weaker or stronger, the same time will undercook or overcook the food. Enter the time and wattage from the package and your oven's wattage, plus a lower power level if you use one, and this converter gives the adjusted time. A chart shows the time across common oven sizes.

Your microwave's wattage is usually printed on a label inside the door, on the back panel or in the manual.

New time = package time × package wattage ÷ your wattage

Effective wattage = oven wattage × power level ÷ 100

The heat delivered to food is roughly the oven's power multiplied by time, so halving the power doubles the time needed. In practice the relationship is approximate: food shape, starting temperature and how much water it contains also matter, and lower power levels on many ovens work by cycling the magnetron on and off rather than running continuously.

A 1100 W instruction in a 700 W oven. 3:00 × 1100 ÷ 700 = 4:43.

An 800 W instruction in a 1000 W oven. 5:00 × 800 ÷ 1000 = 4:00.

Half power. 2:00 at full power on a 1000 W oven takes about 4:00 at 50% power.

A microwave oven's magnetron produces radio waves at about 2.45 GHz that make water molecules in food rotate rapidly, generating heat. The waves penetrate only a couple of centimeters, so the center of thick food is heated by conduction from the outer layers, which is why standing time and stirring matter. The wattage on the label is the cooking power delivered to the food; the electrical power the oven draws from the wall is higher, often 1.5 times as much, because of losses in the electronics.

Because microwaves heat only the food and not a large oven cavity, they are usually the most energy-efficient way to reheat small portions. A 1,500-watt draw for five minutes uses about 0.125 kilowatt-hours, a few cents of electricity at typical US rates. The electricity cost calculator can work out the cost for your own appliance and rate.

Glass and ceramic dishes labeled microwave-safe are the best choice. Avoid metal, which can spark, and thin takeout containers or margarine tubs, which can warp and leach chemicals when hot. A loose cover or vented lid keeps moisture in for more even heating.

Microwaves heat unevenly and can leave cold spots where bacteria survive, so the USDA advises arranging food evenly, covering it, stirring or rotating it partway through, and letting it stand for the time on the package before eating. Reheated leftovers should reach 165 °F (74 °C) throughout, checked with a food thermometer in several places. Follow package instructions closely for frozen meals that contain raw ingredients, and do not heat food in containers that are not labeled microwave-safe.

When converting to a much weaker oven, start checking a little before the calculated time and add 15 to 30 seconds at a time. Older microwaves often deliver less than their rated power. If you cannot find your oven's wattage, compare how long a cup of cold water takes to reach a boil with a microwave whose wattage you know; the time ratio is roughly the inverse of the power ratio.