Convert a microwave time to your wattage
Ready-meal packs, mug-cake recipes and reheating instructions give one time for one wattage, and your microwave rarely matches it. A microwave wattage conversion calculator fixes that: enter the wattage the time was written for, the time itself and your own wattage, and it returns the time to use. In the UK, packs often print a heating category letter instead of watts; pick it from the menu and the matching wattage is filled in for you.
The calculation is one line of arithmetic, shown in the result so you can check it. Everything runs in your browser, nothing is uploaded, and the table underneath lists the same time at seven common wattages so you can compare ovens, a work-place microwave or a friend’s kitchen.
How to use the Microwave Wattage Time Converter
- Type the wattage the time is written for (the pack, recipe or manual), or choose a UK category A–E.
- Type your microwave’s wattage. It is on the rating label inside the door frame or on the back, shown as output power, not the larger “input” figure.
- Enter the time in minutes and seconds.
- Read the converted time, then cook for about three-quarters of it, stir or turn the food, and add time in short steps until it is piping hot.
Microwave Wattage Time Converter features
Shows the working
The result spells out seconds × pack watts ÷ your watts, so there is no black box.
UK category letters
Categories A to E fill in a typical wattage (500, 600, 650, 700 and 800 W). You can overwrite the number if your pack lists its own watts.
Conversion chart
A seven-row table gives the same time at 600 to 1200 W; your wattage is highlighted when it is in the list.
First-check time
A suggested first check at 75% of the time helps you avoid over-cooking when the two wattages are far apart.
Copy button
Copy the answer as text to paste into a note or a message.
How the formula works
A microwave delivers energy, which is power multiplied by time. If a dish needs the same energy whichever oven heats it, then time × watts stays constant, so new time = old time × old watts ÷ new watts. A meal that takes 2:00 at 1000 W holds 120 × 1000 = 120,000 watt-seconds, and a 700 W oven needs 120,000 ÷ 700 = about 171 seconds, or 2:51. A weaker oven needs longer; a stronger one needs less.
A rough way to check your own oven’s output is the water test: heat a known mass of water for a measured time and use power ≈ 4.19 × grams of water × temperature rise in °C ÷ seconds. Use a plastic or glass jug, stir before measuring, and expect a result a little under the label because some energy is lost.
Why the answer is an estimate
- Wattage labels are nominal. Real output varies with the model, the age of the magnetron and the mains voltage.
- Small loads heat differently. One cup of soup or a single potato may behave differently from a family-sized dish; the pack time already assumes a particular load.
- Power levels count. A pack that says “defrost” or “medium” means a lower power setting; convert the time for the same setting.
- Uneven heating. Cold spots remain whatever the arithmetic says, so stir, turn, cover and stand.
Reheating safely
Food safety guidance from the USDA Food Safety and Inspection Service is to reheat leftovers and ready meals to 165 °F (74 °C) all the way through, to cover the food so steam helps even out the heat, and to let it stand for a minute or two. If your converted time leaves the middle cool, keep heating in short bursts; a time on a calculator never replaces a hot, steaming centre. For ovens and air fryers, see the air fryer conversion calculator, and for oven markings the cooking converter.
Microwave Wattage Time Converter FAQ
How do I convert microwave time from 800 W to 1000 W?
Multiply the time by 800 ÷ 1000, which is 0.8. Three minutes at 800 W becomes 2 minutes 24 seconds at 1000 W. Enter both wattages and the time above and the calculator does it for you.
What are the microwave heating categories A to E?
They are letters printed on many UK food packs to mean a nominal output: roughly A 500 W, B 600 W, C 650 W, D 700 W and E 800 W. Some labels use slightly different numbers, so if the pack also prints watts or its own table, use that.
Where do I find my microwave’s wattage?
Look at the rating plate inside the door frame, on the back, or in the manual. Use the output (cooking) power. If only the input power is listed, that is higher than the cooking power, so do not use it.
Why is my food still cold in the middle?
Microwaves heat unevenly and the pack time assumes a particular load and arrangement. Stir or turn the food, spread it out, cover it, and add 15 to 30 seconds at a time until it is steaming hot.
Is the conversion exact?
No. It assumes the oven delivers its stated power and that the food absorbs it the same way. It is a good starting point, and a thermometer reading of 165 °F (74 °C) is the real test for reheated food.
Can I use this for defrosting?
Only if you use the same power setting for both ovens. Defrost settings differ between models, so treat the result as a guide and check the food every minute.