How this calculator works
A microwave delivers a fixed amount of energy per second at its rated output. If a recipe was written for a 900 W oven and yours is 700 W, the same dish needs proportionally longer: multiply the time by the recipe wattage and divide by your wattage. The result assumes both ovens convert the same share of their electricity into microwave energy, which is close enough for reheating and defrosting. Stir or turn the food halfway and check it early the first time, because food shape, starting temperature and oven size all shift the real time.
Formula
new time = recipe time x recipe wattage / your wattage. Going from 900 W to 700 W, four minutes becomes 4 x 900 / 700 = 5.14 minutes, or 5 minutes 9 seconds.
Worked example
A recipe written for a 900 W microwave asks for 4 minutes. In a 700 W oven that becomes 4 x 900 / 700 = 5.14 minutes, about 5 minutes 9 seconds. In a 1,200 W oven the same recipe needs 4 x 900 / 1,200 = 3 minutes exactly.
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Frequently asked questions
Where is the wattage on my microwave?
It is usually on a label inside the door frame or on the back panel, marked as output or cooking power in watts. The input power figure is higher than the cooking power, so use the output rating.
Is a shorter time always better?
No. Cooking too fast can heat the outside of the food before the centre is warm, and liquids can superheat and then erupt when disturbed. Use the converted time as a starting point and let food stand after cooking.
Does this work for defrosting?
It is less reliable because defrost cycles often switch between power levels and pause. Use the conversion for the total time and check the food more often than the formula suggests.
Why do recipes never say the wattage?
Many do, often in the introduction or the notes, and older cookbooks assumed a 700 W or 800 W oven. When the wattage is missing, assume 800 W and reduce the time if your oven is more powerful.