What Is Lukewarm Water? How Hot Is It?

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How often have you used the word ‘lukewarm’ to either describe someone who’s not as excited as you are, or the warmth of water that springs from the head of your shower? After years of deliberate practice, you’ve mastered the art of rotating the knobs just the right amount such that the sudden splash of water is neither too hot to traumatize you nor too cold to leave you unsatisfied.

The warmth of lukewarm or tepid water lies in the Goldilocks zone; just the right amount to freshen your morning and kickstart the day. However, have you ever wondered how hot is right hot? Have you ever wondered how warm is lukewarm?

Singer in Shower

Credits: Ollyy/Shutterstock

At first, the magnitude of warmth seems subjective. If you investigate the origins of the word, you would find that the word “luke” is derived from the Middle English word “lew” which meant slightly warm or “tepid”. Tracing back further, you’ll find that “lew” itself was derived from an Old English adverb “hleowe”, which meant “warm” or “sunny”. Sunny is an appropriate description for the refreshing warmth of the February or early March sunlight, is most associated with lukewarm.

Lukewarm water temperature

However, the definition is not entirely empirical. We have narrowed this warmth to a certain range of temperatures. While “experts” have converged at a consensus that the magic temperature lies between 98º and 105º F, there are skeptics who believe it is marginally colder – lying somewhere between 80º – 90º F.

Without a thermometer at your disposal, you can discern whether a volume of water is lukewarm or not by examining if it is just slightly warmer than or comparable to your own body’s temperature. The difference is, as mentioned, marginal. Pour the warm water on your hand and examine if it is “seatbelt-microwaved-in-a-car hot or just feverish hot. Conventional fevers elevate a body’s temperature to no more than 103º F, so feverish hot is what you’re looking for.

Lukewarm water isn’t just a bathing fetish, it is also ubiquitous in recipes. The most basic of these is the warm milk that infants find so luscious. The temperature of the milk is always recommended to be nothing but lukewarm. This is why the baby-milk test is also recognized as an excellent determiner of lukewarmness.

Little baby drinking milk

(Photo Credit: MS-R / Michael S-R / Flickr)

When cooking a dish that includes yeast, one is required to add lukewarm liquid — usually water or milk — to activate the yeast. Water that is insufficiently warm deprives the yeast of the necessary energy to rise, while overly hot water will over-activate and kill the yeast.

Only liquid that is lukewarm will activate the yeast, which will react with sugar in the dough to create carbon dioxide and cause the dough to rise. Recipes usually suggest that such an optimal temperature can be achieved by mixing one part room-temperature water with two parts boiling water and proudly shouting “voila!” when you’re done.

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About the Author:

Akash Peshin is an Electronic Engineer from the University of Mumbai, India and a science writer at ScienceABC. Enamored with science ever since discovering a picture book about Saturn at the age of 7, he believes that what fundamentally fuels this passion is his curiosity and appetite for wonder.

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