Body Temperature Monitoring Calculator

The calculator starts from a base skin-to-core offset of about -3.5°C, since wrist skin sits well below core temperature even in a healthy person — this is the same reason a wrist thermometer reads differently than an oral or ear reading. That base offset then narrows slightly in warmer ambient conditions and widens in colder ones, but it never approaches zero, so a meaningful gap between skin and core temperature is expected, not a sign anything is wrong.

The calculator adjusts the -3.5°C base offset by 0.05°C for every degree the ambient temperature differs from a 22°C reference — warmer rooms narrow the gap because vasodilation brings warmer blood closer to the skin's surface, while colder rooms widen it as vasoconstriction pulls blood away from the extremities to conserve heat. This means the same core temperature can produce a noticeably different skin-temperature estimate depending on whether you're in a warm or cold environment.

The baseline is calculated the same way as your current estimate but assumes you're resting at midday, factoring in only the ambient offset and circadian effect — not recent exercise or menstrual phase. The deviation figure subtracts that baseline from your actual estimate, so it isolates the effect of exercise heat and cycle phase specifically, rather than mixing them in with the normal daily temperature swing everyone experiences.

It measures how far your estimated skin temperature sits from the calculated fever threshold, relative to your sensor's stated accuracy in degrees. A reading exactly at the threshold produces roughly 50% confidence because it's a coin-flip whether the true temperature is above or below the cutoff, while a reading several sensor-accuracy-widths away from the threshold approaches the 99% cap, since it's much less likely that sensor error alone could put you on the wrong side of the line.