Convert a temperature difference by scaling only: a change of 10 °C equals a change of 18 °F, and a change of 10 °F equals a change of 5.56 °C. Do not add 32 when converting a difference, because 32 adjusts the zero points of temperature readings, not the size of an interval.
Why is a 10°F change not converted like a temperature reading?
A thermostat changes from 20 °C to 25 °C, a difference of 5 °C. Multiply 5 by 9/5 to get a 9 °F difference. If the readings themselves are converted, 20 °C is 68 °F and 25 °C is 77 °F; 77 − 68 is also 9 °F. Adding 32 directly to the 5-degree difference would produce 41 °F, which is not the interval between the two temperatures.
How do I convert a Celsius temperature difference to Fahrenheit?
First decide whether the input is a temperature reading or a temperature change. For a reading, use °F = °C × 9/5 + 32 or the temperature converter. For a difference, use Δ°F = Δ°C × 9/5 and Δ°C = Δ°F × 5/9. State the starting and ending readings when a practical decision depends on them.
Why does the +32 disappear for temperature intervals?
An absolute temperature reading locates a condition on a scale with a chosen zero point. A temperature interval is the numerical difference between two readings on that scale. Celsius and Fahrenheit have different zero points and different degree sizes. One Celsius degree interval corresponds to 1.8 Fahrenheit degree intervals, while the 32-degree offset cancels when one reading is subtracted from another.
What is the difference between a temperature and a temperature change?
Do not apply the normal reading formula to “a 10-degree change.” Do not write 10 °F = 10 °C; their interval sizes differ. Do not confuse a 10 °C increase with a final temperature of 10 °C. Do not use an interval conversion to interpret absolute temperatures near safety, cooking, weather, or material limits without converting the actual reading.
When is an interval conversion not enough?
This conversion describes numerical temperature differences. It does not set a safe cooking temperature, a medical threshold, HVAC control setting, or material operating limit. Those questions require the actual temperature, measurement location, device accuracy, and relevant instructions or standards.
Source for the external fact
NIST states that an interval of 1 Celsius degree corresponds to an interval of 1.8 Fahrenheit degrees in its temperature-units guidance. This is the factor for differences; the familiar +32 belongs only to absolute Celsius-to-Fahrenheit readings.
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