Temperature Interval Converter

A temperature interval — sometimes written ΔT — is the size of a difference or change between two temperatures, not a specific point on a scale. Saying "the oven heated up by 50 degrees" describes an interval; saying "the oven is at 200°C" describes an absolute temperature. This distinction matters enormously for unit conversion: converting an absolute temperature between scales requires both a multiplicative factor and an additive offset (the familiar °F = °C × 9/5 + 32), but converting an interval requires only the multiplicative factor — there is never an offset, because subtracting two absolute readings on either scale automatically cancels any offset term. This converter is built specifically for intervals, and using it on an absolute reading (or using an absolute-temperature formula on an interval) will give the wrong answer.

Kelvin [K] is the SI base unit of temperature and, for interval purposes, is identical in step size to the degree Celsius: a 1 K change is always exactly a 1°C change. Kelvin intervals are the standard in physics, thermodynamics, and heat transfer equations (for example, ΔT in Q = h·A·ΔT or Q = m·c·ΔT), since the kelvin is the coherent SI unit and introduces no offset ambiguity.

Degree Celsius [°C] and degree centigrade [°C] are two names for the same degree-step, historically "centigrade" (hundred steps) was the original name before it was formally renamed "Celsius" in 1948. Both convert to kelvin intervals with a factor of exactly 1 — a temperature rise of 15°C is the same as a temperature rise of 15 K. This converter lists both names separately because older documents, especially pre-1950s scientific literature, still use "centigrade."

Degree Fahrenheit [°F] divides the same water freezing-to-boiling range that Celsius divides into 100 steps into 180 steps instead, making each Fahrenheit degree-step only 5/9 the size of a Celsius or kelvin step. As an interval, this means it takes more Fahrenheit degrees to express the same physical temperature change: 1 K (or 1°C) of interval equals 1.8°F of interval. Converting the other direction, a Fahrenheit interval is multiplied by 0.5555555556 (5/9) to get the equivalent kelvin or Celsius interval. For example, an engine that heats up by 180°F during operation has experienced a temperature rise of 180 × 0.5555555556 = 100 K.

Degree Rankine [°R] is an absolute temperature scale (its zero point coincides with absolute zero, like kelvin) but its degree-steps are sized identically to Fahrenheit degrees. As a result, Rankine intervals use exactly the same 1.8 and 0.5555555556 conversion factors as Fahrenheit intervals relative to kelvin or Celsius. Rankine is mainly used in US thermodynamics and aerospace engineering when an absolute scale with Fahrenheit-sized steps is convenient, such as in compressible flow and gas turbine calculations.

Degree Reaumur [°r] is a largely historical scale, once common in parts of Europe (especially France, Germany, and Russia) for scientific and culinary use, that divides the water freezing-to-boiling range into 80 steps rather than 100. Each Reaumur degree-step is therefore 4/5 the size of a Celsius or kelvin step. Converting a kelvin or Celsius interval to Reaumur multiplies by 0.8, while converting a Reaumur interval back to kelvin or Celsius multiplies by 1.25. A furnace temperature rise of 100 K, for instance, corresponds to a rise of 100 × 0.8 = 80°Ré.

Because none of these conversions involve an offset, the same six factors apply whether the interval is positive (a temperature rise) or negative (a temperature drop) — only the magnitude and sign carry through unchanged. This makes temperature interval conversion especially important in engineering contexts involving rates of change, tolerances, and deltas: HVAC thermostat deadbands, material thermal-expansion coefficients quoted per degree, heat transfer coefficient and specific heat formulas that use ΔT, and reporting a measured temperature swing from a lab instrument calibrated in one scale into another. Mixing up interval conversion with absolute-temperature conversion is one of the most common temperature-related unit errors, which is precisely the confusion this dedicated tool is designed to prevent.

This temperature interval converter supports kelvin [K], degree Celsius [°C], degree centigrade [°C], degree Fahrenheit [°F], degree Rankine [°R], and degree Reaumur [°r]. All conversions are instant, free, offset-free, and precise to 12 significant digits.