Fahrenheit to Celsius Calculator

Background.

A Fahrenheit-to-Celsius calculator is useful whenever a US thermometer, weather report, appliance dial, or technical document must be read in the Celsius scale used elsewhere. Enter the Fahrenheit value and this page returns Celsius plus Kelvin for scientific reference. If the input is a body temperature, this tool performs only the unit conversion; it does not classify fever or tell anyone when to seek care, because clinical thresholds depend on age, symptoms, measurement method, and the guidance being followed.

The conversion is exact by definition, not a rounded approximation. Celsius is related to Fahrenheit by the affine formula °C = (°F − 32) × 5⁄9, fixed because the Celsius degree and the kelvin are identical in size and the scales have a fixed offset (NIST Special Publication 811, Appendix B; BIPM SI Brochure, 9th edition). Plugging in 101.5°F gives 38.6111111111°C. That numerical result can be communicated to a clinician or compared with the specific instructions that apply; the calculator makes no clinical interpretation.

Unlike converting a length or mass, temperature conversion is not a simple multiplication: it carries both a scale factor and an offset, so 0°F is not 0°C. The page explains both operations, shows the numeric example, and notes the one temperature — minus 40 — where the two scales agree.

What is fahrenheit to celsius calculator?

The Fahrenheit and Celsius scales are two different ways of numbering the same physical quantity — thermal energy — using different zero points and different degree sizes. Fahrenheit, defined by Daniel Gabriel Fahrenheit in 1724, sets 32° at water's freezing point and 212° at its boiling point. Celsius sets 0° at water's freezing point and 100° at its boiling point, which means a Celsius degree is 9/5 the size of a Fahrenheit degree. Because the two scales have different zero points, converting between them requires both dividing by a scale factor and subtracting an offset first — the relationship is affine, not linear through the origin, which is why a Fahrenheit-to-Celsius calculator cannot reuse the same 'value × factor' shape that a length or weight converter uses. This calculator also reports the Kelvin equivalent, since Kelvin is the SI base unit for thermodynamic temperature and is anchored to Celsius by a pure offset (K = °C + 273.15) with no scale change at all — one kelvin is exactly one Celsius degree in size.

How to use this calculator.

1. 01Enter the Fahrenheit temperature you want to convert — from a thermometer, an appliance dial, or a weather report.
2. 02Read the Celsius result, computed exactly as (°F − 32) × 5⁄9.
3. 03Check the Kelvin value if you need it for a scientific or engineering context.
4. 04Remember that negative Fahrenheit readings are valid and will convert correctly; only inputs below absolute zero (−459.67 °F) are rejected.

The formula.

The formula °C = (°F − 32) × 5⁄9 has two parts because Fahrenheit and Celsius differ in both zero point and degree size. The offset aligns 32°F with 0°C. The scale factor 5⁄9 follows because 180 Fahrenheit degrees span the same interval as 100 Celsius degrees.

For 101.5°F, subtract 32 to get 69.5, multiply by 5 to get 347.5, and divide by 9 to get 38.6111111111°C. Kelvin then uses K = °C + 273.15, giving 311.7611111111 K. No medical meaning is attached to the converted number. Decimal.js arithmetic preserves the repeating 5⁄9 relationship until the return boundary.

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Institute of Standards and Technology (2008). NIST Special Publication 811 — Guide for the Use of the International System of Units (SI), Appendix B: Conversion Factors.
2. [2]Bureau International des Poids et Mesures (2019). The International System of Units (SI), 9th edition — defines the kelvin via the fixed Boltzmann constant and the exact offset between the Kelvin and Celsius scales.