Temperature Conversion Calculator
Convert absolute readings across Celsius, Fahrenheit, kelvin, and Rankine while enforcing absolute-zero boundaries.
Content updated August 6, 2026
These equations convert absolute scale readings. Temperature intervals use scale factors without the zero-point offsets and are outside this calculator.
Convert one absolute temperature reading among Celsius, Fahrenheit, kelvin, and Rankine. The result table shows all four scales and rejects readings below absolute zero for the selected source scale.
Temperature conversion is affine rather than a simple multiplier: Celsius and Fahrenheit have different zero points as well as different degree sizes. Kelvin and Rankine start at absolute zero but use kelvin-sized and Fahrenheit-sized intervals respectively.
The Temperature Conversion formula
K = °C + 273.15 = (°F + 459.67) × 5/9 = °R × 5/9The source reading is converted to kelvin, checked against absolute zero, then converted to the selected destination scale.
Worked example
For 32 °F, (32 + 459.67) × 5/9 equals 273.15 K; subtracting 273.15 gives exactly 0 °C.
Assumptions, rounding, and limitations
Assumptions
- Inputs are absolute temperature readings, not temperature intervals.
- Celsius, Fahrenheit, kelvin, and Rankine use their standard scale relationships.
- The selected source reading cannot be below its absolute-zero value.
Rounding: The engine applies the exact stated scale equations without intentional intermediate rounding. Displays use up to 12 significant digits.
Limitations
- Does not convert temperature intervals, instrument uncertainty, sensor calibration, historical temperature scales, or physical material behavior.
- Floating-point representation can introduce tiny binary approximations; displayed precision is not a claim about measurement accuracy.
Sources
- SI Units — Temperature — National Institute of Standards and Technology
- SP 330, Section 2: SI Base Units — National Institute of Standards and Technology
Temperature scales combine offsets and interval sizes
NIST gives Celsius to kelvin as K = °C + 273.15 and Celsius to Fahrenheit as °F = °C × 1.8 + 32. Fahrenheit to Celsius reverses both the 32-degree offset and the 1.8 interval factor.
One kelvin has the same interval size as one Celsius degree. One Rankine degree has the same interval size as one Fahrenheit degree, so K = °R × 5/9.
Absolute readings and temperature intervals are different
Offsets apply when converting an absolute scale reading, such as 32 °F to 0 °C. They do not apply to a temperature change: a 10 °C interval equals 10 K and an 18 °F interval.
This calculator converts absolute readings only. It enforces the exact absolute-zero points of −273.15 °C, −459.67 °F, 0 K, and 0 °R.
Frequently asked questions
▶How do I convert Celsius to Fahrenheit?
Multiply the Celsius reading by 1.8, then add 32.
▶How do I convert Fahrenheit to Celsius?
Subtract 32 from the Fahrenheit reading, then divide by 1.8.
▶What is absolute zero in each scale?
Absolute zero is −273.15 °C, −459.67 °F, 0 K, and 0 °R.
▶Where are Celsius and Fahrenheit equal?
They have the same numerical reading at −40: −40 °C equals −40 °F.
▶Can I use this for temperature differences?
No. This page converts absolute readings. Temperature intervals use scale factors without the Celsius or Fahrenheit zero-point offsets.
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Open calculator →Disclaimer: Temperature Conversion Calculator results are estimates for general information and education only, and are not financial, tax, legal or medical advice. Verify important decisions with a qualified professional.