Celsius vs. Fahrenheit vs. Kelvin vs. Rankine: Complete Temperature Scales Guide

Unlike length or mass—where zero means the complete absence of the quantity and conversions only require multiplying by a scale factor—everyday temperature scales (Celsius and Fahrenheit) use arbitrary zero points based on phase changes of water or brine. Consequently, converting between temperature scales requires both a scale multiplier (9/5 or 5/9) and an additive offset (+32 or +273.15).

• Celsius (°C): Part of the metric system; water freezes at 0 °C and boils at 100 °C at 1 standard atmosphere (101.325 kPa). • Fahrenheit (°F): Used in the United States for weather and culinary temperatures; water freezes at 32 °F and boils at 212 °F (a 180-degree span, meaning 1 °C interval = 1.8 °F interval). • Kelvin (K): The base SI thermodynamic unit defined by fixing the Boltzmann constant k. Starts at Absolute Zero (0 K = -273.15 °C) and uses the exact same degree step size as Celsius. • Rankine (°R): The Imperial/US thermodynamic absolute scale starting at Absolute Zero (0 °R = -459.67 °F) while using Fahrenheit-sized degree increments (°R = °F + 459.67).

Because the Fahrenheit scale has 1.8 degrees for every 1 Celsius degree and starts 32 degrees higher at freezing, the two lines intersect at exactly -40°: (-40 × 1.8) + 32 = -72 + 32 = -40. Thus, -40 °C is identically equal to -40 °F.