-40°C=-40°F

To convert -40°C to Fahrenheit, multiply by 9/5 (which is 1.8), then add 32: (-40 × 1.8) + 32 = -72 + 32 = -40°F. The formula accounts for two differences between the scales: the multiplier (9/5) adjusts for the fact that one Celsius degree is larger than one Fahrenheit degree, and the +32 offset aligns the two scales' zero points, since Celsius sets 0° at water's freezing point while Fahrenheit sets 32° there.

The Celsius scale was developed in 1742 by Anders Celsius and anchors its reference points on the behavior of water at standard atmospheric pressure: 0°C is the freezing point and 100°C is the boiling point, giving the scale its intuitive 100-degree range between two universally understood physical states. The Fahrenheit scale, created by Daniel Gabriel Fahrenheit around 1724, originally used a brine/ice mixture as 0° and human body temperature as a higher reference. Today Fahrenheit is used primarily in the United States for everyday weather, cooking temperatures, and home thermostats, while Celsius is the standard everywhere else and in all scientific disciplines.

Knowing that -40°C equals -40°F is practical in many everyday situations. Travelers between the US and the rest of the world frequently need this conversion to interpret local weather forecasts, understand oven temperatures in American recipes, or set the correct thermostat temperature in a hotel. A useful mental shortcut for quick estimates: double the Celsius value and add 30 gives a rough Fahrenheit reading (for example, 20°C ≈ 70°F), though for precise results the formula above is always reliable. Common reference points worth memorizing: 0°C = 32°F (freezing), 20°C = 68°F (cool room), 37°C = 98.6°F (body temperature), and 100°C = 212°F (boiling).