How this works
The two scales don't share a zero point — Celsius freezes water at 0, Fahrenheit at 32 — and Fahrenheit's degree size is smaller (a Fahrenheit degree is 5/9 of a Celsius degree). Convert with °F = °C × 9/5 + 32. Useful when reading US weather forecasts, oven temperatures from American recipes, or fever thresholds in a US medical guide.
The two scales were designed for different purposes. Daniel Fahrenheit invented his scale in 1724 using three anchor points he could reproduce in a workshop: the freezing point of a salt-and-ice brine (0 °F, the coldest he could reliably make), the freezing point of pure water (32 °F), and average human body temperature (he set 96 °F, which later refinements moved to 98.6). Anders Celsius proposed his scale in 1742 around the two phase changes of pure water at sea level — 0 for freezing, 100 for boiling. Celsius is more useful in scientific work; Fahrenheit gives finer granularity in everyday weather (each degree is smaller, so the comfort range from 0 °F to 100 °F roughly maps to "miserable cold" to "miserable hot"). The two scales meet at exactly one point: −40 °C = −40 °F. Above that, °F numbers always run higher; below, they always run lower.
The US is the only large country that still uses Fahrenheit for weather and everyday measurement (Liberia, Belize, the Bahamas and a handful of small territories also use it, but mixed with Celsius). The UK and most of Europe switched to Celsius in the 1960s–70s as part of broader metrication. The US Metric Conversion Act of 1975 made metric "the preferred system" but never mandated it, and consumer-facing temperatures stayed Fahrenheit. Practical implication: any time you read a US recipe, a US thermostat, or an American weather forecast, you need this conversion. Inside US scientific, medical, military, and engineering contexts, Celsius (or Kelvin) is already standard.
The formula
For mental conversion, "double the °C and add 30" gets you within ~2 °F across normal weather temperatures: 20 °C → ~70 °F (actual 68), 30 °C → ~90 °F (actual 86). Useful anchors: −40 °C = −40 °F (the only point where the two scales meet), 0 °C = 32 °F, 100 °C = 212 °F (water boils), 37 °C ≈ 98.6 °F (body temperature).
Example calculation
- −10 °C = 14 °F (cold snap)
- 0 °C = 32 °F (water freezes)
- 20 °C = 68 °F (room temperature)
- 180 °C = 356 °F (typical baking oven)
Frequently asked questions
Why is body temperature 98.6 °F such an oddly specific number?
It's a conversion artefact. The original 19th-century measurement gave 37 °C as average body temperature, which converts to exactly 98.6 °F (37 × 9/5 + 32 = 98.6). The "98.6" is just 37 × 1.8 + 32 expressed in °F. More recent studies suggest average healthy body temperature is closer to 36.6 °C / 97.9 °F, but the older figure remains widely quoted.
My oven only shows °F — what do I set for 200 °C?
200 × 9/5 + 32 = 392 °F. Most US ovens go in 25 °F steps, so 400 °F is the closest setting (≈ 204 °C — close enough for any non-precision baking). Common conversions: 160 °C ≈ 325 °F, 180 °C ≈ 350 °F, 200 °C ≈ 400 °F, 220 °C ≈ 425 °F.
When is a fever in °F? My thermometer reads in Celsius.
US clinical guidelines define fever as ≥100.4 °F oral (or ≥100.4 °F rectal in infants). That's 38 °C exactly — the conversion is the reason for the awkward "0.4". The CDC, NHS, and Infectious Diseases Society of America all use 38 °C / 100.4 °F as the threshold. 37.5–38 °C (99.5–100.4 °F) is "elevated but not yet feverish". 39 °C / 102.2 °F is a moderate fever; 40 °C / 104 °F or higher warrants medical attention. Note that "normal" body temperature varies between 36.1–37.2 °C (97–99 °F) depending on time of day, age, and where you measure.
Why does the US still use Fahrenheit?
Mostly inertia. The US Metric Conversion Act of 1975 declared metric the 'preferred system' but never made it mandatory, leaving each industry to switch (or not) on its own. Science, medicine, military and engineering moved to Celsius/SI; consumer-facing weather, ovens, thermostats and recipes didn't, because the cost of relabelling everything outweighed any individual benefit. Fahrenheit also has a defensible everyday use: each degree is smaller, so 0–100 °F maps roughly to the human comfort range, giving more granularity than 0–37 °C. There's no live debate to switch — surveys consistently show Americans understand Fahrenheit intuitively and Celsius numbers feel abstract, which makes change politically uninteresting.
What are the most common temperatures I should know in both scales?
Reference points worth memorising: −40 °C = −40 °F (the only point where both scales agree); 0 °C = 32 °F (water freezes); 20 °C = 68 °F (typical room); 25 °C = 77 °F (warm summer day); 30 °C = 86 °F (hot); 37 °C = 98.6 °F (body temperature); 38 °C = 100.4 °F (fever threshold); 100 °C = 212 °F (water boils at sea level); 180 °C = 356 °F ≈ 350 °F (most baking); 200 °C = 392 °F ≈ 400 °F (roasting). For freezer/fridge: 4 °C = 39 °F (fridge), −18 °C = 0 °F (freezer).