Temperature conversion is the single most common unit problem for US travelers and expats. Fahrenheit and Celsius don't line up neatly — they have different zero points, different scales, and even different directions at some temperatures. Here's everything you need to move confidently between them.

The exact formulas

Fahrenheit to Celsius: C = (F − 32) × 5/9

Celsius to Fahrenheit: F = C × 9/5 + 32

Notice both formulas have an additive offset (32) and a multiplicative factor (9/5 or 5/9). The offset exists because the two scales were defined relative to different physical phenomena — Celsius starts at water's freezing point (0°C), while Fahrenheit uses a more arbitrary calibration that puts freezing at 32°F and body temperature near 100°F.

Key reference points

  • −40°F = −40°C (the one point where they are equal)
  • 0°F = −17.8°C (very cold)
  • 32°F = 0°C (water freezes)
  • 50°F = 10°C (cool spring day)
  • 70°F = 21°C (room temperature)
  • 77°F = 25°C (warm day)
  • 86°F = 30°C (hot)
  • 100°F = 37.8°C (very hot; close to body temp)
  • 212°F = 100°C (water boils at sea level)

Fast mental approximation

The exact formula is easy to fumble in your head. Try these shortcuts instead:

F to C (rough): Subtract 30, divide by 2.

  • 70°F: (70 − 30)/2 = 20°C. Exact: 21°C. ✓
  • 86°F: (86 − 30)/2 = 28°C. Exact: 30°C. (Off by 2)
  • 50°F: (50 − 30)/2 = 10°C. Exact: 10°C. ✓

C to F (rough): Double, add 30.

  • 25°C: 25 × 2 + 30 = 80°F. Exact: 77°F. (Off by 3)
  • 10°C: 10 × 2 + 30 = 50°F. Exact: 50°F. ✓
  • 30°C: 30 × 2 + 30 = 90°F. Exact: 86°F. (Off by 4)

This shortcut is accurate within about 3 degrees across the normal weather range (−10°C to 35°C). Close enough to know whether to bring a jacket.

Weather in Celsius

  • −10°C: very cold, bitter (14°F)
  • 0°C: freezing (32°F)
  • 10°C: cool, sweater weather (50°F)
  • 15°C: mild (59°F)
  • 20°C: pleasant (68°F)
  • 25°C: warm (77°F)
  • 30°C: hot (86°F)
  • 35°C: very hot (95°F)
  • 40°C: dangerous heat (104°F)

Cooking temperatures

Ovens in US kitchens are marked in Fahrenheit. Most international recipes use Celsius. Common equivalents:

  • 250°F = 121°C (very low, slow roasting)
  • 300°F = 149°C (low)
  • 325°F = 163°C (moderate low)
  • 350°F = 177°C (moderate, most baking)
  • 375°F = 191°C (moderate high)
  • 400°F = 204°C (high, most roasting)
  • 425°F = 218°C (high)
  • 450°F = 232°C (very high, pizza)
  • 500°F = 260°C (broiling)

Body temperature

  • 98.6°F = 37.0°C (normal)
  • 100.4°F = 38.0°C (low-grade fever)
  • 102°F = 38.9°C (moderate fever)
  • 104°F = 40.0°C (high fever)
  • 106°F = 41.1°C (medical emergency)

Celsius uses decimals (37.2°C) where Fahrenheit often just uses whole numbers (99°F). International medical practice is in Celsius.

A word about Kelvin

Scientists use Kelvin (K), which starts at absolute zero (0 K = −273.15°C = −459.67°F). Each Kelvin is the same size as a Celsius degree, just shifted. Convert by: K = C + 273.15.

  • 0 K = absolute zero (−273.15°C)
  • 273.15 K = water freezes
  • 373.15 K = water boils

Why the scales differ

Celsius was designed around water: 0° = freeze, 100° = boil. Clean and simple for metric-minded scientists.

Fahrenheit was defined in the early 1700s using brine ice (0°F) and body temperature (originally meant to be 96°F). It provides finer resolution in the normal weather range — 10°F intervals feel meaningfully different — which is why it persisted in US weather forecasting.

When to use which

Fahrenheit: US weather, US cooking, US health (body temp), American English conversation.

Celsius: Science, international weather, cooking outside the US, most medical contexts worldwide.

Kelvin: Thermodynamics, high-energy physics, cryogenics.

Common conversion mistakes

A few pitfalls to avoid when converting between temperature scales:

  • Forgetting the offset. Length conversion (meters to feet) is pure multiplication — temperature isn't. Always subtract 32 first when going F to C, add 32 last when going C to F.
  • Order of operations on differences. A temperature change of 10°C equals 18°F — NOT 50°F. For deltas, multiply by 9/5 or 5/9 but skip the offset.
  • Freezer vs refrigerator temps. Home freezer is 0°F (−18°C); fridge is 40°F (4°C). Don't mix these up when reading European appliance manuals.
  • Thermostat conversions. Setting a US thermostat to "20" thinking it's Celsius will freeze you out — it's 20°F. Double-check the display unit before adjusting.

Rankine: the other Fahrenheit

Rankine (°R) is an absolute version of Fahrenheit, similar to how Kelvin relates to Celsius. Conversion: °R = °F + 459.67. Used rarely today, mostly in US aerospace engineering. Water freezes at 491.67°R and boils at 671.67°R. Unless you're studying propulsion or legacy thermodynamics texts, you'll never need to convert to Rankine — but it's nice to know what the unit means if you encounter it.

Industrial and scientific thresholds

  • Dry ice sublimation: −78.5°C (−109°F)
  • Liquid nitrogen boil: −196°C (−321°F)
  • Absolute zero: −273.15°C (−459.67°F)
  • Sun's surface: ~5,500°C (9,930°F)
  • Tungsten melting point: 3,422°C (6,192°F) — highest of any pure metal
  • Iron melting point: 1,538°C (2,800°F)

Just convert it

Our temperature converter handles Fahrenheit, Celsius, Kelvin, and Rankine in one click. Enter a value in one scale, see the rest instantly. Use it for travel, cooking, lab work, or just satisfying curiosity about weather elsewhere in the world. Temperature conversion is one of those things you do rarely but can't afford to botch — a tool fixes that.