Temperature Conversion Guide: Celsius, Fahrenheit, and Kelvin

10 min read•June 1, 2026•Reviewed: June 1, 2026

✓Reviewed by Apex Conversion Editorial Team · Last reviewed June 1, 2026

Temperature is unique among physical quantities: unlike length and weight where the two major systems share the same zero point, Celsius and Fahrenheit have completely different zero references. This is why temperature conversion requires two arithmetic steps — a scaling step and a shift step — rather than the simple multiplication used for most other units.

This guide explains all three temperature scales (Celsius, Fahrenheit, Kelvin), provides the exact formulas with worked examples, covers the most important reference temperatures you need to memorize, and answers the ten questions about temperature conversion that come up most often.

What the Units Mean

Celsius (°C) is the metric temperature scale. It sets 0°C at the freezing point of water at standard atmospheric pressure and 100°C at the boiling point — giving 100 equal degrees between these two reference points. Celsius is the everyday standard in almost every country except the United States.

Fahrenheit (°F) is used in the United States (and its territories) for everyday weather and cooking. It places water's freezing point at 32°F and boiling point at 212°F, giving 180 degrees between these points. The scale was originally defined by Daniel Fahrenheit in 1724 using a brine solution (0°F) and human body temperature (~96°F, later revised to 98.6°F).

Kelvin (K) is the SI base unit of thermodynamic temperature. It shares the same degree increment as Celsius but starts at absolute zero (0 K = −273.15°C) — the theoretical point at which all molecular motion ceases. Kelvin values are never preceded by a degree symbol; you write '300 K' not '300°K'. Kelvin is mandatory for all thermodynamic calculations.

Exact Conversion Formulas

━━━ CELSIUS ↔ FAHRENHEIT ━━━
  °F = (°C × 9/5) + 32     →  °F = (°C × 1.8) + 32
  °C = (°F − 32) × 5/9     →  °C = (°F − 32) ÷ 1.8
  WHY TWO STEPS: °C and °F have different degree sizes (1°C = 1.8°F)
  AND different zeros (0°C = 32°F). Both differences must be corrected.

━━━ CELSIUS ↔ KELVIN ━━━
  K  = °C + 273.15          °C = K  − 273.15
  (degree sizes are equal — only the zero point shifts)

━━━ FAHRENHEIT ↔ KELVIN ━━━
  K  = (°F + 459.67) × 5/9  °F = K × 9/5 − 459.67

━━━ QUICK MENTAL ESTIMATES ━━━
  C → F: double °C and add 30  (good for 0–40°C; actual: ×1.8 + 32)
  F → C: subtract 30, halve   (rough guide only)
  Better: subtract 32, multiply by 0.55 (accurate within 2.8%)

Key Reference Temperatures

Memorizing a handful of anchor points makes estimation fast: Absolute zero = 0 K = −273.15°C = −459.67°F. Water freezes at 0°C = 32°F = 273.15 K. Room temperature ≈ 20–22°C = 68–72°F ≈ 293–295 K. Human body temperature = 37°C = 98.6°F = 310.15 K. Fever threshold ≈ 38°C = 100.4°F. Water boils at 100°C = 212°F = 373.15 K.

Cooking reference points: Refrigerator temperature: 3–4°C (37–39°F). Safe food holding temperature: 60°C (140°F). Chicken breast safe internal temperature: 74°C (165°F). Medium-rare steak: 57°C (135°F). Common baking temperature: 180°C (356°F ≈ 350°F). Pizza stone temperature: 260–290°C (500–550°F).

Common Mistakes to Avoid

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Forgetting to subtract 32 before multiplying when converting °F to °C: you must shift the zero point first. A common error is 72°F × (5/9) = 40°C instead of the correct (72 − 32) × 5/9 = 22.2°C.
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Using Celsius in thermodynamic formulas: gas laws (PV=nRT), thermal efficiency, blackbody radiation — all require Kelvin. Always add 273.15 before applying physics or chemistry formulas.
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Treating negative Celsius as a 'reverse' conversion: −10°C = (−10 × 1.8) + 32 = −18 + 32 = 14°F. The formula works exactly the same with negative numbers.
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Assuming 0°C and 0°F are similar: 0°F = −17.8°C — a very cold day. 0°C = 32°F — water's freezing point. The Fahrenheit zero has no physically meaningful anchor like Celsius does.
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Using the 'double and add 30' shortcut for extreme temperatures: the approximation is only accurate for 0–40°C. At 100°C, doubling and adding 30 gives 230°F (actual: 212°F — a 8.5% error).

Frequently Asked Questions

What is 37°C in Fahrenheit?

(37 × 1.8) + 32 = 66.6 + 32 = 98.6°F. Normal human body temperature. Values above 38°C (100.4°F) are generally considered a fever in adults.

What is 350°F in Celsius?

(350 − 32) × 5/9 = 318 × 5/9 = 176.7°C. Approximately 175–180°C, the most common baking temperature for cakes, cookies, and casseroles.

At what temperature are Celsius and Fahrenheit the same?

At −40°. Both scales read −40 at the same temperature: (−40 × 1.8) + 32 = −72 + 32 = −40°F. This is the only point where the two scales converge.

What is room temperature in Kelvin?

Room temperature is approximately 20–22°C = 293.15–295.15 K. Standard laboratory temperature (SATP) is 25°C = 298.15 K. When chemistry problems say 'room temperature,' they often mean 298 K.

How do I convert Fahrenheit to Kelvin directly?

K = (°F + 459.67) × 5/9. Example: 32°F → (32 + 459.67) × 5/9 = 491.67 × 5/9 = 273.15 K. Alternatively, convert to Celsius first, then add 273.15.

Why does Kelvin have no degree symbol?

The degree symbol (°) is used for relative temperature scales where the unit depends on a specific reference point. Kelvin is an absolute scale starting at physical zero, so it stands alone as a unit (like meters or kilograms). Writing 300°K is incorrect; the proper notation is 300 K.

What is the oven temperature for roasting vs. baking?

Low/slow roasting: 120–150°C (250–300°F). Moderate baking: 160–180°C (325–356°F). Hot baking: 200–220°C (400–425°F). High-heat roasting: 230°C (450°F). Broiling/grilling: 230–290°C (450–550°F). Fan-assisted (convection) ovens typically run 10–20°C lower than stated for non-fan recipes.

What is absolute zero?

Absolute zero is 0 K = −273.15°C = −459.67°F. It is the theoretical minimum temperature — the point at which all atomic thermal motion stops. Experimentally, scientists have reached within billionths of a degree above 0 K but have never achieved it.

How does temperature affect sea-level boiling point?

Water boils at 100°C (212°F) only at exactly 1 atmosphere (sea level standard). At higher altitude, atmospheric pressure drops and water boils at a lower temperature: Denver (1,609 m) ≈ 95°C (203°F). Mount Everest (8,849 m) ≈ 69°C (156°F). This is why high-altitude baking recipes adjust cooking times.

What temperature does alcohol freeze at?

Ethanol (drinking alcohol) freezes at −114.1°C (−173.4°F = 159.1 K). That is why spirits do not freeze in a standard freezer (−18°C / 0°F). Vodka (40% ABV) freezes at approximately −27°C (−16.6°F).

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