Temperature Conversion Made Easy

Quick Answer — TL;DR

Temperature conversion is essential for weather, cooking, and science. Use °F = (°C × 9/5) + 32, °C = (°F − 32) × 5/9, and K = °C + 273.15 exactly. For example, 20 °C = 68 °F = 293.15 K. This guide explains the exact definitions, gives 5 worked examples, flags common mistakes, and answers 3 FAQs so you never guess again.

Temperature conversion is one of the most common conversions people need to make. Whether you're checking the weather forecast, cooking, or working in a laboratory, understanding how to convert between Celsius, Fahrenheit, and Kelvin is essential.

The Three Main Temperature Scales

Celsius (°C) is used by most countries worldwide and is based on the freezing (0°C) and boiling (100°C) points of water.

Fahrenheit (°F) is primarily used in the United States. Water freezes at 32°F and boils at 212°F.

Kelvin (K) is the scientific standard, starting at absolute zero. It's primarily used in physics and chemistry.

Watch: Temperature Conversion Explained

Conversion Formulas

Here are the essential formulas you need to know:

• Celsius to Fahrenheit: °F = (°C × 9/5) + 32
• Fahrenheit to Celsius: °C = (°F - 32) × 5/9
• Celsius to Kelvin: K = °C + 273.15

Pro tip: For a quick mental estimate, double the Celsius value and add 30 to get an approximate Fahrenheit value!

Formula & Sources — Exact Factors

Celsius is defined by absolute zero (−273.15 °C = 0 K) and the triple point of water (0.01 °C = 273.16 K). Per BIPM SI Brochure and NIST SP 811, the exact relations are: °F = °C × 9/5 + 32; °C = (°F − 32) × 5/9; K = °C + 273.15; °C = K − 273.15. The Fahrenheit degree is exactly 5/9 of a Celsius degree. Kelvin and Celsius degrees are the same size; only their zero points differ. ISO 80000-5 standardizes symbols: °C, °F, K (no degree symbol for kelvin). Use 273.15 exactly, not 273.

Sources: BIPM SI Brochure 9th ed. defines the kelvin via the Boltzmann constant (2019 redefinition). NIST SP 811 §8 confirms conversion factors. ISO 80000-5:2019 covers thermodynamics. The 1954 definition linked kelvin to water's triple point; since 2019 it is defined via fixed k = 1.380649×10⁻²³ J/K, but 273.15 remains exact for everyday conversion.

Worked Examples (5-step table)

Worked examples — see the calculation shown step by step:

• Example 1 — Room temperature: 20 °C → °F: 20×9/5=36, 36+32=68 °F; → K: 20+273.15=293.15 K.
• Example 2 — Boiling water: 100 °C → 212 °F (100×9/5=180+32); → 373.15 K.
• Example 3 — Body temperature: 37 °C → 98.6 °F (37×1.8=66.6+32); → 310.15 K. Clinical fever threshold 38 °C = 100.4 °F.
• Example 4 — Freezer: −18 °C → −0.4 °F (−18×1.8=−32.4+32); → 255.15 K. Note how negative values require careful sign handling.
• Example 5 — The crossover: −40 °C = −40 °F (solve °C×9/5+32=°C → °C=−40). In kelvin: 233.15 K. Also 0 K = −273.15 °C = −459.67 °F = absolute zero.

Common Mistakes and How to Fix Them

• Mistake: Adding 32 before multiplying. Fix: Always multiply first, then add 32 for °C→°F. Use parentheses: (°C×9/5)+32. Check with 0 °C = 32 °F.
• Mistake: Using 273 instead of 273.15 for kelvin. Fix: 0 °C is 273.15 K exactly. The 0.15 matters in science and calibration; round only at display.
• Mistake: Confusing degree sizes. Fix: 1 °C step = 1 K step = 1.8 °F step. When you need a temperature difference (ΔT), Δ1 °C = Δ1 K = Δ1.8 °F.
• Mistake: Writing '°K'. Fix: The unit is 'K' (kelvin), not '°K', per SI since 1967. Correct: 273.15 K, not 273.15°K.

Frequently Asked Questions (FAQ)

What is the fastest mental trick for Celsius to Fahrenheit?

Double the Celsius, subtract 10% of that doubled value, then add 32. More precisely: °F ≈ (°C×2) − (°C×0.2) + 32. Example: 20 °C → 40 − 4 + 32 = 68 °F exact. The simpler 'double + 30' gives 70 °F, which is 2 °F high — fine for weather but not for cooking or lab work. For Fahrenheit to Celsius, reverse: (°F−32) ÷ 1.8.

Why does 0 °C equal 32 °F and why is −40 the same on both scales?

Daniel Fahrenheit set 0 °F near the coldest lab mixture he could make (ice+salt) and 96 °F near body temperature; Anders Celsius later set 0 °C at freezing and 100 °C at boiling. The two linear scales intersect where °C×9/5+32=°C, solving gives −40. That is the only temperature with the same number on both scales, a handy check for any converter.

When should I use Kelvin instead of Celsius?

Use kelvin for any formula involving absolute temperature: gas laws (PV=nRT), thermodynamics, black-body radiation, and scientific calculations. Kelvin starts at absolute zero, so ratios and multiplications are meaningful. Celsius is fine for everyday weather, cooking, and differences, but never use °C in an equation that requires T in the denominator or as a ratio.

How to Convert Temperature in 3 Steps

1. Step 1: Enter your value and select the source unit (°C, °F, or K) in the converter above — the tool uses the exact NIST factors instantly.
2. Step 2: Read the result in all three scales at once; copy the value you need. For manual checks, apply °F=(°C×9/5)+32 or K=°C+273.15.
3. Step 3: Sanity-check with anchors: 0 °C=32 °F=273.15 K, 100 °C=212 °F=373.15 K, −40 °C=−40 °F. If your result is far from these, re-check the formula order.

Try the Converter & Keep Reading

Try it yourself: open the Temperature Converter or Energy Converter on OmniConverter for instant, high-precision results. For deeper reading, see our complete temperature conversion guide — it pairs perfectly with this guide.

Absolute Zero and Why Kelvin Matters in Science

Kelvin starts at absolute zero, the point where molecular motion stops (0 K = −273.15 °C = −459.67 °F). Unlike Celsius and Fahrenheit, kelvin has no negative values, making ratios meaningful: 300 K is twice as hot as 150 K in terms of thermal energy, but 30 °C is not twice as hot as 15 °C. This is why the ideal gas law, entropy, and any equation with temperature in the denominator requires kelvin. Never plug Celsius into PV=nRT without adding 273.15.

Practical Tips: Rounding and Significant Figures

For weather, round to the nearest whole degree (e.g., 21.3 °C → 70 °F). For cooking, keep one decimal when converting oven settings (176.7 °C = 350 °F). In laboratories, maintain the precision of your input: if you measured 25.00 °C (four significant figures), report 77.00 °F and 298.15 K. Rounding early compounds error; always carry one extra digit and round only the final display. The OmniConverter keeps full precision and rounds at the end.

Sources: BIPM SI Brochure 9th edition (2019), NIST SP 811 Guide for the Use of the SI, ISO 80000 series for quantities and units. All exact factors are per international agreement.

Sources & Further Reading

For authoritative standards, see the NIST — SI Units – Temperature and the Wikipedia — Conversion of scales of temperature for official definitions and conversion factors.