Temperature Converter Online Free
Convert between Celsius, Fahrenheit, Kelvin, and Rankine instantly. The converter shows the exact formula used for each conversion — no signup, runs entirely in your browser.
Formula: (°C × 9/5) + 32 = °F
All Temperature Conversions
| Unit | Value |
|---|---|
| Celsius (°C) | — |
| Fahrenheit (°F) | — |
| Kelvin (K) | — |
| Rankine (°R) | — |
How to Convert Temperature
- 1Select the source scale from the From dropdown — e.g. Celsius.
- 2Enter a temperature value. The result updates instantly and the formula is shown below the converter.
- 3See all scales at once in the all conversions table — useful for quick cross-reference.
Why Temperature Conversion Needs a Formula
Unlike units of length or weight, temperature scales differ in two ways: the size of a degree and the location of zero. Celsius and Fahrenheit both use different degree intervals (a 1 °C change equals a 1.8 °F change) and different zero points (0 °C = 32 °F). Converting between them requires both a multiplication and an addition. Kelvin and Rankine are absolute scales that start at absolute zero, making conversions to and from them simpler (just add or subtract 273.15 or 459.67 respectively).
Temperature Conversion Formula Reference
| From | To | Formula | Example |
|---|---|---|---|
| Celsius | Fahrenheit | (°C × 9/5) + 32 | 20 °C = 68 °F |
| Fahrenheit | Celsius | (°F − 32) × 5/9 | 68 °F = 20 °C |
| Celsius | Kelvin | °C + 273.15 | 20 °C = 293.15 K |
| Kelvin | Celsius | K − 273.15 | 300 K = 26.85 °C |
| Fahrenheit | Kelvin | (°F − 32) × 5/9 + 273.15 | 32 °F = 273.15 K |
| Celsius | Rankine | (°C + 273.15) × 9/5 | 0 °C = 491.67 °R |
| Fahrenheit | Rankine | °F + 459.67 | 32 °F = 491.67 °R |
Quick Mental Temperature Tips
°C to °F Quick Estimate
Double the Celsius and add 30. So 20 °C ≈ (20 × 2) + 30 = 70 °F. The exact answer is 68 °F, so this shortcut is off by about 2°F at room temperature.
Key Reference Points
Freeze: 0 °C = 32 °F. Body temp: 37 °C = 98.6 °F. Boil: 100 °C = 212 °F. Memorising these three anchors lets you sense-check any conversion.
Kelvin for Science
When working with gas laws, radiation, or thermodynamics always use Kelvin. Zero Kelvin is absolute zero — no negative values. Add 273 to Celsius for a quick mental conversion.
Weather Temperatures
Cold day: −10 °C = 14 °F. Cool: 10 °C = 50 °F. Warm: 25 °C = 77 °F. Hot: 35 °C = 95 °F. These benchmarks help you translate forecasts from unfamiliar scales.
Frequently Asked Questions
How do I convert Celsius to Fahrenheit?
Multiply the Celsius value by 9/5 (or 1.8) and add 32. Formula: °F = (°C × 9/5) + 32. For example, 20 °C = (20 × 1.8) + 32 = 36 + 32 = 68 °F.
How do I convert Fahrenheit to Celsius?
Subtract 32 from the Fahrenheit value, then multiply by 5/9. Formula: °C = (°F − 32) × 5/9. For example, 98.6 °F = (98.6 − 32) × 5/9 = 66.6 × 0.556 = 37 °C (body temperature).
What is Kelvin and how does it relate to Celsius?
Kelvin is the SI base unit of temperature. It has the same interval size as Celsius but starts at absolute zero (0 K = −273.15 °C). Formula: K = °C + 273.15. Kelvin is always a positive number and is used in science and engineering.
What is Rankine?
Rankine (°R) is an absolute temperature scale used in some US engineering fields. It has the same degree interval as Fahrenheit, but starts at absolute zero. 0 °R = 0 K = −459.67 °F. Formula: °R = °F + 459.67.
Why can't I use a simple multiplier to convert temperatures?
Unlike length or weight, temperature scales differ in both interval size and zero-point location. Celsius and Fahrenheit have different-sized degrees AND different zeroes (0 °C ≠ 0 °F). You need a two-step formula (scale + offset) rather than a single multiplier.
What are some useful temperature reference points?
Absolute zero: 0 K = −273.15 °C = −459.67 °F. Water freezes: 0 °C = 32 °F = 273.15 K. Body temperature: 37 °C = 98.6 °F. Water boils: 100 °C = 212 °F = 373.15 K. Room temperature: ~20 °C = 68 °F.