Kelvin (K) is the absolute temperature scale of the International System. Unlike Celsius and Fahrenheit, it starts at absolute zero (0 K), where all thermal motion ceases. Here you have the exact formulas, wide table, mental trick and key uses: science, color temperature (lighting/photography) and cryogenics.
Why Kelvin is special
| Scale | Zero | Degree size | Symbol |
|---|---|---|---|
| Kelvin | Absolute zero (0 K) | 1 K = 1 °C | K (no °) |
| Celsius | Water freezes (0 °C) | 1 °C = 1 K | °C |
| Fahrenheit | Ice/salt mix (0 °F) | 1 °F = 5/9 K | °F |
0 K = −273.15 °C = −459.67 °F (absolute zero).
273.15 K = 0 °C = 32 °F (ice).
373.15 K = 100 °C = 212 °F (water boils).
Exact formulas
Kelvin ↔ Celsius (fixed offset)
°C = K − 273.15
K = °C + 273.15
Kelvin ↔ Fahrenheit (via Celsius)
°F = (K − 273.15) × 1.8 + 32
K = (°F − 32) / 1.8 + 273.15
Reference table (K → °C → °F)
| K | °C | °F | Context |
|---|---|---|---|
| 0 | -273.15 | -459.67 | Absolute zero |
| 1 | -272.15 | -457.87 | Liquid helium (boils at 4.2 K) |
| 4.2 | -268.95 | -452.11 | Helium-4 boils (cryogenics) |
| 20 | -253.15 | -423.67 | Liquid hydrogen (boils at 20.3 K) |
| 77.4 | -195.75 | -320.35 | Liquid nitrogen (LN₂) |
| 90.2 | -182.95 | -297.31 | Liquid oxygen (LOX) |
| 111.7 | -161.45 | -258.61 | Liquid methane (LNG) |
| 194.7 | -78.45 | -109.21 | Dry ice (CO₂ sublimates) |
| 273.15 | 0.00 | 32.00 | Water freezes |
| 293.15 | 20.00 | 68.00 | Room comfort |
| 310.15 | 37.00 | 98.60 | Human body |
| 373.15 | 100.00 | 212.00 | Water boils |
| 450 | 176.85 | 350.33 | Low oven |
| 500 | 226.85 | 440.33 | Neapolitan pizza |
| 573 | 299.85 | 571.73 | High oven |
| 1000 | 726.85 | 1340.33 | Red-hot steel |
| 2000 | 1726.85 | 3140.33 | Candle flame / tungsten |
| 3000 | 2726.85 | 4940.33 | Halogen bulb |
| 5500 | 5226.85 | 9440.33 | Daylight (color temp) |
| 6500 | 6226.85 | 11240.33 | Overcast daylight / sRGB monitor |
| 10000 | 9726.85 | 17540.33 | Deep blue sky |
Color temperature (lighting, photography, screens)
Color temperature is expressed in Kelvin (ideal black body). It’s not thermal temperature, it’s the color of light.
| K | Appearance | Typical use |
|---|---|---|
| 1700–1900 | Deep red/amber | Candle, flame |
| 2000–2500 | Warm orange | Vintage bulb, sunset |
| 2700–3000 | Warm white | Home, restaurants, “warm white” |
| 3500–4100 | Neutral white | Offices, retail, “neutral white” |
| 5000–5500 | Daylight (D50/D55) | Graphic arts, photography, “daylight” |
| 6000–6500 | Cool white / Daylight | sRGB monitors, overcast, “cool white” |
| 7000–10000 | Bluish | Shade, blue sky, “north light” |
Rule: Low K = warm (red/amber); High K = cool (blue).
In LED: “2700 K” = warm; “6500 K” = cool.
Cryogenics: boiling points (1 atm)
| Substance | K | °C | Use |
|---|---|---|---|
| Helium-4 | 4.22 | -268.93 | Superconductors, quantum |
| Hydrogen | 20.27 | -252.88 | Rocket fuel |
| Neon | 27.10 | -246.05 | Specialized cryogenics |
| Nitrogen | 77.36 | -195.79 | LN₂ (flash freezing, dermatology, overclocking) |
| Oxygen | 90.19 | -182.96 | LOX (rockets, medical) |
| Argon | 87.30 | -185.85 | Welding, insulation |
| Methane | 111.65 | -161.50 | LNG (gas transport) |
Mental trick
°C ≈ K − 273 (error 0.15 °C, negligible except precision metrology)
| K | Trick °C | Exact °C |
|---|---|---|
| 300 | 27 | 26.85 |
| 293 | 20 | 19.85 |
| 310 | 37 | 36.85 |
| 5500 | 5227 | 5226.85 |
For color temperature (2000–10000 K) the approximation is enough.
Conclusion
- Kelvin = absolute scale (0 K = absolute zero). No “degrees”.
- K ↔ °C: add/subtract 273.15 (same degree size).
- K ↔ °F: via Celsius.
- Color temp: expressed in K (2700 K warm, 6500 K cool).
- Cryogenics: LN₂ 77 K, LOX 90 K, LNG 111 K.
- Tool: unit converter — pick “Temperature” and convert K ↔ °C ↔ °F instantly.