Temperature is the one common conversion that is not a simple multiplication. Fahrenheit and Celsius have different zero points as well as different degree sizes, so you have to shift and scale. This converter takes a single reading and shows it on all four scales in use today — Fahrenheit, Celsius, Kelvin and Rankine — with each formula written out, the nearest familiar reference point, and an option to convert temperature differences correctly.
How to use the temperature converter
- Type the temperature in Temperature. Negative values are fine.
- Choose the scale it is measured in under Scale.
- Read all four scales on the tape. The headline shows the most likely target (Celsius for a Fahrenheit input, and vice versa).
- Tick Convert a temperature change if your number is a difference, such as “the oven ran 25 °F hot” or “the river warmed by 3 °C.”
Values below absolute zero (−459.67 °F or −273.15 °C) are rejected, because nothing can be colder.
Temperature conversion formulas
The factor 9/5 (1.8) is the ratio of degree sizes: water’s freezing-to-boiling range spans 100 Celsius degrees but 180 Fahrenheit degrees. The offsets come from where each scale puts zero.
Worked example: a 72 °F room
Celsius: (72 − 32) × 5/9 = 40 × 0.5556 = 22.22 °C
Kelvin: 22.2222 + 273.15 = 295.37 K
Rankine: 72 + 459.67 = 531.67 °R
For a temperature change, skip the offset. If a thermostat setback lowers the room by 8 °F, that is a drop of 8 × 5/9 = 4.44 °C, not (8 − 32) × 5/9.
Temperature conversion table
| °C | °F | K | What it feels like |
|---|---|---|---|
| −40 | −40 | 233.15 | Scales meet; severe arctic cold |
| −18 | −0.4 | 255.15 | Home freezer |
| 0 | 32 | 273.15 | Water freezes |
| 4 | 39.2 | 277.15 | Refrigerator |
| 10 | 50 | 283.15 | Cool spring day |
| 20 | 68 | 293.15 | Room temperature |
| 25 | 77 | 298.15 | Warm room |
| 30 | 86 | 303.15 | Hot summer day |
| 37 | 98.6 | 310.15 | Normal body temperature |
| 40 | 104 | 313.15 | High fever; hot tub limit |
| 100 | 212 | 373.15 | Water boils at sea level |
| 177 | 350.6 | 450.15 | Moderate oven (350 °F) |
| 232 | 449.6 | 505.15 | Hot oven (450 °F) |
The four scales
Fahrenheit (°F), proposed by Daniel Gabriel Fahrenheit in 1724, is the everyday scale in the United States and a few other countries. Water freezes at 32 °F and boils at 212 °F at sea level.
Celsius (°C), used almost everywhere else and in science, puts water’s freezing point at 0 °C and boiling point at about 100 °C. Today it is defined from the kelvin: °C = K − 273.15.
Kelvin (K) is the SI unit of temperature. It starts at absolute zero, the point where thermal motion is at its minimum, and since 2019 it has been defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ J/K. Gas laws and radiation formulas require kelvins.
Rankine (°R) is the absolute counterpart of Fahrenheit, used mainly in US engineering thermodynamics. 0 °R is absolute zero, and one Rankine degree equals one Fahrenheit degree.
Quick estimates
To convert Celsius to Fahrenheit in your head, double it and add 30: 20 °C → 70 °F (exact: 68). For the reverse, subtract 30 and halve: 80 °F → 25 °C (exact: 26.7). The shortcut is closest between about 0 °C and 25 °C.
For one-direction charts, use Celsius to Fahrenheit or Fahrenheit to Celsius; for scientific work, Celsius to Kelvin.
Frequently asked questions
How do I convert Fahrenheit to Celsius?
Subtract 32, then multiply by 5/9. For example, 72 °F − 32 = 40, and 40 × 5/9 = 22.2 °C. To go from Celsius to Fahrenheit, multiply by 9/5 (1.8) and add 32.
At what temperature are Celsius and Fahrenheit equal?
At −40. Setting F = C in F = 1.8C + 32 gives C = −40, so −40 °C and −40 °F are the same temperature.
Why doesn't Kelvin use a degree symbol?
Since 1967 the kelvin has been an SI base unit like the meter or second, so it is written simply as K, not °K. Kelvin and Celsius degrees are the same size; only the zero point differs, with 0 K = −273.15 °C.
What is the Rankine scale?
Rankine is an absolute scale with Fahrenheit-sized degrees. Zero is absolute zero, and water freezes at 491.67 °R. US engineers use it in thermodynamics and aerospace calculations that need an absolute temperature in Fahrenheit units.
How do I convert a temperature change rather than a reading?
Tick the temperature-change box. A change of 10 °C equals a change of 18 °F, because only the degree size (×1.8) matters; the 32° offset cancels out when you subtract two readings.