Temperature Converter

How to use the temperature converter

Type a temperature and choose the scale it is in: Fahrenheit (°F), Celsius (°C), kelvin (K) or Rankine (°R). The converter shows the value in all the other scales, the formula with your numbers filled in, and a short Fahrenheit-to-Celsius (or Celsius-to-Fahrenheit) chart around your value. It rejects anything below absolute zero, the coldest temperature possible.

Fahrenheit to Celsius formula

°C = (°F − 32) × 5/9

°F = °C × 9/5 + 32

K = °C + 273.15

°R = °F + 459.67

The two everyday scales differ in two ways. Their zero points are different (water freezes at 0 °C but 32 °F), and their degrees are different sizes (100 Celsius degrees span the same range as 180 Fahrenheit degrees, a ratio of 5 to 9). That is why you must subtract 32 before scaling, and add 32 after scaling in the other direction. Kelvin uses Celsius-sized degrees starting at absolute zero, and Rankine does the same with Fahrenheit-sized degrees.

Worked example

Convert a normal body temperature of 98.6 °F to Celsius:

1. Subtract 32: 98.6 − 32 = 66.6
2. Multiply by 5/9: 66.6 × 5 ÷ 9 = 37 °C
3. In kelvin: 37 + 273.15 = 310.15 K

And an oven setting of 180 °C to Fahrenheit: 180 × 9/5 = 324, then 324 + 32 = 356 °F, which recipes round to 350 °F.

Body temperature and fever chart

Normal body temperature varies from person to person and through the day, and readings depend on where you measure (mouth, ear, forehead or armpit). For public health purposes the CDC defines a fever as a measured temperature of 100.4 °F (38 °C) or higher. Ask a doctor about a fever in a baby under 3 months, a very high temperature or one that lasts more than a couple of days.

Oven temperature conversion chart

For a fan or convection oven, many recipes lower the Celsius temperature by about 20 °C. Safe minimum internal temperatures for meat from the USDA are 145 °F (62.8 °C) for whole cuts of beef, pork and fish, 160 °F (71.1 °C) for ground meat and 165 °F (73.9 °C) for poultry.

Weather temperature chart

Key reference points

• Water freezes at 0 °C (32 °F) and boils at 100 °C (212 °F) at sea level. At altitude it boils at a lower temperature.
• −40 is the same in both scales.
• Absolute zero is 0 K, −273.15 °C or −459.67 °F.
• Room temperature is usually taken as about 20–22 °C (68–72 °F).

Kelvin and Rankine

Scientists use kelvin, the SI unit of temperature, because it starts at absolute zero and has no negative values. A kelvin step is the same size as a Celsius degree, so a room at 25 °C is 298.15 K. Note that it is written “K”, not “°K”. Rankine does the same for Fahrenheit-sized degrees and still appears in some U.S. engineering work: 77 °F is 536.67 °R. Both scales matter when a formula multiplies or divides temperatures, as in the gas laws, because ratios only make sense on an absolute scale.

Quick mental conversion

For a rough answer, subtract 30 from °F and halve it (80 °F becomes about 25 °C), or double °C and add 30 (20 °C becomes about 70 °F). The shortcut is closest around 10 °C (50 °F), where it is exact, and drifts by a few degrees at very hot or cold temperatures. For other units such as length, weight and volume, use the unit converter. If you are working out what it costs to heat or cool a room, the electricity cost calculator turns appliance wattage into a monthly bill.