Temperature Converter

How to use this calculator#

1. 1
   Decide whether you have a point or an intervalA thermostat reading, an oven setting and a weather forecast are all points on the scale, so they need the +32 offset. A temperature rise, a daily range or a tolerance band is an interval and uses the 1.8 ratio alone.
2. 2
   Set the source scale before typingThe number 100 is boiling water in Celsius, a warm summer day in Fahrenheit, and −173 °C in Kelvin. Nothing in the digits tells the converter which you meant.
3. 3
   Keep the decimal on anything clinical or scientific38 °C is 100.4 °F. Rounding that to 100 °F crosses below the threshold most fever guidance uses, so a half-degree of rounding can change the reading of a symptom.
4. 4
   Sanity-check against a known anchorWater freezes at 0 °C / 32 °F, boils at 100 °C / 212 °F, and −40 is the same number on both scales. If your answer sits on the wrong side of the nearest anchor, you inverted the formula.

The formula#

The Celsius and Fahrenheit formulas#

Temperature scales do not share a zero point, so they cannot be converted by a single multiplication the way lengths can. To go from Celsius to Fahrenheit, multiply by 9/5 and then add 32: F = (C x 1.8) + 32. To reverse it, subtract 32 first and then multiply by 5/9: C = (F - 32) / 1.8. Kelvin is far simpler — add 273.15 to the Celsius value.

These anchor points are worth knowing: -40 C = -40 F, the single temperature where the two scales meet; 0 C = 32 F, water freezing; 20 C = 68 F, room temperature; 37 C = 98.6 F, body temperature; 100 C = 212 F, water boiling at sea level; and 180 C = 356 F, a standard baking oven. Absolute zero sits at -273.15 C, 0 K, or -459.67 F.

Where the four scales came from#

Daniel Fahrenheit built his 1724 scale around a brine freezing point at zero and human body temperature near 96. Anders Celsius proposed his scale in 1742 with 0 as boiling and 100 as freezing, and it was inverted into today's form shortly after his death. Kelvin, introduced by William Thomson in 1848, begins at absolute zero, so it never goes negative and underpins all of physics.

Rankine is the absolute scale built on Fahrenheit-sized degrees, and it still appears in US engineering thermodynamics. Zero Rankine is absolute zero, which is why 0 F equals 459.67 R. Fahrenheit itself survives for everyday weather only in the United States, its territories and a small handful of other countries; almost everywhere else reports the weather in Celsius.

Common mistakes and quick estimates#

The biggest error is treating a temperature difference as if it were a temperature. A rise of 10 C is a rise of 18 F, not 50 F — the +32 offset applies only to points on the scale, never to intervals. When converting a change in temperature, a temperature range or a rate of change per hour, use the 1.8 factor on its own and leave the offset out entirely.

For a fast mental estimate, double the Celsius figure and add 30: 25 C gives 80, against a true 77 F. The shortcut drifts at extremes, so keep it for weather. With fevers, hold on to the decimal — 38 C is 100.4 F, and rounding that to 100 F hides the exact threshold most clinical guidance uses to define a fever.

Worked examples#

Reference tables#

Common mistakes#

• Applying the +32 offset to a temperature differenceThis is the single biggest error in temperature conversion. A 5 °C rise is a 9 °F rise, not 41 °F. Anything expressed as a change, a range, a tolerance or a rate per hour uses only the 1.8 factor. Get it wrong on a process spec and every control limit is 32 degrees out.
• Dividing before subtracting when going °F to °CFor 98.6 °F the correct order gives (98.6 − 32) ÷ 1.8 = 37.0 °C. Dividing first gives (98.6 ÷ 1.8) − 32 = 22.8 °C — you would read a normal body temperature as a cool room.
• Writing a degree symbol on KelvinThe SI unit is 'kelvin', symbol K, with no degree sign and no capital when spelled out. Writing °K marks a result as non-standard, and in published or regulated work it is treated as an error.
• Trusting the double-and-add-30 shortcut outside weatherIt gives 25 °C ≈ 80 °F against a true 77 °F, which is fine for deciding what to wear. At oven temperatures it collapses: 180 °C becomes 390 °F against a true 356 °F, hot enough to burn the top of a cake in the time the recipe allows.

Frequently asked questions#

Key terms#

Absolute zero

The lowest possible temperature, where a system holds its minimum thermal energy: 0 K, −273.15 °C, −459.67 °F, 0 °R. Nothing colder can exist, which is why Kelvin and Rankine never go negative.

Kelvin

The SI base unit of thermodynamic temperature, defined since 2019 by fixing the Boltzmann constant. A kelvin and a Celsius degree are the same size, so the two scales differ only by the 273.15 offset.

Rankine

The absolute scale built on Fahrenheit-sized degrees, still used in some US engineering thermodynamics. 0 °R is absolute zero, so 0 °F = 459.67 °R.

Interval versus point

A point is a position on a scale (it is 20 °C outside). An interval is a difference between two points (it warmed by 20 °C). Points need the offset; intervals only need the ratio.

Standard pressure

The 100 °C boiling point of water assumes 101.325 kPa at sea level. Water boils near 93 °C at 2,000 m of altitude, which is why high-altitude baking and cooking times differ.

Sources#