Celsius to Fahrenheit Calculator

About the Celsius to Fahrenheit Calculator

Celsius is the temperature scale used for weather, cooking and medicine in almost every country, while Fahrenheit remains the everyday scale in the United States and a handful of other places. Travellers checking a forecast, cooks following a European recipe in an American oven, parents reading a thermometer with the other scale, and students learning the formula all need to turn degrees Celsius into degrees Fahrenheit.

This Celsius to Fahrenheit calculator converts °C to °F exactly and shows the formula step by step. It also gives the temperature in kelvin, compares the exact answer with the popular double-and-add-30 estimate, and describes what the temperature means in everyday terms.

How to Use the Celsius to Fahrenheit Calculator

Enter the temperature in degrees Celsius. Negative numbers are fine for cold weather and freezers, down to absolute zero at −273.15 °C.

The Fahrenheit value appears straight away, with each step of the working.

The Formula

  °F = °C × 9/5 + 32
     = °C × 1.8 + 32

There are two parts. Multiplying by 9/5 adjusts for the size of the degrees: a Celsius degree is 1.8 times as large as a Fahrenheit degree, because the scales divide the gap between water freezing and boiling into 100 and 180 parts. Adding 32 shifts the zero point, because water freezes at 0 °C but at 32 °F.

Step-by-Step Example

25 °C.

  × 9/5:   25 × 1.8 = 45
  + 32:    45 + 32 = 77 °F

So 25 °C is 77 °F, a warm summer day.

180 °C, a common oven setting.

  180 × 1.8 = 324
  324 + 32 = 356 °F

−10 °C, a cold winter day.

  −10 × 1.8 = −18
  −18 + 32 = 14 °F

Weather Temperatures

  −20 °C  =  −4 °F        15 °C  =  59 °F
  −10 °C  =  14 °F        20 °C  =  68 °F
    0 °C  =  32 °F        25 °C  =  77 °F
    5 °C  =  41 °F        30 °C  =  86 °F
   10 °C  =  50 °F        35 °C  =  95 °F
                          40 °C  = 104 °F

A handy rhyme for remembering a few: "30 is hot, 20 is nice, 10 is cold, 0 is ice."

The Double-and-Add-30 Shortcut

For a quick answer in your head, double the Celsius and add 30. At 10 °C it is exact: 20 + 30 = 50 °F. Between about 0 and 25 °C it stays within a few degrees: 25 °C gives 80 °F against the true 77. Outside that range it drifts badly — at 180 °C it gives 390 °F instead of 356 — so never use it for oven temperatures or medical readings.

Oven Temperatures

  140 °C  =  284 °F   (often given as 275 °F)
  160 °C  =  320 °F   (325 °F)
  180 °C  =  356 °F   (350 °F)
  200 °C  =  392 °F   (400 °F)
  220 °C  =  428 °F   (425 °F)
  240 °C  =  464 °F   (475 °F)

Recipes round to the nearest 25 °F because oven dials are marked that way. For a fan oven, recipes usually lower the Celsius figure by about 20 degrees.

Body Temperature

A normal body temperature of 37 °C is 98.6 °F. A fever usually starts at 38 °C, 100.4 °F, and 39 °C is 102.2 °F. Because each Celsius degree is 1.8 Fahrenheit degrees, a rise of half a degree Celsius is nearly a whole degree Fahrenheit, so be careful when comparing readings on different thermometers.

Why the Formula Works

Two fixed points define both scales. Water freezes at 0 °C and 32 °F, and boils at sea level at 100 °C and 212 °F. Between those points lie 100 Celsius degrees but 180 Fahrenheit degrees, so each Celsius degree equals 180 ÷ 100 = 1.8 Fahrenheit degrees. To convert, scale the Celsius reading by 1.8 to count the Fahrenheit degrees above freezing, then add the 32 that Fahrenheit already has at the freezing point. The same reasoning, run backwards, gives the formula for Fahrenheit to Celsius.

Where the Scales Cross

Because the Fahrenheit scale starts lower and climbs faster, the two scales meet at one point: −40. At −40 °C the formula gives −40 × 1.8 + 32 = −40 °F. Above that point the Fahrenheit figure is always the larger number, and below it the Celsius figure is. At everyday temperatures the difference is large: 30 °C is 86 °F, and a scorching 45 °C is 113 °F, figures that sound far hotter to anyone used to Fahrenheit.

Freezers, Fridges and Food Safety

Food-safety guidance is often published in both scales. A fridge should be at or below 5 °C, 41 °F, and a freezer at −18 °C, which is almost exactly 0 °F. Cooked chicken should reach 74 °C, 165 °F, in the thickest part, and hot food kept for serving should stay above 63 °C, about 145 °F. A food thermometer that shows both scales removes the need to convert at all.

Understanding Your Result

The headline is the temperature in degrees Fahrenheit.

The kelvin line gives the temperature on the scientific scale, °C + 273.15.

The quick estimate line shows the double-and-add-30 answer and how far it is from the exact one.

The what it means line describes the temperature in everyday terms.

When Should You Use This Calculator?

Use it to read weather forecasts when travelling to the US.

Use it to set an American oven for a metric recipe.

Use it for body and fever temperatures on thermometers set to the other scale.

Use it in science and maths classes to practise the formula.

Common Mistakes

Adding 32 before multiplying. Multiply by 1.8 first, then add 32.

Using the shortcut for ovens. Double-and-add-30 is far out at cooking temperatures.

Adding 32 to a temperature change. A rise of 10 °C is a rise of 18 °F.

Multiplying by 5/9 instead of 9/5. Celsius to Fahrenheit uses 9/5, which makes the number larger.

Rounding the multiplication. Use 1.8 exactly; 2 is only for rough estimates of the weather.