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Convert Celsius, Fahrenheit, Kelvin and Rankine — with reference points, oven temperatures including the fan-forced adjustment, and a check that stops you going below absolute zero.
| °C | °F | K | |
|---|---|---|---|
| −273.15 | −459.67 | 0 | Absolute zero |
| −40 | −40 | 233.15 | The scales cross here |
| −18 | 0 | 255.15 | Freezer |
| 0 | 32 | 273.15 | Water freezes |
| 20 | 68 | 293.15 | Room temperature |
| 37 | 98.6 | 310.15 | Body temperature |
| 100 | 212 | 373.15 | Water boils |
| 180 | 356 | 453.15 | Moderate oven |
Every other unit conversion is a single multiplication. Metres to feet, kilograms to pounds, litres to gallons — all just a factor, because zero means the same thing in both systems.
Temperature scales do not share a zero. Zero Celsius is 32 Fahrenheit, and neither is the coldest possible temperature. So converting needs both a ratio and an offset.
| Scale | Zero is | Water freezes | Water boils | Used for |
|---|---|---|---|---|
| Celsius (°C) | Water's freezing point | 0 | 100 | Everyday use almost everywhere, including Australia |
| Fahrenheit (°F) | A brine freezing point | 32 | 212 | United States, everyday use |
| Kelvin (K) | Absolute zero | 273.15 | 373.15 | Science — the SI base unit |
| Rankine (°R) | Absolute zero | 491.67 | 671.67 | Some US engineering |
Fahrenheit's numbers look arbitrary because they are. Daniel Fahrenheit set his zero at the coldest temperature he could reliably reproduce — a mixture of ice, water and salt — and calibrated the upper end against body temperature. Celsius chose water's two phase changes instead, which is why its numbers are round.
Absolute zero is the point at which a system holds the minimum possible energy. It has never been reached, though laboratories have come extraordinarily close.
| Scale | Absolute zero |
|---|---|
| Kelvin | 0 K |
| Celsius | −273.15 °C |
| Fahrenheit | −459.67 °F |
| Rankine | 0 °R |
This is why the calculator rejects anything colder. A converter that happily returns −300 °C in kelvin is producing a number that cannot describe anything real, and on a page used for homework that matters.
The conversion most likely to affect your dinner, and the one with an extra step most tables leave out.
| Description | Conventional °C | Fan-forced °C | °F | Gas mark |
|---|---|---|---|---|
| Very slow | 120 | 100 | 250 | ½ |
| Slow | 150 | 130 | 300 | 2 |
| Moderately slow | 160 | 140 | 325 | 3 |
| Moderate | 180 | 160 | 350 | 4 |
| Moderately hot | 200 | 180 | 400 | 6 |
| Hot | 220 | 200 | 425 | 7 |
| Very hot | 240 | 220 | 475 | 9 |
Gas marks are a British scale still seen in older cookbooks. They are not linear at the bottom of the range, which is why the table lists them rather than offering a formula.
Normal body temperature is usually quoted as 37 °C or 98.6 °F, though in practice it varies between people, across the day, and with where the measurement is taken.
| °C | °F | Generally described as |
|---|---|---|
| 35.0 | 95.0 | Below normal range |
| 36.5–37.5 | 97.7–99.5 | Typical range for a healthy adult |
| 38.0 | 100.4 | Commonly used threshold for fever |
| 39.0 | 102.2 | High fever |
| 40.0+ | 104.0+ | Very high |
| Situation | °C | °F |
|---|---|---|
| Freezer | −18 | 0 |
| Water freezes | 0 | 32 |
| Fridge | 4 | 39 |
| Cool Melbourne morning | 10 | 50 |
| Room temperature | 20–22 | 68–72 |
| Warm Sydney day | 28 | 82 |
| Hot Australian day | 35 | 95 |
| Body temperature | 37 | 98.6 |
| Extreme heat warning | 40+ | 104+ |
| Water boils | 100 | 212 |
| Use | What actually matters |
|---|---|
| Weather | Rough accuracy is fine. "Double it and add 30" converts an American forecast well enough to know what to wear |
| Cooking | Precision matters, and so does the oven type — the fan-forced adjustment moves the number more than any rounding error would |
| Science | Kelvin, because it starts at absolute zero and so behaves proportionally in gas laws and thermodynamics |
| Engineering | Kelvin or Rankine for absolute calculations; Celsius or Fahrenheit on specifications and datasheets |
| Health readings | One decimal place, since the clinically meaningful range is narrow — and context matters more than the number |
| Travel | Knowing that 30 °C is a warm day and 100 °F is a hot one covers most of it |
How do you convert Celsius to Fahrenheit?
Multiply by 9/5 and add 32, so 20 °C becomes 68 °F. The multiplication accounts for Fahrenheit degrees being smaller, and the +32 shifts the starting point because the two scales place zero differently. For a rough estimate, double the Celsius figure and add 30.
How do you convert Fahrenheit to Celsius?
Subtract 32 first, then multiply by 5/9. So 98.6 °F becomes (98.6 − 32) × 5/9 = 37 °C. The order matters — multiplying before subtracting gives the wrong answer.
What temperature is the same in Celsius and Fahrenheit?
Minus forty. It is the single point where the two scales cross, so −40 °C and −40 °F are the same temperature. It also makes a quick sanity check on any conversion tool.
What is absolute zero?
The lowest temperature physically possible, where a system holds minimum energy. It is 0 K, −273.15 °C or −459.67 °F. Nothing can be colder, which is why this converter rejects any value below it rather than returning a meaningless number.
Why does science use Kelvin?
Because it starts at absolute zero, which makes it proportional — doubling the kelvin value genuinely doubles the thermal energy, something that is not true of Celsius or Fahrenheit. That matters in gas laws and thermodynamics, where ratios of temperature appear in the equations.
Why is there no degree symbol on Kelvin?
Because kelvin is a unit rather than a point on a graduated scale, so it is written 300 K, not 300 °K. The convention was formalised in 1967. Celsius and Fahrenheit keep the degree symbol because they measure position on a scale with an arbitrary zero.
What is the difference between conventional and fan-forced oven temperatures?
A fan-forced oven circulates hot air, cooking faster and more evenly, so the usual adjustment is to reduce the temperature by 15 to 20 °C. A recipe calling for 180 °C conventional becomes about 160 to 165 °C fan-forced. Manufacturers vary in what they recommend and some ovens compensate automatically, so the manual is worth checking. Australian recipes assume conventional unless they state otherwise.
What are gas marks?
A British oven scale still found in older cookbooks. Gas mark 4 is about 180 °C or 350 °F, and gas mark 6 is around 200 °C. The scale is not linear at the low end, so a conversion table is more reliable than a formula.
Is Celsius the same as centigrade?
Yes, the same scale under two names. It was called centigrade — meaning a hundred steps — until 1948, when it was renamed after Anders Celsius to avoid confusion with a French unit of angle that used the same word.
What is normal body temperature?
Usually quoted as 37 °C or 98.6 °F, though a healthy adult typically ranges between about 36.5 and 37.5 °C, varying through the day and with the measurement site. Whether a reading is concerning depends on age, symptoms and circumstances, so speak to a doctor rather than relying on a threshold.
How do I convert a temperature difference?
Use the ratio only, without the offset. A rise of 10 °C is a rise of 18 °F, because you multiply by 9/5 and do not add 32. The offset applies to positions on the scale, not to the size of a change — this is one of the most common errors in physics problems.
What is Rankine?
An absolute scale using Fahrenheit-sized degrees, so 0 °R is absolute zero and water freezes at 491.67 °R. It appears in some American engineering and thermodynamics work, standing in the same relation to Fahrenheit as kelvin does to Celsius.
Temperature is the odd one out among unit conversions, and the reason is simple: the scales disagree about where zero is. Every mistake on the list above comes back to that — forgetting the offset, applying it to a difference, or assuming a negative kelvin is possible.
Two figures are worth carrying around: −40 is the same in both common scales, and a fan-forced oven wants roughly 15 to 20 °C less than the recipe says.
For other measurement types, the unit converter handles ten categories in one tool.