Why do both scales show the same value at −40?

Setting x = x × 9/5 + 32 gives x = −40. It is the only point where Celsius and Fahrenheit read the same.

Is Kelvin written with a degree sign?

No. The correct notation is 300 K, without a degree sign, because the kelvin is a full SI unit and not just a degree scale.

What is normal body temperature in Fahrenheit?

About 97.9–98.6 °F, which is 36.6–37 °C. The true norm differs a little from person to person, and the place of measurement matters too.

Is double and add 30 an exact method?

No, it is only an estimate. It is fine for weather, but for precise work use the full formula °F = °C × 9/5 + 32.

Celsius, Fahrenheit and Kelvin: Formulas and Examples

Temperature is measured on three scales: Celsius, Fahrenheit and Kelvin. This guide gives exact formulas, explains where each scale is used and shows how to estimate a conversion quickly without a calculator.

Three scales, three purposes

The Celsius scale (°C) is used in Lithuania and most of the world: weather forecasts, cooking and medicine. The Fahrenheit scale (°F) is used in everyday life mainly in the United States and a few other countries. Kelvin (K) is the scientific and engineering unit, and it is written without a degree sign: 300 K, not 300 °K.

A Celsius degree and a kelvin are the same size; only the zero point differs. A Fahrenheit degree is smaller: one Celsius degree equals 1.8 Fahrenheit degrees.

Exact formulas

Celsius to Fahrenheit: °F = °C × 9/5 + 32. Fahrenheit to Celsius: °C = (°F − 32) × 5/9. Celsius to Kelvin: K = °C + 273.15. Kelvin to Celsius: °C = K − 273.15.

Example: 25 °C is 25 × 1.8 + 32 = 77 °F and 298.15 K. Another: 100 °F is (100 − 32) × 5/9 = 37.8 °C. Mind the brackets: subtract 32 first and only then multiply.

Mental math

To roughly convert Celsius to Fahrenheit, double the number and add 30. For example, 20 °C gives 40 + 30 = 70 °F, while the true value is 68 °F. This is only an approximation: the error grows the further you get from about 10 °C, but it is fine for judging the weather.

In reverse, subtract 30 from Fahrenheit and halve the result: 70 °F gives 20 °C, while the true value is 21.1 °C. When you need a reliable result, for example in cooking or when measuring body temperature, use the real formula or a converter.

Useful reference temperatures

Situation	°C	°F	K
Absolute zero	−273.15	−459.67	0
Point where both scales agree	−40	−40	233.15
Water freezes	0	32	273.15
Room temperature (roughly)	20	68	293.15
Normal body temperature (about)	36.6 – 37	97.9 – 98.6	309.75 – 310.15
Water boils (at sea level)	100	212	373.15

Oven and kitchen temperatures

American recipes give oven temperatures in Fahrenheit. Common equivalents: 160 °C ≈ 320 °F, 180 °C ≈ 356 °F (recipes often round this to 350 °F), 200 °C ≈ 392 °F and 220 °C ≈ 428 °F.

Keep in mind that ovens vary: the stated temperature can be slightly off, and with a fan (convection) recipes often advise lowering the temperature by roughly 20 °C. In such cases, watch for the signs that the dish is done rather than trusting the number alone.

A short history

Daniel Gabriel Fahrenheit proposed his scale around 1724, with water freezing at 32 and boiling at 212 degrees. Anders Celsius proposed a hundred-degree scale in 1742, but at first it ran the other way: boiling was 0 and freezing was 100. The scale was soon flipped into the form we know today.

The Kelvin scale is named after the British physicist William Thomson, later Lord Kelvin. Its zero is absolute zero, the lowest theoretically possible temperature, about −273.15 °C. In the current SI, the kelvin is defined through a physical constant rather than the properties of water.

Worked examples

To make the formulas tangible, a few calculations. −10 °C: −10 × 1.8 = −18, adding 32 gives 14 °F. 30 °C: 30 × 1.8 + 32 = 86 °F. 50 °F: (50 − 32) × 5/9 = 10 °C. 350 °F: (350 − 32) × 5/9 ≈ 176.7 °C. 450 °F: (450 − 32) × 5/9 ≈ 232.2 °C. A fridge at about 4 °C is about 39 °F, and a freezer at −18 °C is almost exactly 0 °F (precisely −0.4 °F).

Knowing that every 10 °C corresponds to an 18 °F change, you can build your own landmark table: 0 °C = 32 °F, 10 °C = 50 °F, 20 °C = 68 °F, 30 °C = 86 °F, 40 °C = 104 °F.

How the quick trick goes wrong

The rule of doubling and adding 30 is handy, but its error grows as you move away from moderate temperatures. Compare:

°C	Quick method (x2 + 30)	True value, °F	Difference, °F
0	30	32	2
10	50	50	0
20	70	68	2
30	90	86	4
40	110	104	6
−10	10	14	4

Common mistakes

The first mistake is confusing a temperature with a temperature difference. If the air warmed by 10 °C, that is an 18 °F change, not 50 °F, because the 32 is added only when converting an actual temperature, not a difference. The second is forgetting the brackets or doing the steps in the wrong order, for example multiplying by 5/9 before subtracting 32.

Third: the Kelvin notation. Write K without a degree sign and do not confuse it with a Celsius degree. Fourth: oven scales. In some countries recipes use gas marks, which are neither Fahrenheit nor Celsius degrees, so they must be converted using the manufacturer's table.

Checklist

Do you know which scale the value is in (°C, °F or K)?
Are you converting a temperature or a temperature difference?
Did you apply the formula in the right direction?
Did you sanity-check the result against landmarks: 0 °C = 32 °F, 20 °C = 68 °F, 100 °C = 212 °F?