Temperature is the conversion people get wrong most often, because the two scales do not start at the same zero and the popular mental shortcut quietly drifts. This guide gives the exact formula, shows how far the shortcut is off across the range people actually use, and covers the rounding mistake that turns a normal body temperature into a fever.

You can check any value in the UrsaKit temperature converter, which handles the offset correctly instead of treating the units as a simple ratio.

01How to convert Celsius to Fahrenheit

To convert Celsius to Fahrenheit, multiply by 1.8 and add 32. That is the whole method: no lookup table, no adjustment for the range, and the same answer whether you are converting a body temperature or an oven setting.

°F = °C × 9/5 + 32
°C = (°F − 32) × 5/9

20°C  ->  20 × 1.8 + 32  =  68°F
100°C ->  100 × 1.8 + 32 =  212°F   (water boils)
0°C   ->  0 × 1.8 + 32   =  32°F    (water freezes)

Use 1.8 instead of 9/5 when you are doing it in your head, and 9/5 in code where the fraction documents the scale ratio. Both are the same number.

02Why it is not a simple ratio

Length conversions are a single multiplication because both scales start at zero. Temperature scales do not:

• Celsius puts 0 at the freezing point of water and 100 at its boiling point, at standard pressure (NIST).
• Fahrenheit puts 32 at freezing and 212 at boiling, so one Celsius degree is 1.8 Fahrenheit degrees.
• The +32 is the offset that aligns the two zero points.

That offset is the reason a unit converter cannot treat temperature like metres and feet. A converter that multiplies without adding the offset gives 36°F for 20°C, which is not cold weather — it is a bug.

Temperature needs two operations, not one.

Convert the scale (×1.8) and then move the zero point (+32). Missing the offset is the single most common conversion error, and it produces plausible-looking numbers, which is worse than an obvious one.

03The 2x + 30 shortcut and its drift

The shortcut doubles the Celsius value and adds 30. It is exact at 10°C and wrong everywhere else:

For weather, being off by two degrees does not change what you wear. For a fever, a dose of medicine or an oven, it does.

Never use the shortcut for anything you will act on.

At 37°C the shortcut says 104°F, a number that would read as a high fever. At 220°C it says 470°F, which would burn a bake. The trick is a conversation aid, not a conversion.

04Celsius to Fahrenheit chart

05Kelvin and absolute zero

Kelvin uses the same degree size as Celsius but starts at absolute zero, so the conversion is a pure offset (BIPM):

K  = °C + 273.15
°C = K − 273.15

0°C   -> 273.15 K
100°C -> 373.15 K
0 K    -> -273.15°C -> -459.67°F   (absolute zero)

Kelvin is the scale to use in physics and thermodynamics, because ratios of temperature only make sense from absolute zero. Doubling 20°C is not twice as hot; doubling 293.15 K is.

06Fahrenheit back to Celsius

98.6°F -> (98.6 − 32) × 5/9 = 66.6 × 5/9 = 37°C
350°F  -> (350 − 32) × 5/9  = 318 × 5/9  = 176.67°C

Subtract the offset before multiplying, not after. Doing the operations in the wrong order is the second most common mistake, and it produces a result that is wrong by 32 degrees per step rather than by the offset alone.

07Oven and cooking temperatures

Recipe books are not conversion tables. They were written for the dials that existed, so their Fahrenheit values are rounded:

If a European recipe says 180°C, an American reader sets 350°F and both are close enough because domestic ovens hold temperature in a wide band. When the exact value matters — sugar work, chocolate tempering, proofing — use the calculated number, not the rounded label.

08Where rounding goes wrong

The rule is simple: convert from the original value, keep full precision through the calculation, and round once at the end, at the precision the task actually allows.

09Converting in the browser

1. Open the temperature category — The temperature converter holds Celsius, Fahrenheit and Kelvin in one place, so no value is converted twice.
2. Type into any field — The other units update as you type, and the offset is applied, not just the ratio.
3. Check the crossing point — Enter -40 in either field: the result in the other is also -40, which is the quickest way to confirm a converter is doing the offset correctly.
4. Switch categories when you need more — The unit converter covers length, volume, weight, speed, area, power and pressure, for the same reason: temp and mass are where the mental maths fails.

Temperature is the case that proves a point about unit conversion in general — knowing which conversions have an offset is more valuable than memorising a multiplier.

10Sources

• SI Units – Temperature — NIST (accessed 2026-10-08)
• The International System of Units (SI Brochure) — BIPM (accessed 2026-10-08)