Temperature scales: Celsius, Fahrenheit, Kelvin and conversions

Apr 27, 2026 5 min read

“100°F” — what does that feel like? Most of the world uses Celsius, but Fahrenheit dominates in the US. This article walks through the scales and their conversions.

The main scales

Scale	Symbol	Water freeze	Water boil	Absolute zero
Celsius	°C	0°C	100°C	-273.15°C
Fahrenheit	°F	32°F	212°F	-459.67°F
Kelvin	K	273.15 K	373.15 K	0 K
Rankine	°R	491.67°R	671.67°R	0°R

Celsius — water-anchored

Defined by Anders Celsius (Sweden, 1742):

0°C — water freezing point.
100°C — water boiling point.
Linearly divided into 100.

Intuitive and used by almost every country except the US and a few others.

Fahrenheit — body-temperature anchored

Defined by Daniel Fahrenheit (Germany, 1724):

0°F — coldest mixture he could make (ice + salt + water).
96°F — human body temperature (later refined to 98.6°F).
32°F — water freezing (a derived consequence).
212°F — water boiling.

He wanted body temperature near 100, but measurement precision left awkward numbers. Used by the US, Caribbean nations, and a handful of others.

Kelvin — absolute scale

Defined by Lord Kelvin (UK, 1848):

0 K — absolute zero (no molecular motion).
Same step size as Celsius.
No “degree” — just K.

Standard in science and engineering. Physics formulas (ideal gas law, etc.) use Kelvin.

Rankine — Fahrenheit’s absolute scale

William Rankine (UK, 1859):

0°R — absolute zero.
Same step size as Fahrenheit.
Used in some US engineering, especially thermodynamics.

R = F + 459.67

Conversions

Celsius ↔ Fahrenheit

F = C × 9/5 + 32
C = (F - 32) × 5/9

Examples:

25°C → 25 × 1.8 + 32 = 77°F (warm summer day).
100°F → (100 − 32) × 5/9 = 37.8°C (close to body temp).

Celsius ↔ Kelvin

K = C + 273.15
C = K - 273.15

Examples:

0°C = 273.15 K.
25°C = 298.15 K.
−273.15°C = 0 K (absolute zero).

Fahrenheit ↔ Kelvin

K = (F - 32) × 5/9 + 273.15
F = (K - 273.15) × 9/5 + 32

Direct conversion is uncommon — usually go via Celsius.

Mental-math approximations

Quick “F to C”:

C ≒ (F - 30) / 2

70°F → 20°C (actual 21°C).
80°F → 25°C (actual 26.7°C).

Less accurate but instant.

Reverse (C to F):

F ≒ C × 2 + 30

20°C → 70°F (actual 68°F).
25°C → 80°F (actual 77°F).

Why -40 is the same in both

Solve F = C × 1.8 + 32 for F = C:

C = 1.8C + 32
-0.8C = 32
C = -40

-40°C = -40°F. Shows up in materials testing and weather.

Kelvin notation

Kelvin is just K — “degrees Kelvin” and “°K” are wrong:

Correct — 273.15 K, 0 K.
Wrong — 273.15°K, degrees Kelvin.

It’s an absolute scale (with a true zero), not a relative one like Celsius or Fahrenheit. The ”°” was officially dropped in 1967.

Sense of temperature

Celsius:

Temp	Feel
-10°C	deep winter
0°C	freezing
10°C	cold
20°C	comfortable
25°C	warm
30°C	hot
35°C	very hot
40°C	dangerous

Fahrenheit:

Temp	Feel
32°F	freezing
50°F	cool
70°F	comfortable
80°F	warm
90°F	hot
100°F	body temp

Cooking

Oven temperatures:

Use	Celsius	Fahrenheit	Gas mark
Low / dry	100°C	212°F	1/4
Warm	150°C	300°F	2
Moderate	180°C	350°F	4
Hot	220°C	425°F	7
Very hot	250°C	480°F	9

US/UK recipes use Fahrenheit; conversion is common.

Why Fahrenheit looks weird

“Water freezes at 32°F” feels arbitrary because Fahrenheit isn’t anchored to water:

0°F — coldest reproducible mixture.
~100°F — human body.

It’s anchored to human-scale weather: the 30–100°F range covers most of where people live, and that range is divided into pleasant round numbers. Water just lands wherever.

Country usage

Celsius (°C) — Japan, Europe, Russia, Latin America, most of Africa, most of Asia.
Fahrenheit (°F) — US, Liberia, Cayman Islands, Bahamas.
Both — Canada, UK (official Celsius, colloquial Fahrenheit too).

About 95% of the world uses Celsius.

Summary

Celsius — water freeze/boil at 0/100, intuitive.
Fahrenheit — historical, anchored to human comfort range.
Kelvin — scientific standard, absolute zero at 0.
Mental approximations get you close.
-40°C = -40°F (the unique crossover point).

For arbitrary temperature conversion, the temperature converter on this site handles all four scales.