Temperature Converter — Celsius to Fahrenheit to Kelvin Online

Temperature is one of the most fundamental physical quantities, yet it is measured on four different scales that never quite agree. A weather forecast in the US says 95°F; you need to know that is 35°C — a hot summer day. A chemistry textbook specifies a reaction temperature of 500K; you need to convert that to Celsius for your lab setup. A British recipe says "Gas Mark 6"; you need the equivalent in degrees Celsius for your oven.

This guide covers all four major temperature scales — Celsius, Fahrenheit, Kelvin, and Rankine — with formulas, worked examples, and practical reference tables for cooking, weather, and science. Use our free temperature converter for instant, accurate conversions between any two scales.

The 4 Temperature Scales

Temperature scales differ in two things: the zero point and the step size (degree magnitude). Understanding these differences explains why converting temperature requires addition/subtraction as well as multiplication — unlike most unit conversions which only need multiplication.

Scale	Symbol	Zero Point	Water Freezes	Water Boils	Main Use
Celsius	°C	Water freezes	0°C	100°C	Most of the world (weather, cooking)
Fahrenheit	°F	Brine freezes	32°F	212°F	USA, some Caribbean nations
Kelvin	K	Absolute zero	273.15 K	373.15 K	Science and engineering
Rankine	°R	Absolute zero	491.67°R	671.67°R	US engineering thermodynamics

The Celsius and Fahrenheit scales are relative — their zeros are defined by physical reference points. The Kelvin and Rankine scales are absolute — their zero (0 K / 0°R) is absolute zero, the theoretically coldest possible temperature at which molecular motion essentially stops (−273.15°C / −459.67°F).

Celsius Scale Explained

The Celsius scale (formerly called Centigrade) was proposed by Swedish astronomer Anders Celsius in 1742, though his original scale was inverted (100 at freezing, 0 at boiling). After Celsius's death, the scale was reversed to its current form. The Celsius scale is defined by two reference points at standard atmospheric pressure: 0°C at the freezing point of water and 100°C at the boiling point.

Celsius is the primary temperature scale used in most countries for weather forecasting, medicine, cooking, and everyday life. In the human body temperature context, normal body temperature is approximately 37°C; 38°C and above is a fever. Celsius is part of the International System of Units (SI), though the SI base unit for thermodynamic temperature is the Kelvin.

The Celsius degree is the same size as the Kelvin — 1°C change = 1 K change. The only difference is the zero point: 0°C = 273.15 K. This makes Celsius to Kelvin conversion the simplest of all: just add 273.15.

Fahrenheit Scale Explained

The Fahrenheit scale was proposed by German-Polish physicist Daniel Gabriel Fahrenheit in 1724. He set the zero point at the lowest temperature achievable with an ice-salt mixture (used in refrigeration at the time) and 96°F at his estimate of human body temperature. In this scale, water freezes at 32°F and boils at 212°F.

The Fahrenheit scale is still widely used in the United States for weather, cooking, and informal temperature references. Most other English-speaking countries (UK, Australia, Canada) officially use Celsius but retain some cultural familiarity with Fahrenheit — many older Britons still think in Fahrenheit for summer temperatures.

From a practical standpoint, Fahrenheit provides finer resolution for everyday ambient temperatures: the range from freezing to boiling is 180°F, while in Celsius it is only 100°C. This means each Fahrenheit degree represents a smaller temperature change — useful for precise weather descriptions but less elegant mathematically.

Kelvin Scale Explained

The Kelvin scale is the SI base unit for temperature, named after Scottish physicist William Thomson, 1st Baron Kelvin (Lord Kelvin), who proposed an absolute temperature scale in 1848. Unlike Celsius and Fahrenheit, Kelvin uses no degree symbol — temperatures are expressed as "300 kelvin" or "300 K", not "300°K".

Absolute zero (0 K = −273.15°C) is the temperature at which a system has minimum thermodynamic energy. It is a theoretical lower limit — no physical system can actually reach absolute zero, though laboratory experiments have come extraordinarily close (within a billionth of a degree). At absolute zero, classical kinetic theory predicts that molecular motion would cease, and gas pressure would drop to zero.

Kelvin is used exclusively in scientific contexts: thermodynamics, astrophysics, chemistry, and physics. The temperature of the Sun's surface is about 5,778 K (5,505°C). The temperature of interstellar space is about 2.7 K. Cryogenic temperatures (liquid nitrogen = 77 K, liquid helium = 4 K) are always expressed in Kelvin. In colour temperature for lighting, 6500K means "daylight colour" and 3000K means "warm white."

Rankine Scale Explained

The Rankine scale is an absolute temperature scale based on the Fahrenheit degree. It was proposed by Scottish engineer Macquorn Rankine in 1859. Rankine zero (0°R) is the same absolute zero as Kelvin, but the degree size matches Fahrenheit, not Celsius.

0°R = 0 K = −459.67°F = −273.15°C. Water freezes at 491.67°R and boils at 671.67°R. The Rankine scale is used primarily in American engineering, particularly in thermodynamics and heat transfer calculations where the traditional US customary units are used alongside absolute temperature.

For most everyday and scientific purposes, Rankine has been largely superseded by Kelvin. However, it remains in use in some US aerospace and chemical engineering contexts. Converting between Rankine and Kelvin simply requires multiplying or dividing by 1.8.

All Conversion Formulas

Temperature conversion formulas are unique because they involve both multiplication and addition (for the zero offset between scales). Here are all the formulas you need:

Convert From	Convert To	Formula
Celsius (°C)	Fahrenheit (°F)	°F = (°C × 9/5) + 32
Fahrenheit (°F)	Celsius (°C)	°C = (°F − 32) × 5/9
Celsius (°C)	Kelvin (K)	K = °C + 273.15
Kelvin (K)	Celsius (°C)	°C = K − 273.15
Fahrenheit (°F)	Kelvin (K)	K = (°F + 459.67) × 5/9
Kelvin (K)	Fahrenheit (°F)	°F = K × 9/5 − 459.67
Celsius (°C)	Rankine (°R)	°R = (°C + 273.15) × 9/5
Rankine (°R)	Celsius (°C)	°C = (°R × 5/9) − 273.15
Kelvin (K)	Rankine (°R)	°R = K × 1.8
Rankine (°R)	Kelvin (K)	K = °R / 1.8

Worked examples:

• Body temperature 37°C → °F = (37 × 9/5) + 32 = 66.6 + 32 = 98.6°F
• Oven temperature 350°F → °C = (350 − 32) × 5/9 = 318 × 0.556 = 176.7°C
• Absolute zero 0 K → °C = 0 − 273.15 = −273.15°C
• Room temperature 25°C → K = 25 + 273.15 = 298.15 K

Weather Temperature Reference

Understanding weather temperatures in both Celsius and Fahrenheit is practically useful — especially when reading international weather forecasts or weather history records.

Celsius	Fahrenheit	Weather Description
−40°C	−40°F	Extreme arctic cold (unique: same in both scales!)
−20°C	−4°F	Very severe winter, frostbite risk
−10°C	14°F	Cold winter day (Europe, northern India winters)
0°C	32°F	Freezing point, ice and snow likely
10°C	50°F	Cool day, jacket needed
20°C	68°F	Pleasant, mild weather
25°C	77°F	Warm and comfortable
30°C	86°F	Hot summer day
37°C	98.6°F	Normal human body temperature
40°C	104°F	Extreme heat, dangerous without shade/hydration
45°C	113°F	Severe heatwave (summer in Rajasthan, Gulf states)

A useful memory point: −40°C = −40°F (the two scales intersect at exactly this value). At 28°C ≈ 82°F, the Celsius value in degrees roughly equals 82% of the Fahrenheit value — useful as a rough cross-check.

Cooking Temperature Guide

Cooking temperatures are commonly specified in Celsius (most of the world), Fahrenheit (US recipes), and Gas Mark (British oven temperatures). Here is a complete cooking temperature reference:

Description	Celsius	Fahrenheit	Gas Mark
Very low / Keep warm	120°C	250°F	Gas Mark ½
Low / Slow cook	150°C	300°F	Gas Mark 2
Moderate	180°C	350°F	Gas Mark 4
Moderately hot	190°C	375°F	Gas Mark 5
Hot	200°C	400°F	Gas Mark 6
Very hot	220°C	430°F	Gas Mark 7
Extremely hot	240°C	465°F	Gas Mark 9
Pizza / bread baking	250°C+	480°F+	Gas Mark 10

Food safety temperatures are also critical: poultry must reach 75°C (165°F) internally to kill pathogens. Ground beef must reach 71°C (160°F). Fish should reach 63°C (145°F). For food storage, 0–4°C (32–40°F) is the safe refrigerator range; below −18°C (0°F) is the safe freezer range.

Scientific Uses of Temperature Scales

Science relies exclusively on Kelvin because thermodynamic equations require an absolute temperature scale. The ideal gas law (PV = nRT), for instance, requires temperature in Kelvin — using Celsius would give nonsensical results near 0°C and negative values below it.

Some scientifically important temperature reference points:

• Absolute zero: 0 K = −273.15°C. The coldest possible temperature.
• Superconductivity: Conventional superconductors require temperatures near absolute zero (4–10 K). High-temperature superconductors work up to ~133 K (−140°C).
• Liquid nitrogen: Boiling point 77 K (−196°C). Used for cryogenic preservation.
• Liquid helium: Boiling point 4.2 K (−269°C). Used in MRI machines and quantum computing.
• Sun's surface: ~5,778 K (5,505°C). The core is approximately 15 million K.
• Lightning bolt: ~30,000 K — about five times hotter than the Sun's surface.
• CMB (Cosmic Microwave Background): 2.725 K — the temperature of the residual radiation from the Big Bang.

Colour temperature in photography and lighting is expressed in Kelvin. Candlelight is around 1,800K (very warm/orange); daylight is 5,500–6,500K (neutral white); overcast sky is 7,000–8,000K (slightly blue).

Frequently Asked Questions