Home / Learn / Physics / Temperature scalesTool #L208 (formerly #1346)

Temperature scales

Drag one thermometer and read Celsius, Fahrenheit and kelvin side by side, with the conversion worked out. See a gas thermometer's straight lines all reach zero pressure at −273.15 °C, and at Class 11 add Rankine and make a scale of your own.

Keep it one tap away for everyday use.

Bookmark Temperature scales?

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  1. Drag
  2. Read scales
  3. Zero line
  4. Check
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One temperature, three scales

Drag the thermometer up or down, use the slider, or type a temperature in any scale.

Why −273.15 °C?

Trap some gas in a closed flask so its volume cannot change, and read its pressure at several temperatures. The points lie on a straight line. Carry the line on to colder temperatures (dashed) and it reaches zero pressure at −273.15 °C, whatever gas and however much of it. The gas would turn liquid first, but every line points to the same place: the coldest anything can be.

Try this

  • Press −40°: Celsius and Fahrenheit read the same number. Can you show why with the formula?
  • Press Absolute zero: kelvin reads 0. Try typing −300 °C.
  • Warm by 10 °C and watch the kelvin reading: it also goes up by exactly 10. A Fahrenheit degree is smaller (9 of them for every 5 Celsius degrees).
  • On the gas graph, change how much gas is in the flask: the line gets steeper or flatter but still meets zero at −273.15 °C.
  • At Class 11, make a scale with ice point 20 and steam point 220, then find the temperature where it agrees with Celsius.

Check yourself

What it does

Three thermometers stand side by side, marked in Celsius, Fahrenheit and kelvin. They share one liquid level, because they are measuring the same temperature; only the numbers painted on them differ. Drag the level up and down (or type a temperature in any scale) and every reading changes together, with the conversion formulas filled in with your numbers.

A second picture answers the question every student asks: why does the kelvin scale start at −273.15 °C? It shows a constant-volume gas thermometer. Pressure against temperature is a straight line, and if you carry the line on to colder temperatures it reaches zero pressure at −273.15 °C, whatever the amount of gas. That point is absolute zero, the zero of the kelvin scale.

It serves Class 7 and 8 Science (heat and measuring temperature), Class 9 (matter and the kelvin scale) and Class 11 Physics, Thermal Properties of Matter (temperature scales, the ideal-gas equation and absolute temperature). See heating curve for what happens to water as it warms, kinetic theory for what temperature means for the molecules, and gas law calculator to put kelvin to work in the gas laws.

How to use it

  1. Level: Class 8–10 shows Celsius, Fahrenheit and kelvin. Class 11 adds the Rankine scale and a card where you make your own scale.
  2. Drag on the thermometer, move the Temperature slider, or type a value and choose its scale. The other readings follow.
  3. The landmark buttons jump to absolute zero, −40°, ice melting, body temperature and water boiling.
  4. On the gas graph, the Gas in the flask slider changes how much gas is trapped (its pressure at 0 °C). The three lines are half, the same and one and a half times that amount.
  5. At Class 11, type the ice point and steam point of your own scale. The page gives the reading on your scale and the one temperature where it agrees with Celsius.

Worked example

The page opens at 25 °C. Then F = 1.8 × 25 + 32 = 77 °F and K = 25 + 273.15 = 298.15 K. With 100 kPa of gas in the flask at 0 °C, the pressure at 25 °C is 100 × 298.15 / 273.15 = 109.2 kPa.

Press −40°: F = 1.8 × (−40) + 32 = −40 °F, so the two scales agree there. K = 233.15 K.

Class 11: a scale reading 20 at melting ice and 220 at boiling water reads 20 + 200 × 25 / 100 = 70 at 25 °C. It agrees with Celsius where C = 20 + 2C, so at C = −20 °C.

Check yourself

Five fresh questions each time: Celsius to Fahrenheit, Fahrenheit to Celsius, kelvin to Celsius, a reading on a made-up scale, and where two scales read the same. Answers are checked on the page and the working is shown. Nothing is saved.

Tips

  • A change of 1 °C is a change of 1 K, but a change of 1.8 °F. Temperature differences convert without the 32 or the 273.15.
  • Write kelvin as K, never °K, and never negative.
  • Gas laws (pV = nRT and the rest) only work with temperatures in kelvin.

FAQ

Has anything reached absolute zero? No. Laboratories have cooled atoms to within a billionth of a kelvin, but absolute zero itself can be approached, never reached.

Why does the gas line stop being solid below −50 °C? Real gases turn liquid before they get that cold, so the dashed part is the line carried on (extrapolated), not a measurement.

Why 9/5? Between melting ice and boiling water there are 100 Celsius degrees but 180 Fahrenheit degrees (32 to 212), and 180/100 = 9/5.

Where the numbers come from

  • Celsius, Fahrenheit, kelvin and Rankine relations: F = 9/5 C + 32, K = C + 273.15, °R = 9/5 K; 0 K = −273.15 °C by the definition of the kelvin (SI Brochure, BIPM).
  • Constant-volume gas thermometer: p proportional to absolute temperature for an ideal gas (Gay-Lussac's law): standard physics.
  • Landmarks are rounded general values: water boils at 100 °C at standard sea-level pressure, body temperature about 37 °C, lead melts at about 327.5 °C.
  • Checked against an independent Python calculation of the same formulas.

Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.

Last updated 2026-11-09.

Change log for Temperature scales

  1. Add #1346 Temperature scales and absolute zero Details on GitHub

Every change to fibub

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