Charles's Law Calculator
At constant pressure and constant amount of gas, volume divided by absolute temperature stays constant. Enter any 3 of V₁, T₁, V₂, T₂ below and the calculator solves the missing one instantly — temperatures you enter in °C or °F are converted to kelvin automatically.
Fill any 3 of the 4 fields — the empty one is solved automatically.
Charles constant k = V₁/T₁ = — m³/K
How it works — worked example
- Convert temperatures to kelvin first: T₁ = 20 °C = 293.15 K, T₂ = 80 °C = 353.15 K. Never plug °C directly into the law.
- Compute the Charles constant: k = V₁/T₁ = 2.0 L / 293.15 K = 0.00682245 L/K.
- Rearrange for the unknown: V₂ = k × T₂ = 0.00682245 × 353.15 = 2.4093 L.
- Heating the gas from 20 °C to 80 °C expands it from 2.0 L to 2.4093 L — a factor of 353.15/293.15 = 1.2047, about 20.5% growth.
Assumptions: pressure and the amount of gas stay constant (a sealed-but-flexible container), and the gas behaves ideally. Real gases deviate at very high pressures or near condensation — see the FAQ.
Frequently asked questions
What is Charles’s law?
Charles’s law says that for a fixed amount of gas at constant pressure, volume is directly proportional to absolute temperature: V₁/T₁ = V₂/T₂. Heat a gas and it expands; cool it and it shrinks — by the same fraction as the temperature change in kelvin. It was discovered by Jacques Charles in the 1780s and published by Joseph Louis Gay-Lussac in 1802, and it is one of the three classical gas laws.
Why must the temperature be in kelvin?
This is the classic trap: the law only works with an absolute scale. Going from 20 °C to 80 °C is not a fourfold increase — in kelvin it is 293.15 K to 353.15 K, a factor of only 1.2047, so the volume grows by about 20.5%, not 300%. Plugging °C straight in gives nonsense. The calculator above converts °C and °F to kelvin for you before applying V₁/T₁ = V₂/T₂.
How do you calculate the final volume (V₂)?
Rearrange the law to V₂ = V₁T₂ / T₁. Example: 2.0 L of gas at 20 °C (293.15 K) is heated to 80 °C (353.15 K), so V₂ = 2.0 × 353.15 / 293.15 = 2.4093 L — the gas expands by the same 1.2047 factor as the absolute temperature. Enter any three values in the calculator above and it solves the fourth.
How do you find the final temperature (T₂)?
Use T₂ = T₁V₂ / V₁. Example: 1.0 L of gas at 0 °C (273.15 K) expands to 1.5 L, so T₂ = 273.15 × 1.5 / 1.0 = 409.725 K = 136.575 °C. The calculator shows the answer in whichever unit you pick — °C, °F or K — plus a full conversion table.
What units can I use for volume and temperature?
Volume: m³, L, mL or ft³ — you can even mix units, e.g. V₁ in ft³ with V₂ in mL, because everything is converted to SI internally. Temperature: °C, °F or K, converted to kelvin first. Exact references: 1 m³ = 1000 L; 1 ft³ = 28.3168 L = 28316.8 mL; °F to kelvin is (T − 32) × 5/9 + 273.15, so 68 °F = 20 °C = 293.15 K.
What must stay constant for Charles’s law to apply?
Two things: the pressure and the amount of gas. Think of a sealed-but-flexible container — a balloon, or a syringe with a freely moving plunger — so no gas escapes and the pressure cannot build up. If the pressure changes while you heat the gas, V₁/T₁ will not equal V₂/T₂ and the calculator’s consistency check will flag the mismatch (for varying pressure at constant temperature, use Boyle’s law instead).
Can temperature be zero or negative kelvin in the calculator?
No — all temperatures must be above absolute zero (> 0 K). Zero kelvin is unreachable in practice, and negative kelvin has no physical meaning here, so the calculator rejects both with an honest error. Volumes must be positive numbers as well.
Where is Charles’s law used in real life?
Anywhere gas warms or cools at roughly constant pressure: hot-air balloons (heating the air inside expands it, lowering its density), human breathing (warm exhaled air takes up more space), car tires reading lower pressure on cold mornings, a basketball left out in the cold going soft, and thermometers and piston engines designed around thermal expansion.
Free tool by NRTechStudio