Gas Laws Calculator (Boyle, Charles, Combined)
Boyle's, Charles's, Gay-Lussac's and the combined gas law, solved for the missing value.
What is Gas Laws?
The simple gas laws are the ideal gas law with one variable held constant. Boyle's law varies pressure and volume at fixed temperature, Charles's law varies volume and temperature at fixed pressure, Gay-Lussac's varies pressure and temperature at fixed volume, and the combined law varies all three at once.
How it works
Only the fields a given law actually uses are shown, so a temperature that has nowhere to go cannot be entered under Boyle's law. Temperatures must be absolute and are labelled as kelvin, since the laws are proportional statements and a Celsius reading would break the proportionality. Pressures and volumes may be in any consistent unit — they cancel on each side of the equation.
- Enter what you know. Every field recalculates as you type — there is no submit button. Leave the quantity you want blank and the page solves for it.
- Read the answer and the working. The result appears above the fold; underneath it is the formula with your numbers substituted, so the arithmetic can be checked rather than trusted.
- Check the units and the size. Units are shown on every value and selectable per field, so a result in kJ, kPa or km/h is converted rather than assumed. Sanity-check the magnitude against a known landmark such as 1 g/cm³ for water.
Examples
A textbook problem with a known answer
Every tool here is checked against standard example problems — the molar mass of C6H12O6 is 180.16 g/mol, and free fall from 20 m takes about 2.02 s at g = 9.8 m/s².
A result you want to reason backwards from
Leave the quantity you already know blank instead: given the final answer, most tools recover the starting value — the mass needed for a target concentration, or the height a fall must have had.
Common mistakes
Forgetting that a formula is only as good as its subscripts
H2O and H2O2 are different compounds. The formula field shows its parse — element by element — so a mis-typed subscript is visible before it reaches the arithmetic.
Mixing units in the middle of a calculation
The most common source of a wrong answer is a value in grams next to a formula expecting kilograms, or Celsius next to a gas constant expecting kelvin. This category converts on the way in and labels what it used.
Frequently asked questions
Which law should I use?
Whichever quantity stays constant. If a gas is compressed at a steady temperature, that is Boyle's law; if it is heated in a sealed rigid container, that is Gay-Lussac's; if both pressure and volume change, use the combined law.
Why do temperatures have to be in kelvin?
Because these laws say volume or pressure is proportional to temperature, and that is only true on an absolute scale. Doubling 20 °C to 40 °C does not double the volume — doubling 293 K to 586 K does.
Can I mix units between the two states?
Within a law, the two pressures must use the same unit and the two volumes must use the same unit, because they form a ratio. Any consistent choice works, since the units cancel.