What value of R does this use?

The universal gas constant R = 8.314462618 J/(mol·K), the exact CODATA value in SI units. Because every input is converted to SI base units (pascals, cubic meters, kelvin, moles) before solving, this single R is always the right one — you never need the 0.08206 L·atm/(mol·K) form or 62.36 L·mmHg/(mol·K). After solving, the answer is converted back into whatever unit you pick, so you still see pressures in atm or volumes in liters if that is what your problem uses.

Why must temperature be in kelvin?

The ideal gas law is a relationship between absolute quantities — pressure, volume, and amount are all measured from a true zero, and temperature has to match. Kelvin starts at absolute zero (0 K = −273.15 °C), where molecular motion stops, so doubling the kelvin temperature really does double the average kinetic energy. Celsius and Fahrenheit start at arbitrary points (the freezing point of water, a brine mixture), so using them directly would put a meaningless offset into PV = nRT. This tool lets you type °C or °F for convenience but converts to kelvin before computing, and it refuses any temperature at or below 0 K because no real gas can exist there.

When does the ideal gas law stop being accurate?

It assumes gas molecules are point particles with no volume of their own and no attraction or repulsion between them. That holds well at low pressure and high temperature, where molecules are far apart and moving fast. It breaks down at high pressure (the molecules' own volume becomes a meaningful fraction of the container) and at low temperature near the point where the gas would condense (intermolecular attractions matter, and the gas is about to become a liquid). For those conditions chemists switch to a real-gas equation such as van der Waals. For most homework, lab work near room conditions, and back-of-envelope estimates, the ideal gas law is accurate to a few percent.

What is STP, and does it match the default here?

STP means "standard temperature and pressure". The current IUPAC definition is 0 °C (273.15 K) and 100 kPa (1 bar), at which one mole of an ideal gas occupies 22.711 L. The older, still widely taught definition uses 1 atm (101.325 kPa), giving a molar volume of 22.414 L. The default scenario in this tool is 1 mol at 273.15 K in 22.414 L, which solves to almost exactly 1 atm — the classic textbook molar volume. Switch the pressure unit to bar and the volume to 22.711 L to work in the modern IUPAC definition instead.

Can I solve for any of the four quantities?

Yes. Use the "Solve for" buttons to choose P, V, n, or T. The field you are solving for is disabled and shows "= ?", and you fill in the other three. The tool rearranges PV = nRT for you — P = nRT/V, V = nRT/P, n = PV/RT, or T = PV/nR — and guards against impossible inputs such as dividing by a zero amount or temperature.

Find the volume of gas produced in a reaction

A stoichiometry problem tells you a reaction releases 0.25 mol of CO₂ at 25 °C and 1 atm, and asks for the volume. Solve for V, type n = 0.25 mol, T = 25 with the unit set to °C, and P = 1 atm. The tool converts 25 °C to 298.15 K, solves V = nRT/P, and returns about 6.11 L. Because the temperature unit dropdown lets you stay in Celsius, you never make the classic mistake of plugging 25 straight into a formula that expected kelvin.

Convert a tank pressure between conditions

You have a 50 L cylinder of nitrogen at 200 bar and 15 °C, and you want to know how many moles it holds. Solve for n, set V = 50 L, P = 200 with the unit on bar, and T = 15 °C. The result, around 418 mol, times the molar mass of N₂ (28.0 g/mol) gives you the mass of gas in the tank — a quick check against the cylinder's stated capacity.

Check a lab measurement of molar volume

In a gas-laws lab you measured that 0.040 mol of a gas filled 1.00 L at 101 kPa. Solve for T to back out the temperature your sample must have been at: enter n = 0.040, V = 1.00 L, P = 101 kPa, and read the kelvin result (about 304 K, or 31 °C with the unit switched). If that is far from your recorded room temperature, your mole count or volume reading needs a second look.

Size a gas storage volume for a target amount

You need to store exactly 2.0 mol of helium for a demonstration and your regulator holds it at 3 atm and 20 °C. Solve for V with n = 2.0, P = 3 atm, T = 20 °C to get roughly 16.0 L, then pick a cylinder at least that large. Switching the result unit to mL or m³ lets you match whatever the equipment spec uses.

Teach the inverse relationships visually

Demonstrating Boyle's and Charles's laws to a class, you fix two quantities and vary a third. Solve for V, keep n and T constant, and step P from 1 atm to 2 atm to 4 atm — the volume halves each time, showing the inverse P–V relationship. Then solve for V again, hold n and P, and raise T from 200 K to 400 K to show volume doubling. Each step is one share link you can drop into a slide.

Ideal Gas Law Calculator — solve PV = nRT for any unknown

Solve PV = nRT for pressure, volume, amount, or temperature — pick any unknown, mix any units, runs in your browser

Runs locally
Category Calculator
Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.

Solve for

Pressure

Volume

Amount of substance

mol

Temperature

PV = nRT · R = 8.314462618 J/(mol·K)

Result · Pressure (P)

0.999999

What this tool does

Enter any three of the four quantities in the ideal gas law — pressure P, volume V, amount of substance n, and temperature T — pick which one is unknown, and this calculator returns the fourth from PV = nRT. The hard part of these problems is rarely the algebra; it is the units. Here each field carries its own unit: pressure in Pa, kPa, atm, bar, mmHg, or psi; volume in L, mL, or m³; temperature in K, °C, or °F. Everything is converted to SI base units internally, solved with the exact universal gas constant R = 8.314462618 J/(mol·K), and converted back to a unit you choose for the answer, so you never have to look up a different R for atm·L versus kPa·m³. Temperature is always taken as absolute: a value in °C or °F is shifted to kelvin before it touches the equation, and any temperature at or below absolute zero is rejected because it is not a real gas state. The full calculation lives in the URL, so a share link reproduces exactly what you computed, and one click copies the result with its unit. No data leaves your browser tab.

Tool details

Input: Text + Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result + Copy; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 11 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Calculator · Student; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How Ideal Gas Law Calculator fits into your work

Use it for fast estimates, comparisons, and planning numbers before you make the final call.

Calculation jobs

Getting a realistic range before a purchase, plan, workout, or schedule decision.
Comparing scenarios by changing one input at a time.
Turning rough assumptions into a number you can discuss.

Calculation checks

Double-check units, dates, rates, and rounding assumptions.
Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
Save the inputs that produced an important result so you can reproduce it later.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Find the volume of gas produced in a reaction
A stoichiometry problem tells you a reaction releases 0.25 mol of CO₂ at 25 °C and 1 atm, and asks for the volume. Solve for V, type n = 0.25 mol, T = 25 with the unit set to °C, and P = 1 atm. The tool converts 25 °C to 298.15 K, solves V = nRT/P, and returns about 6.11 L. Because the temperature unit dropdown lets you stay in Celsius, you never make the classic mistake of plugging 25 straight into a formula that expected kelvin.
Convert a tank pressure between conditions
You have a 50 L cylinder of nitrogen at 200 bar and 15 °C, and you want to know how many moles it holds. Solve for n, set V = 50 L, P = 200 with the unit on bar, and T = 15 °C. The result, around 418 mol, times the molar mass of N₂ (28.0 g/mol) gives you the mass of gas in the tank — a quick check against the cylinder's stated capacity.
Check a lab measurement of molar volume
In a gas-laws lab you measured that 0.040 mol of a gas filled 1.00 L at 101 kPa. Solve for T to back out the temperature your sample must have been at: enter n = 0.040, V = 1.00 L, P = 101 kPa, and read the kelvin result (about 304 K, or 31 °C with the unit switched). If that is far from your recorded room temperature, your mole count or volume reading needs a second look.
Size a gas storage volume for a target amount
You need to store exactly 2.0 mol of helium for a demonstration and your regulator holds it at 3 atm and 20 °C. Solve for V with n = 2.0, P = 3 atm, T = 20 °C to get roughly 16.0 L, then pick a cylinder at least that large. Switching the result unit to mL or m³ lets you match whatever the equipment spec uses.
Teach the inverse relationships visually
Demonstrating Boyle's and Charles's laws to a class, you fix two quantities and vary a third. Solve for V, keep n and T constant, and step P from 1 atm to 2 atm to 4 atm — the volume halves each time, showing the inverse P–V relationship. Then solve for V again, hold n and P, and raise T from 200 K to 400 K to show volume doubling. Each step is one share link you can drop into a slide.

Common pitfalls

Plugging Celsius or Fahrenheit straight into the equation. PV = nRT needs absolute temperature; 25 °C is 298.15 K, not 25. This tool converts for you when you choose the °C or °F unit, but if you do the algebra by hand, always go to kelvin first.
Mismatching the gas constant with the units. The number 0.08206 only works with L, atm, mol, and K; 8.314 only works with Pa, m³, mol, and K. Mixing them silently gives an answer off by orders of magnitude. Converting everything to SI and using one R avoids this entirely.
Forgetting that mL and L differ by 1000, or that 1 m³ is 1000 L. A volume entered in the wrong unit throws the whole result off. Pick the unit from the dropdown rather than scaling the number yourself.

Privacy

Every conversion and the PV = nRT solve run as plain JavaScript inside your browser tab — there is no server call and nothing you enter is logged. The one thing to know: the shareable URL encodes your inputs and units in the query string (e.g. ?solve=P&v=22.414&t=273.15), so if you paste a "share link" somewhere, the destination server's access log will record those values. For chemistry homework that is harmless; if a value is sensitive, copy the result manually instead of sharing the URL.

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Ideal Gas Law Calculator — solve PV = nRT for any unknown

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How Ideal Gas Law Calculator fits into your work

Calculation jobs

Calculation checks

Good next steps

Real-world use cases

Find the volume of gas produced in a reaction

Convert a tank pressure between conditions

Check a lab measurement of molar volume

Size a gas storage volume for a target amount

Teach the inverse relationships visually

Common pitfalls

Privacy

FAQ

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