STP Calculator (Standard Temperature and Pressure)

Convert a gas volume to standard conditions and back to moles.

Supports 21 volume units, three temperature scales and 12 pressure units.

Updated October 13, 2026

CreatorFrank Zhao

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Introduction

A gas volume on its own says very little. Five litres of nitrogen in a warm room and five litres of the same gas in an ice bath are different amounts of substance, and only one of them matches the number in a data table. This calculator restates a measured volume on the standard reference state — 0 °C and one atmosphere — and reports both the volume it would occupy there and the amount of gas in moles.

It is the normalising step that sits between a measurement and everything else: comparing two samples, quoting a yield, checking a supplier's "normal cubic metres" against your own meter, or turning a burette reading into moles. The three measured boxes take 21 volume units, three temperature scales and twelve pressure units, and the calculator runs in both directions: fill in any three of volume, temperature, pressure and amount of gas, and it solves the fourth.

The idea worth taking away first

Standard conditions are a convention, not a property of the gas. Nothing happens to the sample when you normalise it — you are holding the same amount of substance and describing it against a fixed reference point. That is why every "volume at STP" number must travel with its reference state, and why two data sets can disagree by more than a per cent without either one of them being wrong.

Any three of four

Volume, temperature, pressure, amount of gas

22.41396954 L/mol

Molar volume this calculator normalises to

12 pressure units

All treated as absolute pressure

Quick start

Set the three unit menus

Litres, Celsius and hectopascals are the opening defaults; the menu only changes how a number is written, never the number behind it. If your reading came off a gauge, note the scale you will convert from — the pressure box wants absolute pressure, not the overpressure above the room.

Enter the measured state

Volume, temperature and absolute pressure, in the Measured conditions card. The results need all three: a volume without a temperature and a pressure has no defined amount of gas behind it.

Read the two results in the standard card

Volume is what the same gas would occupy at 0 °C and 1 atm, and No. of moles is how much gas you have. Both are derived, so they follow whatever you type above them.

Or start from the side you already know

Type the amount of gas, leave one of the measured boxes empty, and that empty box is solved instead. A standard volume on its own returns the moles straight away — the conversion between them is just the molar volume.

A first calculation: 5 L at 350 K and 1 atm

A sample sits at 350 K (about 77 °C) and exactly one atmosphere, measured at 5 L. Normalising it is a ratio of the two temperatures and the two pressures:

V_{\mathrm{STP}}

=

V \times \frac{T_{0}}{T} \times \frac{P}{P_{0}}

=

5 \times \frac{273.15}{350} \times \frac{101325}{101325}

=

3.9021429\ \mathrm{L}

The amount of gas comes from the same three numbers through the gas constant:

n

=

\frac{P\,V}{R\,T}

=

\frac{101325 \times 5}{8314.46261815324 \times 350}

=

0.17409423\ \mathrm{mol}

The page shows exactly these numbers

The standard volume reads 3.9021429 and the amount reads 0.17409423. The standard volume is smaller than the 5 L you measured because the sample was warmer than 0 °C while sitting at exactly the standard pressure — gas expands when it is heated, so cooling it back to the reference temperature shrinks it. Change only the pressure and the direction flips.

The calculation

Two relations cover everything on the page, and both are written with the exact molar gas constant rather than a rounded one.

n = \frac{P\,V}{R\,T}

Volume in litres, temperature in kelvin, pressure in pascals. Because the pressure menu works in pascals and the volume menu in litres, the constant is used in its litre form, $8.31446261815324\ \tfrac{\mathrm{Pa\,L}}{\mathrm{mol\,K}}$ . That is the same $R$ as $8.314462618\ \mathrm{J/(mol\,K)}$ , simply written for these units — the NIST value is exact, so nothing here is rounded to 0.08206 or 62.36.

The second relation is the one that does the normalising, and it needs no gas constant at all because the constant cancels on both sides:

V_{\mathrm{STP}}

=

V \times \frac{T_{0}}{T} \times \frac{P}{P_{0}}

\quad\text{with}\quad

T_{0} = 273.15\ \mathrm{K},\ P_{0} = 101325\ \mathrm{Pa}

Together they imply the molar volume that links the two cards, which is also how a standard volume turns back into moles:

V_{\mathrm{m}}

=

\frac{R\,T_{0}}{P_{0}}

=

22.41396954\ \mathrm{L/mol}

\quad\text{and}\quad

n = \frac{V_{\mathrm{STP}}}{V_{\mathrm{m}}}

The symbols on this page

V,\ T,\ P

The measured volume, absolute temperature and absolute pressure you type in.

n

Amount of gas in moles — the quantity that survives every change of temperature and pressure.

V_{\mathrm{STP}}

What that same amount of gas would occupy at 0 °C and 1 atm.

T_{0},\ P_{0}

The reference state: 273.15 K and 101 325 Pa.

R

Molar gas constant, 8.31446261815324 J/(mol·K), exact.

V_{\mathrm{m}}

Molar volume at the reference state, 22.41396954 L/mol.

One assumption, not several

The gas is treated as ideal, which means the compressibility factor $Z$ is taken as 1: $P\,V = Z\,n\,R\,T$ with $Z = 1$ . Everything else on the page is arithmetic, not assumption.

Absolute pressure only

A tyre gauge, a regulator and a lab manometer often report pressure relative to the atmosphere. Add the local atmospheric pressure before typing — the calculator has no way to know which one you are holding.

Four boxes, one empty slot

Volume, temperature, pressure and amount of gas form one connected set: three of them pin the fourth. The standard volume rides along through the amount of gas, so it moves whenever the moles move.

If the amount of gas is not what you want next, the same molar mass of a gas calculator turns a pressure, a volume, a temperature and a weighed mass into an identity for the gas.

What standard conditions means

The 22.4 L that everyone quotes for a mole of gas is one reference state out of several, and the number moves with it. This calculator uses 0 °C and 101 325 Pa, the classical chemical reference state and the one that gives 22.41396954 L/mol.

Every other row below is the same ideal-gas relation at a different pair of reference values — divide $R\,T$ by $P$ and the molar volume falls out:

Reference state	Where it is used	Molar volume at ideal gas
0 °C, 101.325 kPa	This calculator; the classical STP of chemistry textbooks	22.41396954 L/mol
0 °C, 100 kPa	The 100 kPa reference that replaced one atmosphere in many standards	22.71095464 L/mol
15 °C, 101.325 kPa	Gas metering and trade in several countries	23.64483004 L/mol
20 °C, 101.325 kPa	Many laboratory and industrial tables	24.05511687 L/mol
25 °C, 101.325 kPa	Room-temperature reference at one atmosphere	24.46540370 L/mol
25 °C, 100 kPa	The standard ambient reference (SATP)	24.78957030 L/mol

The first two values are tabulated, not derived here: the NIST CODATA listing gives the molar volume of an ideal gas as 22.413 969 54 × 10⁻³ m³/mol at 273.15 K and 101.325 kPa, and 22.710 954 64 × 10⁻³ m³/mol at 273.15 K and 100 kPa. The rest of the column follows from the same relation, so a supplier quoting 15 °C conditions is about 5.5 % away from a 0 °C figure for the same gas.

Converting between two reference states

You do not need to pass through moles. Two reference states are related by the ratio of their absolute temperatures and pressures, so the correction is a single multiplication:

V_{2}

=

V_{1} \times \frac{T_{2}}{T_{1}} \times \frac{P_{1}}{P_{2}}

A 22.41397 L figure at 0 °C and 1 atm becomes 23.64483004 L at 15 °C and the same pressure — the same gas, described against a warmer reference point.

Worked examples

1. A cylinder at 150 bar

A 10 L cylinder of compressed gas stands at 22 °C (295.15 K) with 150 bar on the regulator, which is 15 000 000 Pa absolute.

n

=

\frac{15\,000\,000 \times 10}{8314.46261815324 \times 295.15}

=

61.12435459\ \mathrm{mol}

V_{\mathrm{STP}}

=

61.12435459 \times 22.41396954

=

1\,370.039422\ \mathrm{L}

The same 10 L at room pressure holds 0.41289502 mol, so the cylinder is carrying roughly 148 times as much gas as its physical volume suggests — the pressure ratio, not the volume, is what did the work.

This is where the ideal-gas assumption shows up

At 150 bar the pressure box is flagged, because real gases deviate from $Z = 1$ by percent-level amounts at that density. The number stays on screen, but read it as an estimate: the deviation is why industrial metering uses a compressibility factor measured for the actual gas rather than the ideal-gas relation.

2. Backwards from an amount of gas

You need 0.5 mol of a gas and you want to know whether it fits in a 15 L vessel at room temperature, 25 °C and one atmosphere. Enter the moles, the temperature and the pressure, leave the volume empty, and the measured volume is solved:

V

=

\frac{n\,R\,T}{P}

=

\frac{0.5 \times 8314.46261815324 \times 298.15}{101325}

=

12.232702\ \mathrm{L}

It fits with about 2.8 L to spare. The same 0.5 mol is 11.206985 L at standard conditions, and both numbers appear together on the page.

3. Checking a textbook answer

The classic exercise asks for the volume of 5 g of oxygen at STP. The molar mass of $\mathrm{O_2}$ is 32 g/mol, so first the mass becomes an amount of gas:

n

=

\frac{5}{32}

=

0.15625\ \mathrm{mol}

\quad\Rightarrow\quad

V_{\mathrm{STP}} = 0.15625 \times 22.41396954

=

3.5021827\ \mathrm{L}

A textbook that answers 3.5 L is using the rounded 22.4 L/mol and is right to the precision it states. The difference is 0.06 %, and it only becomes visible once you quote four significant figures — so quote the molar volume you used. If the mass is what you have, the grams to moles calculator does that first step with the periodic table built in.

Tips and common mistakes

Three traps that change the answer by more than rounding

Gauge pressure typed as absolute. 100 L at 20 °C carrying a 5 bar gauge reading is really 6.01325 bar absolute, which is 24.67091 mol. Typing 5 bar as though it were absolute gives 20.513791 mol — about 17 % low, with nothing on the page to warn you.
A Celsius reading typed into a kelvin box. The temperature menu is part of the measurement, not decoration: 100 L at one atmosphere is 4.0874045 mol at 298.15 K but 48.746385 mol at 25 K, nearly twelve times more gas. The calculator warns when a reading falls between 0 and 20 K, because almost nothing is a gas there except helium — that catches a 15 °C reading typed as 15, but it cannot catch 25.
Tablespoons and teaspoons. Those two menu entries are the 15 mL and 5 mL culinary sizes, which is what food labelling uses; the US customary spoonful is 14.7868 mL and 4.9289 mL, exactly half and one sixth of a US fluid ounce. On a large recipe conversion that is a 1.4 % difference per spoonful.

Two sanity checks worth doing by hand

Watch the ratio. The normalising step multiplies your volume by $(T_{0}/T) \times (P/P_{0})$ . Measured warmer than 0 °C and at about an atmosphere, the standard volume must come out smaller; measured at pressure above one atmosphere, smaller again. If the result moves in the other direction, one of the three boxes is not what you think it is.
Compare with the molar volume before you trust a big number. Divide the moles into the measured volume: an ideal gas sits near 22.4 L/mol at room temperature and one atmosphere. A quotient a hundred times smaller means the sample is compressed, and that is your cue to treat the result as an estimate rather than a measurement.
Pressure is the sum, not the reading. Mercury scales on this page are absolute too: 760 Torr and 1013.25 hPa are the same atmosphere, and 29.92 inHg is the imperial version of it.

How far to trust the result

This is a normalising tool with a single physical assumption, and it is honest about where that assumption starts to cost accuracy. Four limits cover almost every case.

Real gases are not ideal

Above about 100 bar (10 MPa) the pressure box is flagged and the result is still shown, because that is where $Z = 1$ stops being a good description. Measured deviation for common gases reaches a few per cent there and grows quickly; a compressibility-corrected equation of state is the right tool for anything tighter.

Below 20 K almost nothing is a gas

A reading between 0 and 20 K is flagged: at ordinary pressures only helium is still a gas there, and for every other substance the sample is a liquid or a solid. If you are genuinely working with helium, treat the number as a rough estimate; otherwise the temperature scale is the likelier explanation.

The reference state is fixed at 0 °C and 1 atm

The page cannot be re-based in the interface. If your contract, standard method or meter uses 15 °C, 20 °C or 100 kPa, normalise to this calculator's state and then apply the reference-state ratio — and name the state in whatever you write down.

Volume only: no mass, no composition, no flow

Moles are not grams, and a normalised volume is not a mass flow. Turning moles into a mass needs the substance's molar mass, and metering a stream needs its composition. For billing, custody transfer or safety-critical sizing, follow the applicable standard and the calibrated instrument rather than this page.

What the tool is good for

Estimating, teaching, checking someone else's arithmetic and comparing measurements taken on different days. Those uses need two or three significant figures, and that is exactly what an ideal-gas normalisation delivers away from the extremes.

Questions

Why is the standard-condition volume different from the volume I measured?

Because you measured at a different temperature or pressure. The two are two views of the same amount of gas, related by $V_{\mathrm{STP}} = V\,(T_{0}/T)\,(P/P_{0})$ . Five litres measured at 350 K and one atmosphere is 3.9021429 L at standard conditions, because the sample was 77 K warmer than the reference; measure the same gas at one atmosphere in an ice bath and the two volumes agree to the last digit.

Is 22.4 L/mol the same as the molar volume this calculator uses?

It is the same reference state rounded to three significant figures. At 0 °C and 101 325 Pa the ideal molar volume is 22.41396954 L/mol, so a textbook answer built on 22.4 L/mol is low by about 0.06 % — invisible at two or three digits, and worth knowing when you have to match a number to four. Use 22.71095464 L/mol instead if your reference state is 0 °C and 100 kPa.

When does the ideal-gas assumption stop being good enough?

At high pressure, at low temperature, and for gases whose molecules attract each other strongly — ammonia, water vapour and heavy hydrocarbons are the usual offenders. Two practical checks: the pressure box is flagged above 100 bar, and if the temperature is anywhere near the point where your substance condenses, the sample is no longer a gas at all. To find that point, look up the boiling point of the liquid at the pressure you measured and compare it with your working temperature.

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