STP Conditions Calculator

Convert the volume of a gas from any pressure and temperature to standard temperature and pressure (STP).

Normalize Gas Volume to Standard Temperature & Pressure & Compute Moles

Standard:

Measured Operating Gas Conditions Ref: 101.325 kPa, 273.15 K

Pressure (P)

Volume (V)

Temperature (T)

Desired Output Volume Unit:

V_stp = (PVT_stp)/(P_stpT)

Live Result

1 atm, 0 °C

Volume at STP

13.742 L

Amount (n)

0.6131 mol

Step-by-Step Mathematical Derivation:

1. Formula: V_stp = (P × V × T_stp) / (P_stp × T)

• Operating: P = 1.5 atm, V = 10.0 L, T = 298.15 K

2. V_stp = 13.742 L

3. Moles: n = PV / RT = 0.6131 moles

This STP calculator converts the volume of a gas from any pressure and temperature to standard temperature and pressure (STP). Enter the pressure, volume, and temperature you measured, choose an STP definition, and the tool solves V(STP) = (P1 × V1 × T(STP)) / (P(STP) × T1) with full steps. It also returns the moles of gas at STP and lets you run the calculation in reverse, from a known STP volume to the volume at other conditions. It accepts atm, kPa, mmHg, torr, bar, and psi for pressure, L, mL, and m³ for volume, and K, °C, and °F for temperature, and it converts temperature to Kelvin automatically.

Quick Reference

Item	Value
IUPAC STP (current)	273.15 K (0 °C) and 100 kPa (1 bar)
Older STP (many textbooks)	273.15 K (0 °C) and 101.325 kPa (1 atm)
Formula	V(STP) = (P1 × V1 × T(STP)) / (P(STP) × T1)
Based on	Combined gas law, P1V1/T1 = P2V2/T2
Molar volume at IUPAC STP	22.711 L/mol
Molar volume at older STP	22.414 L/mol
Temperature scale required	Kelvin (K = °C + 273.15)

What Are STP Conditions?

STP stands for standard temperature and pressure. It is a fixed reference state that lets scientists compare gas volumes, densities, and amounts measured in different places on different days. A gas volume means little without its temperature and pressure, because the same amount of gas occupies very different volumes in a cold lab and a hot factory. STP removes that ambiguity.

The current IUPAC definition of STP is a temperature of 273.15 K (0 °C) and an absolute pressure of 100 kPa (1 bar). IUPAC adopted this in 1982. Before that, IUPAC defined standard pressure as 1 atm (101.325 kPa), and many textbooks, exams, and chemistry courses still use that older value. Both versions use 0 °C, so the difference is only the pressure.

Because gases follow the combined gas law, converting a measured volume to STP is a direct application of P1V1/T1 = P2V2/T2, where state 2 is the standard state. The relationship follows from the ideal gas law and the kinetic molecular theory, and it holds for a fixed amount of gas.

Why STP Matters

Comparing gas volumes

A volume at STP reflects the amount of gas, not the weather in the lab.

Stoichiometry

Reaction problems often give or ask for gas volumes at STP, using molar volume to link volume and moles.

Density comparisons

Gas densities are tabulated at STP so different gases can be compared fairly.

Industry reporting

Gas flow and storage are often reported as standard volumes so that quantities are comparable.

STP Conditions Compared: STP, NTP, SATP, and Others

Several "standard" states exist. Always check which one your textbook, instructor, or standard requires.

Standard	Temperature	Pressure	Molar Volume of an Ideal Gas
STP (IUPAC, since 1982)	273.15 K (0 °C)	100 kPa (1 bar)	22.711 L/mol
STP (older IUPAC, many textbooks)	273.15 K (0 °C)	101.325 kPa (1 atm)	22.414 L/mol
NTP (normal temperature and pressure, NIST)	293.15 K (20 °C)	101.325 kPa (1 atm)	24.055 L/mol
SATP (standard ambient temperature and pressure)	298.15 K (25 °C)	100 kPa (1 bar)	24.790 L/mol
SATP (older, 1 atm)	298.15 K (25 °C)	101.325 kPa (1 atm)	24.465 L/mol
ISA sea level (International Standard Atmosphere)	288.15 K (15 °C)	101.325 kPa (1 atm)	23.645 L/mol

💡 Which one should you use? Use the definition given in your problem or course. If none is given, modern IUPAC STP (273.15 K and 100 kPa) is the current international standard, but many chemistry classes still expect 273.15 K and 1 atm. This calculator lets you pick any of these or enter custom values.

Equivalent Values of Standard Pressure

Standard Pressure	atm	kPa	mmHg	bar	psi
IUPAC standard (1 bar)	0.98692	100	750.06	1.000	14.504
Older standard (1 atm)	1.00000	101.325	760	1.01325	14.696

The STP Conversion Formula

Start with the combined gas law:

P1V1 / T1 = P2V2 / T2

Let state 2 be the standard state, so P2 = P(STP), T2 = T(STP), and V2 = V(STP). Solve for V(STP):

V(STP) = (P1 × V1 × T(STP)) / (P(STP) × T1)

To go the other way, from a known STP volume to the volume at other conditions:

V2 = (P(STP) × V(STP) × T2) / (P2 × T(STP))

To find the amount of gas at STP:

n = V(STP) / Vm

where Vm is the molar volume at the chosen standard state.

Variable Table

Variable	Meaning	Common Units
P1	Measured pressure	atm, kPa, mmHg, torr, bar, psi
V1	Measured volume	L, mL, m³
T1	Measured temperature	K (converted from °C or °F)
P(STP)	Standard pressure	100 kPa or 101.325 kPa
T(STP)	Standard temperature	273.15 K
V(STP)	Volume at STP (unknown)	Same unit as V1
Vm	Molar volume at STP	L/mol
n	Moles of gas	mol

How to Use This STP Calculator

Choose the mode

Select "convert to STP" or "convert from STP."

Pick the STP definition

Choose IUPAC STP (273.15 K, 100 kPa), older STP (273.15 K, 1 atm), NTP, SATP, or custom values.

Enter the measured pressure

Type P1 and choose atm, kPa, mmHg, torr, bar, or psi.

Enter the measured volume

Type V1 and choose L, mL, or m³.

Enter the measured temperature

Type T1 in K, °C, or °F. The tool converts it to Kelvin.

Click Calculate

The calculator returns the volume at STP and the moles of gas.

Read the steps

Check the substituted equation to see how the answer was found.

Run a sense check

A gas measured at low pressure or high temperature should have a smaller volume at STP, and a gas measured at high pressure or low temperature should have a larger one.

⚠️ Note: Use absolute pressure. If your pressure is a gauge reading, add atmospheric pressure first. If the gas was collected over water, subtract the vapor pressure of water before converting to STP.

What Can This Calculator Calculate?

Volume at STP

Convert a measured gas volume to the standard state.

Volume from STP

Convert a known STP volume to the volume at other conditions.

Moles at STP

Find the amount of gas from its STP volume using molar volume.

Gas density at STP

Estimate density from molar mass and molar volume.

For a general problem with any unknown, use the Combined Gas Law Calculator. For moles, mass, or a single state, use the Ideal Gas Law Calculator. For temperature conversions, use the Temperature Calculator.

STP Rearranged Formulas

All forms come from cross-multiplying P1V1/T1 = P(STP)V(STP)/T(STP).

Solve For	Formula
Volume at STP	V(STP) = (P1 × V1 × T(STP)) / (P(STP) × T1)
Measured volume	V1 = (P(STP) × V(STP) × T1) / (P1 × T(STP))
Measured pressure	P1 = (P(STP) × V(STP) × T1) / (V1 × T(STP))
Measured temperature	T1 = (P1 × V1 × T(STP)) / (P(STP) × V(STP))
Moles at STP	n = V(STP) / Vm

Read the STP volume formula as a set of ratios:

V(STP) = V1 × (P1 / P(STP)) × (T(STP) / T1)

Multiply the measured volume by a pressure correction and a temperature correction. If the gas was measured at higher pressure than standard, the first ratio is above 1 and the volume grows. If it was measured at a higher temperature than 273.15 K, the second ratio is below 1 and the volume shrinks.

STP Calculation Examples

Example 1

Convert a Volume to STP (Older Definition, 1 atm)

Problem: A gas occupies 5.00 L at 2.00 atm and 27 °C. Find its volume at STP (273.15 K and 1.00 atm).

Step 1: Convert T1 to Kelvin T1 = 27 + 273.15 = 300.15 K

Step 2: Write the formula V(STP) = (P1 × V1 × T(STP)) / (P(STP) × T1)

Step 3: Substitute V(STP) = (2.00 × 5.00 × 273.15) / (1.00 × 300.15) = 2731.5 / 300.15

Answer: V(STP) ≈ 9.10 L

Using the ratio form: 5.00 × (2.00 / 1.00) × (273.15 / 300.15) = 5.00 × 2 × 0.910 = 9.10 L. The pressure is twice standard, which doubles the volume at STP, and the small temperature difference trims it slightly.

Example 2

Lab Gas Collection (mL, mmHg, °C)

Problem: A gas sample occupies 250 mL at 740 mmHg and 22 °C. Find its volume at STP (273.15 K and 760 mmHg), then the moles of gas.

Step 1: Convert T1 to Kelvin T1 = 22 + 273.15 = 295.15 K

Step 2: Substitute V(STP) = (740 × 250 × 273.15) / (760 × 295.15) = 50,532,750 / 224,314

Step 3: Find moles n = V(STP) / Vm = 0.2253 L / 22.414 L/mol

Answer: V(STP) ≈ 225.3 mL and n ≈ 0.01005 mol

The units mL and mmHg carry through because the measured and standard states use the same units. Only temperature needed conversion.

Example 3

IUPAC STP (100 kPa)

Problem: A gas occupies 1.50 L at 150 kPa and 50 °C. Find its volume at IUPAC STP (273.15 K and 100 kPa).

Step 1: Convert T1 to Kelvin T1 = 50 + 273.15 = 323.15 K

Step 2: Substitute V(STP) = (150 × 1.50 × 273.15) / (100 × 323.15) = 61,458.75 / 32,315

Answer: V(STP) ≈ 1.90 L

The same problem under the older 1 atm standard gives 1.88 L, which shows why you must use the standard your course specifies.

Example 4

Convert From STP to Other Conditions

Problem: A gas occupies 10.0 L at STP (273.15 K and 1.00 atm). What volume does it occupy at 25 °C and 0.900 atm?

Step 1: Convert T2 to Kelvin T2 = 25 + 273.15 = 298.15 K

Step 2: Write the formula V2 = (P(STP) × V(STP) × T2) / (P2 × T(STP))

Step 3: Substitute V2 = (1.00 × 10.0 × 298.15) / (0.900 × 273.15) = 2981.5 / 245.835

Answer: V2 ≈ 12.13 L

The gas expands because the pressure is lower and the temperature is higher than standard.

Example 5

Moles and the Effect of the STP Definition

Problem: A gas occupies 44.8 L at STP. How many moles is it under each definition?

Older STP (Vm = 22.414 L/mol) n = 44.8 / 22.414 = 2.00 mol

IUPAC STP (Vm = 22.711 L/mol) n = 44.8 / 22.711 = 1.97 mol

Answer: Older STP: 2.00 mol | IUPAC STP: 1.97 mol

The two answers differ by about 1.3%. On an exam, use the molar volume your course gives. Textbook problems that give a volume at STP and expect a clean whole number of moles usually use 22.4 L/mol.

Molar Volume at STP

The molar volume of an ideal gas is the volume of one mole at a given temperature and pressure. It follows from the ideal gas law, Vm = RT / P, where R is the gas constant (8.314 J/(mol·K), or 8.314 L·kPa/(mol·K)). At the same temperature and pressure, one mole of any ideal gas occupies the same volume. This is Avogadro's law, and it is why one mole holds the same number of particles (the Avogadro constant, 6.022 × 10²³) whichever gas you choose.

Condition	Temperature	Pressure	Molar Volume
IUPAC STP	273.15 K	100 kPa	22.711 L/mol
Older STP	273.15 K	101.325 kPa	22.414 L/mol
NTP	293.15 K	101.325 kPa	24.055 L/mol
SATP (1 bar)	298.15 K	100 kPa	24.790 L/mol
SATP (1 atm)	298.15 K	101.325 kPa	24.465 L/mol

🔬 Loschmidt constant: At 273.15 K and 101.325 kPa, an ideal gas contains about 2.687 × 10²⁵ molecules per cubic meter. This is called the Loschmidt constant and follows from the molar volume and the Avogadro constant.

Gas Density at STP

Once you know the molar volume, density follows from molar mass: Density = molar mass / molar volume Values below use 22.414 L/mol (0 °C and 1 atm) and treat each gas as ideal.

Gas	Molar Mass (g/mol)	Density at 0 °C and 1 atm (g/L)
Hydrogen (H₂)	2.016	0.0899
Helium (He)	4.003	0.1786
Nitrogen (N₂)	28.014	1.250
Air (average)	28.97	1.292
Oxygen (O₂)	31.998	1.428
Carbon dioxide (CO₂)	44.01	1.964

Under IUPAC STP (22.711 L/mol), every density is about 1.3% lower. For example, air is about 1.275 g/L. Real gas densities differ slightly from these ideal values. For carbon dioxide, the measured density at 0 °C and 1 atm is about 1.977 g/L.

STP Units and Conversions

The conversion is a ratio, so any pressure unit and any volume unit work if the measured and standard states use the same ones.

Pressure Units

Pressure Unit	Equivalent
1 atm	101.325 kPa
1 atm	760 mmHg
1 atm	760 torr
1 atm	1.01325 bar
1 atm	14.696 psi
1 bar	100 kPa

Volume Units

Volume Unit	Equivalent
1 L	1000 mL
1 mL	1 cm³
1 m³	1000 L

Note: Temperature must be in Kelvin. Convert with K = °C + 273.15 or K = (°F − 32) × 5/9 + 273.15. Use absolute pressure, which equals gauge pressure plus atmospheric pressure.

Standard Volumes in Industry

Outside the classroom, "standard" volumes are used to report gas quantities in a comparable way, but the reference conditions vary by industry and region.

Normal cubic meter (Nm³)

Usually a cubic meter of gas at 0 °C and 1.01325 bar, though some suppliers use other references.

Standard cubic meter (Sm³)

Often defined at 15 °C and 1.01325 bar, but definitions vary.

Standard cubic feet (scf) and SCFM

Common in the US for compressed air and natural gas, often referenced to 60 °F and 14.696 psi, though some suppliers use 68 °F.

Actual cubic feet per minute (ACFM)

The real flow at operating conditions, which must be converted to standard flow with the same gas law method.

Always confirm the reference conditions in a supplier specification or contract before comparing standard volumes, since two "standard" volumes can differ by several percent.

When Can You Use This STP Calculator?

Use the STP conversion when all of the following are true:

Constant amount of gas

The number of moles stays the same between the measured and standard states.

Same gas sample

You are converting one sample from measured conditions to a reference state.

Known measured conditions

You know pressure, volume, and temperature.

Absolute units

Temperature is in Kelvin and pressure is absolute.

A gas

STP molar volume applies to gases, not liquids or solids.

⚠️ Limits: The conversion and the molar volume values assume ideal gas behavior. Near STP, most common gases behave close to ideal, but gases that liquefy easily, such as ammonia or sulfur dioxide, and gases at high pressure deviate more. In those cases, the van der Waals equation or a compressibility factor (Z) gives better results. Note also that water is a liquid at 0 °C, so "a gas at STP" does not apply to water vapor at that temperature.

Common STP Calculation Mistakes

Mistake	Why It Fails	Fix
Using the wrong STP definition	1 bar and 1 atm differ by 1.3%, and 0 °C and 25 °C differ a lot	Check which standard your course or standard requires
Confusing STP with SATP or room temperature	SATP is 25 °C, not 0 °C	Confirm 273.15 K for STP
Using °C or °F	Gas laws need absolute temperature	Convert with K = °C + 273.15
Using 22.4 L/mol for all conditions	22.414 L/mol only applies at 273.15 K and 1 atm	Use the molar volume for your chosen standard
Using gauge pressure	Ignores atmospheric pressure	Add about 1 atm (14.7 psi) to get absolute
Mixing pressure units	atm and kPa in one ratio give a wrong answer	Convert all pressures to one unit
Ignoring water vapor	Gas collected over water includes vapor pressure	Subtract the vapor pressure of water first
Inverting the ratios	Puts standard values where measured ones belong	Write V(STP) = V1 × (P1 / P(STP)) × (T(STP) / T1)
Applying STP to a changing amount of gas	Moles must not change	Use the ideal gas law if gas is added or lost
Rounding too early	Small errors grow through the calculation	Keep extra digits and round at the end

STP Calculator vs Other Gas Law Tools

Tool	Use It When
STP Conditions Calculator (this page)	You need a gas volume, moles, or density at a standard reference state
Combined Gas Law Calculator	Any two states of a gas, solving for P, V, or T
Ideal Gas Law Calculator	You need moles, mass, or a single state (PV = nRT)
Pressure Calculator	You need final pressure after volume and temperature changes
Volume Calculator	You need final volume after pressure and temperature changes
Temperature Calculator	You need final temperature, or Kelvin, Celsius, and Fahrenheit conversions
Boyle's, Charles's, and Gay-Lussac's Law Calculators	Only two variables change and the third is constant

Knowledge Base

STP Calculator FAQs

Frequently asked questions about standard temperature and pressure, molar volume, and NTP/SATP comparisons.

What are STP conditions?

STP stands for standard temperature and pressure. The current IUPAC definition is 273.15 K (0 °C) and 100 kPa (1 bar). Many textbooks still use 273.15 K and 1 atm (101.325 kPa).

What is the temperature at STP?

The temperature at STP is 273.15 K, which is 0 °C or 32 °F.

What is the pressure at STP?

Under the current IUPAC definition, it is 100 kPa (1 bar, about 0.987 atm). Under the older definition, it is 101.325 kPa (1 atm, 760 mmHg).

What is the molar volume at STP?

It is 22.711 L/mol at IUPAC STP (273.15 K and 100 kPa) and 22.414 L/mol at 273.15 K and 1 atm. Use the value your course specifies.

How do you convert a gas volume to STP?

Use V(STP) = (P1 × V1 × T(STP)) / (P(STP) × T1). Convert the measured temperature to Kelvin, use matching pressure units, substitute the values, and solve.

How do you find moles of gas at STP?

Divide the volume at STP by the molar volume. For example, 44.8 L at 273.15 K and 1 atm equals 44.8 / 22.414, or about 2.00 mol.

What is the difference between STP and NTP?

STP uses 0 °C, while NTP (as defined by NIST) uses 20 °C and 1 atm. The molar volume at NTP is about 24.055 L/mol, compared with about 22.4 L/mol at older STP.

What is the difference between STP and SATP?

STP uses 273.15 K (0 °C), and SATP uses 298.15 K (25 °C). SATP is closer to normal lab conditions, and its molar volume is about 24.79 L/mol at 100 kPa.

Why did IUPAC change the STP pressure from 1 atm to 100 kPa?

IUPAC changed the standard pressure to 100 kPa (1 bar) in 1982 to align with SI units, since 1 bar is a round number in kilopascals. Many textbooks kept 1 atm out of tradition.

Does 22.4 L/mol always apply?

No. It applies only at 273.15 K and 1 atm. At other conditions, the molar volume differs. This calculator finds the correct molar volume for any standard state.

Can I use STP conversion if the amount of gas changes?

No. The combined gas law requires a constant number of moles. If gas is added or removed, use the ideal gas law.

Is water at STP a gas?

No. At 0 °C, water is a solid or liquid, so the gas molar volume does not apply to water at STP. It applies to true gases such as nitrogen, oxygen, and carbon dioxide.

Why do I need the vapor pressure of water for gases collected over water?

A gas collected over water is a mixture of the gas and water vapor. The total pressure equals the sum of both partial pressures, so subtract the vapor pressure of water at the collection temperature to get the pressure of the dry gas.

Is the STP calculator accurate for real gases?

It is accurate for gases that behave close to ideal, which includes most common gases near STP. Gases that liquefy easily or gases at high pressure deviate from ideal behavior.