Use this STP Calculator to convert any gas sample to Standard Temperature and Pressure conditions. Find volume at STP and number of moles instantly using the Combined Gas Law and ideal gas law.

What is STP? (Standard Temperature and Pressure)

STP stands for Standard Temperature and Pressure. It is a universal reference condition used in chemistry to compare gas behavior across different experiments, laboratories, and textbooks.

Which definition to use? Unless your textbook or exam specifies otherwise, use the traditional definition: 273.15 K and 1 atm with molar volume 22.4 L/mol. This is what most general chemistry courses expect.

STP Formula and Derivation

The STP formula comes directly from the Combined Gas Law, which relates pressure, volume, and temperature of a fixed amount of gas between two states:

P₁V₁ / T₁ = P₂V₂ / T₂

To convert any gas sample to STP, set State 2 equal to STP conditions (P₂ = 1 atm, T₂ = 273.15 K) and solve for V₂:

V_STP = V × (T_STP / T) × (P / P_STP)
V_STP = V × (273.15 / T) × (P / 1)
V_STP = V × (273.15 / T) × P

Where:

• V — initial volume of gas (liters)
• T — initial temperature in Kelvin (°C + 273.15)
• P — initial pressure in atm (divide mmHg by 760; divide kPa by 101.325)
• V_STP — volume at Standard Temperature and Pressure (liters)

Pressure Conversion Reference

To find the number of moles once you have V_STP:

n = V_STP / 22.4

How to Calculate Volume of Gas at STP — 4 Steps

1. Convert temperature to Kelvin
   Add 273.15 to the Celsius temperature. Never plug Celsius directly into gas law formulas — this is the most common mistake in STP problems.
   Formula: T(K) = T(°C) + 273.15
2. Convert pressure to atm
   If pressure is given in mmHg, kPa, bar, or psi, convert to atm using the table above. STP pressure is 1 atm, so matching units makes the calculation straightforward.
3. Apply the STP formula
   V_STP = V × (273.15 / T) × P
   Multiply the original volume by the temperature correction ratio (always ≤1 if T > 273.15 K) and the pressure ratio (≥1 if P > 1 atm).
4. Calculate moles
   n = V_STP / 22.4
   Divide the STP volume by 22.4 L/mol to get the number of moles of gas.

STP Calculation — Worked Examples

Example 1: Nitrogen Gas at High Temperature and Pressure

Problem: You have 10.0 liters of nitrogen gas (N₂) at 50°C and 2.00 atm. Find the volume at STP and the number of moles.

Step 1 — Convert temperature to Kelvin

T = 50 + 273.15 = 323.15 K

Step 2 — Pressure already in atm

P = 2.00 atm (no conversion needed)

Step 3 — Apply STP formula

V_STP = V × (273.15 / T) × P
V_STP = 10.0 × (273.15 / 323.15) × 2.00
V_STP = 10.0 × 0.8452 × 2.00
V_STP = 16.90 liters

Step 4 — Calculate moles

n = V_STP / 22.4 = 16.90 / 22.4 = 0.754 moles

Result: At STP, 10.0 L of N₂ at 50°C and 2 atm occupies 16.90 liters and contains 0.754 moles. The volume increased because doubling pressure compresses the gas, but the STP correction re-expands it — and the net effect is larger because the pressure correction (×2) outweighs the temperature correction (×0.845).

Example 2: Oxygen Gas Given in mmHg and Celsius

Problem: A sample of oxygen gas (O₂) occupies 5.00 liters at 25°C and 950 mmHg. Find its volume at STP.

Step 1 — Convert temperature

T = 25 + 273.15 = 298.15 K

Step 2 — Convert pressure from mmHg to atm

P = 950 / 760 = 1.2500 atm

Step 3 — Apply STP formula

V_STP = 5.00 × (273.15 / 298.15) × 1.2500
V_STP = 5.00 × 0.9161 × 1.2500
V_STP = 5.00 × 1.1451
V_STP = 5.73 liters

Step 4 — Calculate moles

n = 5.73 / 22.4 = 0.256 moles

Result: The oxygen sample occupies 5.73 liters at STP and contains 0.256 moles. Because the original pressure (950 mmHg) was higher than STP (760 mmHg), the volume increases when pressure drops. The temperature correction slightly reduces this because 25°C is warmer than 0°C.

Example 3: Carbon Dioxide from kPa — Working Backwards from Moles

Problem: How many liters does 0.500 moles of CO₂ occupy at STP? Then find the volume at 30°C and 150 kPa.

Part A — Volume at STP

V_STP = n × 22.4 = 0.500 × 22.4 = 11.2 liters

Part B — Volume at 30°C and 150 kPa (rearrange STP formula)

Convert: T = 303.15 K, P = 150 / 101.325 = 1.4803 atm

V = V_STP × (T / 273.15) × (1 / P)
V = 11.2 × (303.15 / 273.15) × (1 / 1.4803)
V = 11.2 × 1.1099 × 0.6755
V = 8.39 liters

Result: 0.500 moles of CO₂ occupies 11.2 liters at STP and 8.39 liters at 30°C and 150 kPa. Higher pressure compresses the gas more than the temperature increase can expand it.

Molar Volume at STP — Why 22.4 L/mol?

The 22.4 L/mol value is not a definition — it is a calculated result from the ideal gas law:

PV = nRT
V = nRT / P
V = (1 mol × 0.08206 L·atm/mol·K × 273.15 K) / 1 atm
V = 22.414 liters

This calculation uses the universal gas constant R = 0.08206 L·atm/(mol·K). The result, 22.414 L, is rounded to 22.4 L in most textbooks. Any ideal gas — whether H₂, O₂, CO₂, or Ar — occupies exactly this volume at STP because the ideal gas law does not depend on the identity of the gas, only on temperature and pressure.

Key point: Real gases deviate slightly from 22.4 L/mol at STP due to intermolecular forces and molecular volume. The deviation is smallest for small, nonpolar molecules (He, H₂, N₂) and largest for large polar molecules (CO₂, SO₂, NH₃).

STP vs NTP vs SATP — Full Comparison

Three different standards exist because different fields (physics, engineering, biology) settled on different reference temperatures. Always confirm which standard your course or problem uses.

The pressure difference between old STP (1 atm = 101.325 kPa) and new IUPAC STP (1 bar = 100 kPa) is only 1.325 kPa — about 1.3%. This changes the molar volume from 22.4 to 22.711 L/mol. For most problems below the level of research chemistry, the difference is negligible.

Common Mistakes in STP Calculations

Why is STP Used in Chemistry?

STP gives chemists a universal baseline for comparing gas measurements across different laboratories and experimental conditions. Without a shared standard:

• Gas volumes measured at different temperatures and pressures would be incomparable between labs or textbooks.
• Stoichiometry calculations involving gases would require knowing the exact conditions of every measurement instead of applying a fixed molar volume.
• The 22.4 L/mol shortcut could not be used — every mole-to-volume conversion would require the full ideal gas law calculation.
• Reporting gas flow rates, emissions data, and industrial gas quantities would have no common reference point.

STP was also chosen because 0°C is a reproducible, precisely defined temperature (the ice-water equilibrium point at 1 atm) that any laboratory can achieve and verify without specialized equipment.

At STP, gases behave most closely to the ideal gas model. Low temperature reduces kinetic energy, meaning molecules spend more time interacting — but 273 K is still high enough that most common gases (N₂, O₂, H₂, noble gases) remain well above their condensation points and behave nearly ideally. The deviation from ideal behavior at STP is less than 0.1% for most diatomic gases.

Frequently Asked Questions

What is STP in chemistry?

STP stands for Standard Temperature and Pressure: 273.15 K (0°C) and 1 atm in most textbook definitions. It is the universal reference condition for gas calculations, allowing volumes and mole quantities to be compared consistently across experiments.

What is the STP formula for volume?

The STP formula derived from the Combined Gas Law is: V_STP = V × (273.15 / T) × P, where V is the initial volume in liters, T is the initial temperature in Kelvin, and P is the initial pressure in atm. This corrects the volume to the conditions 273.15 K and 1 atm.

What is the STP temperature?

The standard temperature at STP is 273.15 K = 0°C = 32°F — the freezing point of pure water at sea level. Both the old (1 atm) and new IUPAC (1 bar) definitions share the same temperature; only the pressure definition changed in 1982.

What is the volume of 1 mole of gas at STP?

At STP using the 1 atm definition, 1 mole of any ideal gas occupies 22.4 liters. This is derived directly from the ideal gas law: V = nRT/P = (1)(0.08206)(273.15)/1 = 22.414 L. Under the IUPAC 1 bar definition, the molar volume is 22.711 L/mol.

Is STP the same as room temperature?

No. STP is 0°C (273.15 K), the freezing point of water — well below typical room temperature of 20–25°C. Room temperature conditions correspond to NTP (20°C, 1 atm) or SATP (25°C, 100 kPa) depending on the context.

What is the difference between STP and NTP?

STP = 273.15 K (0°C) and 1 atm, giving a molar volume of 22.4 L/mol. NTP (Normal Temperature and Pressure, NIST definition) = 293.15 K (20°C) and 1 atm, giving a molar volume of 24.04 L/mol. NTP is closer to actual laboratory ambient conditions.

What is the new IUPAC STP standard?

Since 1982, IUPAC defines STP as 273.15 K and 100 kPa (1 bar) rather than 1 atm. This shifts the molar volume from 22.4 to 22.711 L/mol. The change matters for precise research work but is negligible (<1.5% difference) for most general chemistry calculations. Always check which standard your textbook uses before applying a molar volume value.

Why does temperature need to be in Kelvin for STP calculations?

All gas law equations require absolute temperature in Kelvin because they are derived from the kinetic theory of gases, where temperature represents the average kinetic energy of molecules. At 0 K, molecular motion theoretically stops. Using Celsius would give negative or zero temperatures that produce meaningless (or undefined) results in ratios like 273.15/T.