chemistry

Standard Temperature and Pressure Calculator

At standard temperature and pressure (STP), gases behave predictably, making it straightforward to compare their properties. Chemistry students, engineers, and laboratory technicians rely on STP calculations to normalise measurements and predict behaviour across different conditions. Whether you're verifying experimental data or solving coursework problems, this tool converts between your measured conditions and the standardised reference point of 273.15 K and 1 atmosphere.

Last updated: April 27, 2026

Creators Davide Borchia, PhD

Reviewers Hanna Pamuła and Dominik Czernia

8,666 people find this calculator helpful

Understanding STP: The Reference Point

Standard temperature and pressure define a fixed baseline for comparing gases. These conditions are:

Temperature: 273.15 K (0 °C or 32 °F) — the freezing point of water
Pressure: 1 atm (101.325 kPa, 760 Torr, or 760 mm Hg) — atmospheric pressure at sea level

At STP, one mole of any ideal gas occupies exactly 22.4 litres, a relationship known as molar volume. This fixed ratio is invaluable because it lets you move between volume and mole measurements instantly.

Different fields sometimes use alternative reference conditions. In modern chemistry, STP often refers to 25 °C (298.15 K) and 100 kPa — better suited to laboratory work. Always verify which definition applies to your experiment or data source.

Converting Between Conditions

When a gas is measured at non-standard conditions, you can calculate its volume at STP using the combined gas law. The formula accounts for both temperature and pressure differences:

V_STP = V × (T_STP / T) × (P / P_STP)

n = (P × V) / (R × T)

VSTP — Volume at standard conditions (litres)
V — Measured volume (litres)
TSTP — Standard temperature, 273.15 K
T — Measured temperature (Kelvin)
P — Measured pressure (atm or kPa)
PSTP — Standard pressure, 1 atm or 101.325 kPa
n — Number of moles
R — Gas constant, 8.314 J/(mol·K)

The Ideal Gas Law Foundation

All STP calculations rest on the ideal gas law:

PV = nRT

This relationship shows that pressure (P), volume (V), number of moles (n), and temperature (T) are interconnected. The gas constant R is 8.314 J/(mol·K) in SI units, or 0.0821 L·atm/(mol·K) when working with litres and atmospheres.

Related principles emerge from this equation:

Boyle's Law: At constant temperature, pressure and volume are inversely proportional (P₁V₁ = P₂V₂)
Charles's Law: At constant pressure, volume scales with absolute temperature (V₁/T₁ = V₂/T₂)
Gay-Lussac's Law: At constant volume, pressure scales with temperature (P₁/T₁ = P₂/T₂)

Practical Considerations and Common Pitfalls

Several real-world factors can affect the accuracy of your STP calculations.

Always use absolute temperature — Temperature must be in Kelvin, not Celsius or Fahrenheit. Convert using K = °C + 273.15. Forgetting this conversion is the most frequent error and will skew your results significantly.
Confirm which STP standard applies — Classical STP uses 0 °C and 1 atm. Modern chemistry often uses 25 °C and 100 kPa. Check your textbook, lab protocol, or data source to avoid miscalculation. Mixing standards will produce incorrect molar volume conversions.
Real gases deviate at extremes — The ideal gas law assumes gas particles have negligible volume and no intermolecular forces. High pressures, low temperatures, or gases near their condensation point violate these assumptions. For precise work, consider the Van der Waals equation.
Pressure unit conversion matters — Pressure can be given in atm, kPa, Torr, bar, or psi. Always convert to your chosen system before calculation. Mixing units (e.g., using kPa with R in L·atm units) introduces errors.

Worked Example

Suppose you measure a gas sample at 350 K, occupying 5.0 litres at 850 Torr. What volume does it occupy at STP?

Step 1: Convert pressure to the same unit. 850 Torr = 850 / 760 = 1.118 atm.

Step 2: Apply the conversion formula with T_STP = 273.15 K and P_STP = 1 atm:

V_STP = 5.0 × (273.15 / 350) × (1.118 / 1)

V_STP = 5.0 × 0.7805 × 1.118 = 4.36 litres

Step 3: To find the number of moles, use PV = nRT. At STP with 4.36 litres:

n = (1 × 4.36) / (0.0821 × 273.15) = 0.195 moles

Frequently Asked Questions

What defines standard temperature and pressure in chemistry?

STP represents a fixed reference state: 273.15 K (0 °C) and 1 atm (101.325 kPa). These values correspond to water's freezing point at sea-level atmospheric pressure. Modern chemistry sometimes uses an alternative definition of 25 °C and 100 kPa for practical laboratory work. At the classical STP, one mole of ideal gas occupies 22.4 litres, a benchmark used extensively in stoichiometric calculations and gas comparisons.

Why is STP useful in chemistry?

STP provides a universal reference point for comparing gas properties independent of local conditions. Because gas volume and pressure are highly temperature and pressure dependent, standardising these values lets chemists and engineers communicate measurements unambiguously. For example, when a chemical supplier states that a gas has a certain density, they typically reference STP conditions so you know exactly what to expect under your own laboratory conditions.

How do I convert a gas volume from lab conditions to STP?

Use the combined gas law: VSTP = V × (TSTP / T) × (P / PSTP). Measure your gas volume (V), temperature (T in Kelvin), and pressure (P in consistent units). Divide by measured temperature and multiply by the ratio of measured pressure to standard pressure (1 atm or 101.325 kPa). Always ensure temperature is in Kelvin; convert Celsius by adding 273.15.

What is the difference between STP and other standard conditions like NTP?

STP uses 273.15 K and 1 atm as its baseline. Normal Temperature and Pressure (NTP) is similar but uses 293.15 K (20 °C) and 1 atm instead. Some industries define their own standards; for instance, natural gas calculations may use 288.15 K and 101.325 kPa. Always verify which standard your field applies because the molar volume and calculation results will differ accordingly.

Can I use the ideal gas law for real gases?

The ideal gas law works well for most gases at moderate pressures and temperatures far from condensation. However, real gases deviate noticeably at high pressure, low temperature, or when approaching their boiling point. For greater accuracy under extreme conditions, use the Van der Waals equation, which accounts for molecular volume and intermolecular attractions. For everyday chemistry and undergraduate coursework, the ideal gas law is sufficiently precise.

How many litres does 5 grams of oxygen occupy at STP?

Oxygen (O₂) has a molar mass of 32 g/mol. First, find the number of moles: 5 g ÷ 32 g/mol = 0.156 moles. At STP, one mole occupies 22.4 litres, so 0.156 moles occupies 0.156 × 22.4 = 3.5 litres. This direct relationship between moles and volume at STP eliminates the need to apply the gas law explicitly when you already know the molar mass.

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