Standard Temperature and Pressure Calculator
Convert a gas sample measured at known temperature and pressure to its equivalent volume and amount at STP.
Gas sample inputs
In standard conditions
State comparison
| State | Volume (L) | Temperature (K) | Pressure (atm) |
|---|---|---|---|
| Measured state | 5.000 | 350.00 | 1.1184 |
| STP equivalent | 4.364 | 273.15 | 1.0000 |
How to use the standard temperature and pressure calculator
What this calculator does
This calculator converts the volume of a fixed amount of gas from a measured temperature and absolute pressure to the volume it would occupy at standard temperature and pressure. It also estimates the number of moles by dividing the STP volume by 22.4 liters per mole, matching the convention used by this calculator. The model applies the combined gas law and treats the gas as ideal. It does not identify the gas, correct for humidity, account for compressibility, or replace a real-gas equation when pressure is high or temperature is close to condensation.
When to use it
Use it to normalize laboratory gas-volume measurements before comparing experiments, convert a collected gas sample to a common reporting basis, check textbook combined-gas-law exercises, or estimate an amount of gas from a volume reported at nonstandard conditions. The IUPAC definition of standard conditions is useful context because different disciplines may use different standard pressures; this calculator explicitly uses 273.15 K and 1 atmosphere.
How to calculate
- The calculator opens with a complete demonstration: 5 L at 350 K and 850 Torr. Its results and example Excel workbook are available immediately.
- Replace Volume, Temperature, and Pressure with your measured values. Choose the matching unit beside each field; changing a unit converts the current value rather than merely relabeling it.
- Read Volume at STP as the equivalent volume at 273.15 K and 1 atm. Read Number of moles as the corresponding amount based on 22.4 L/mol, and use Volume factor to see how strongly the original volume was scaled.
- Review the State comparison table to compare the measured state with the normalized STP state in common base units.
- Select Download Excel to export the current inputs, assumptions, outputs, and comparison table. Reset clears the demonstration data and may disable the export until all three required measurements are complete and valid again.
Input guide
Volume is required and accepts a positive finite decimal with a period as the decimal separator. Choose liters, milliliters, or cubic meters; for example, 5 L. Increasing volume increases the STP volume and moles proportionally. Do not enter a mass or a negative volume. Temperature is required and accepts kelvins, degrees Celsius, or degrees Fahrenheit; for example, 350 K. The converted absolute temperature must be greater than zero kelvin. At fixed volume and pressure, a higher measured temperature produces a smaller normalized STP volume. A common error is treating Celsius as though it were already an absolute scale. Pressure is required and accepts Torr, atmospheres, kilopascals, or bar; for example, 850 Torr. It must be absolute pressure, not gauge pressure. At fixed volume and temperature, higher pressure produces a larger STP-equivalent volume because the same gas would expand when brought to 1 atm.
Output guide
Volume at STP is the primary conversion result in liters. It is driven by all three inputs and is an ideal-gas estimate. A factor below one means the normalized volume is smaller than the measured volume; a factor above one means it is larger. Number of moles is the STP volume divided by 22.4 L/mol and is therefore an estimate tied to that molar-volume convention. Volume factor is the dimensionless multiplier applied to the entered volume. The State comparison table reports each state's volume in liters, temperature in kelvins, and pressure in atmospheres. The measured row is a unit-normalized restatement of your input; the STP row is the calculated equivalent.
Worked example
For the startup values, convert 850 Torr to 850 ÷ 760 = 1.1184 atm. Then apply the combined gas law: the STP volume is 5 × (273.15 ÷ 350) × (850 ÷ 760) = 4.364 L. Dividing 4.364 L by 22.4 L/mol gives about 0.195 mol. The volume factor is 4.364 ÷ 5 = 0.8728, so the sample's equivalent volume at STP is about 87.28% of its measured volume.
Formula and assumptions
V(STP) = V₁ × (273.15 K ÷ T₁) × (P₁ ÷ 1 atm)
The relationship follows from the ideal gas law, with the amount of gas held constant. NIST's molar gas constant reference provides the physical constant behind the ideal-gas relationship. For high-accuracy work, verify the standard-state convention required by your laboratory, regulation, or dataset and consider real-gas corrections.
Common mistakes and interpretation
Use absolute pressure, convert temperature to kelvins before applying gas-law ratios, and keep the same amount of gas throughout the comparison. Gauge pressure must be adjusted for ambient atmospheric pressure before entry. Very high pressures, very low temperatures, or gases near phase change can depart materially from ideal behavior. The result is most useful as a normalization and learning tool, not as proof that a particular real gas occupies exactly the predicted volume.