STP stands for Standard Temperature and Pressure, a set of reference conditions used in chemistry to make gas measurements comparable. The standard values are 0 °C (273.15 K) for temperature and 100 kPa (1 bar) for pressure, as defined by the International Union of Pure and Applied Chemistry (IUPAC). These fixed conditions give scientists and students a common baseline when reporting how gases behave, because gas volume changes dramatically with temperature and pressure.
Why STP Exists
A balloon of oxygen in a cold lab takes up less space than the same balloon on a hot day. If two researchers measure the same gas under different conditions and report raw numbers, their results look different even though the amount of gas is identical. STP solves this by giving everyone the same reference point. When you see a gas volume reported “at STP,” you know exactly what temperature and pressure were assumed, so you can compare it to any other measurement also reported at STP.
The Exact Values
The current IUPAC definition sets standard temperature at 273.15 K (0 °C, or 32 °F) and standard pressure at exactly 100,000 pascals, which equals 1 bar. Under these conditions, one mole of an ideal gas occupies 22.71 liters. This number, called the molar volume, is one of the most useful constants in gas calculations.
If you’ve seen 22.4 liters per mole in a textbook, that figure comes from an older standard. IUPAC previously defined standard pressure as 1 atmosphere (101,325 Pa), and many textbooks, especially older ones, still use that value. The difference between 1 bar and 1 atm is only about 1.3%, but it’s enough to shift the molar volume from 22.41 L to 22.71 L. IUPAC now recommends discontinuing the use of 1 atm as the standard pressure.
Not Everyone Uses the Same Numbers
One source of confusion is that different organizations define “standard” conditions differently. The U.S. National Institute of Standards and Technology (NIST), for example, uses 20 °C (293.15 K) and 101.325 kPa (1 atm) as its standard conditions. That’s a warmer temperature and a slightly higher pressure than the IUPAC definition, which means NIST’s molar volume comes out to about 22.41 L/mol instead of 22.71 L/mol.
Engineers in the oil and gas industry often use yet another set of conditions, typically 60 °F (15.56 °C) and 14.696 psi. The takeaway: always check which standard a source is using before plugging numbers into a calculation. In most chemistry courses, the IUPAC definition is the one that matters.
STP vs. SATP
You may also encounter SATP, which stands for Standard Ambient Temperature and Pressure. SATP uses 25 °C (298.15 K) and 1 bar. The temperature is closer to a typical room, making it more practical for reactions that happen at everyday conditions rather than at the freezing point of water. Some newer IUPAC references use SATP for thermodynamic data, while STP remains the go-to for reporting gas volumes.
How STP Is Used in Gas Calculations
The ideal gas law, PV = nRT, relates pressure (P), volume (V), the number of moles (n), the gas constant (R), and temperature (T). At STP, you already know P and T, so if you know any one of the remaining variables, you can solve for the others. For quick estimates, you can skip the full equation entirely: just multiply the number of moles by the molar volume (22.71 L at the current IUPAC standard) to get the volume, or divide a known volume by 22.71 to find the number of moles.
This shortcut works only at STP. If the gas is at a different temperature or pressure, you need the full ideal gas law. The temperature must always be in kelvins, not Celsius, because the gas law is built on an absolute temperature scale where zero means zero molecular motion.
Other Meanings of STP
Outside of chemistry, the acronym STP shows up in a few other contexts. In medicine, STP can refer to Standardized Treatment Protocols, which are step-by-step guides for managing conditions like high blood pressure. These protocols specify which medications to start, what blood pressure targets to aim for, and how often to follow up. Dozens of such protocols exist worldwide, and public health organizations use them to improve consistency in patient care.
STP is also a street name for DOM, a synthetic psychedelic compound chemically known as 2,5-dimethoxy-4-methylamphetamine. It acts on the same serotonin receptor targeted by other psychedelics like LSD, mescaline, and psilocybin. The nickname “STP” dates to the 1960s counterculture. It is a controlled substance in most countries.
For the vast majority of people searching this term, though, Standard Temperature and Pressure in chemistry is the relevant definition, and understanding the specific values (0 °C, 1 bar, 22.71 L/mol) is the core knowledge you need.