5 pressure measurement myths that ruin your calculations

Pressure is simply force applied over an area. But measuring it means wading through an alphabet soup of units: psi, bar, kPa, mmHg, atm, and more. If you check a tire in Ohio, you read psi. If you rent a car in Germany, the pump reads in bar or kPa. In a hospital, blood pressure is measured in mmHg, but a ventilator uses inches of water.

Because these units grew out of completely different fields—meteorology, engineering, medicine—translating between them causes a lot of confusion. Let’s look at the actual math behind these units and clear up a few common misunderstandings along the way.

Myth 1: Bar and Atmosphere (atm) are the exact same thing

People often treat “bar” and “atm” as interchangeable because they are so close in value. They are close, but they are not identical.

One standard atmosphere (atm) represents the average atmospheric pressure at sea level on Earth. By definition, 1 atm equals exactly 101,325 Pascals (Pa). A bar, on the other hand, is a metric unit created to make the math cleaner. It is defined as exactly 100,000 Pascals.

That leaves a difference of about 1.3 percent. If you are just estimating the weather, you might not notice. But in chemistry or precision manufacturing, that gap matters. Imagine an industrial compressor filling a tank to 200 bar. If a technician assumes bar and atm are identical, they might record 200 atm. The true conversion is roughly 197.38 atm. That 2.6 atm error translates to a difference of over 38 pounds per square inch (psi). In high-pressure fluid systems, an unexpected 38 psi can easily blow a seal or trigger a safety valve.

Myth 2: Torr and mmHg measure different things

If you work around lab equipment, vacuum pumps often read in Torr. If you work in healthcare, blood pressure is universally measured in millimeters of mercury (mmHg). Because they appear in entirely different contexts, a persistent myth is that they measure fundamentally different properties and require a complicated mathematical formula to convert.

In reality, they are the same thing. The Torr is named after Evangelista Torricelli, the Italian physicist who invented the barometer. By definition, one Torr is exactly 1/760 of a standard atmosphere. Historically, a standard atmosphere was defined by the pressure needed to support a column of mercury exactly 760 millimeters high.

Because of that shared history, the math lines up perfectly. One Torr equals approximately 133.322 Pascals. One mmHg also equals 133.322 Pascals. For practical purposes in engineering, meteorology, and medicine, you can treat 1 Torr as exactly equal to 1 mmHg. You do not need a calculator to switch between them; just swap the label.

Myth 3: You can easily guess the conversion between psi and kPa

In the United States, pounds per square inch (psi) is the default. We use it for bicycle tires, air compressors, and water pipes. Much of the rest of the world uses kilopascals (kPa) or bar. Because the numbers can look somewhat similar on a dial, a common mistake is assuming a simple 10-to-1 or round-number ratio to do the math in your head.

The exact conversion factor is that 1 psi equals 6,894.757 Pascals. Since a kilopascal is just a thousand Pascals, 1 psi equals roughly 6.895 kPa.

If your car requires 32 psi in its tires, and you pull up to an air pump in Europe that only reads in kPa, you cannot just multiply by 10 and fill it to 320 kPa. The correct math is 32 multiplied by 6.895, which gives you about 220.6 kPa. Guessing will leave you with dangerously over-inflated tires. If you need to estimate on the fly, multiplying your psi by 7 gets you much closer (32 × 7 = 224 kPa), but using the exact multiplier is always safer.

Myth 4: Imperial units like inches of water are obsolete

It is easy to assume that units based on columns of liquid—like inches of water column (inH2O) or inches of mercury (inHg)—are historical relics that have been entirely replaced by the Pascal. While the Pascal is the global scientific standard, fluid-based units remain highly specialized and are actively used every day.

Inches of water is the standard measurement in HVAC for air duct pressure, and in respiratory medicine for ventilators and spirometers. The reason is scale. The pressures moving air through a house or a pair of lungs are incredibly small. One inch of water equals just 249.089 Pascals, or a tiny 0.036 psi. It is much easier for an HVAC technician to read “0.5 inH2O” on a diagnostic gauge than to parse out 124.5 Pascals.

Similarly, aviation and meteorology still rely heavily on inches of mercury for barometric pressure. When a pilot receives an altimeter setting from air traffic control, it is given in inHg. One inch of mercury equals exactly 3,386.389 Pascals. These units stuck around because they perfectly fit the scale of the specific jobs they do.

Myth 5: The conversion math changes depending on the substance

A surprising number of people believe that converting pressure units for a liquid (like water in a plumbing system) requires a different formula than converting units for a gas (like compressed air). This is false. Pressure is always just force divided by area, regardless of what medium pushes against that area.

A pressure of 14.7 psi exerted by the air in the atmosphere is mathematically identical to 14.7 psi exerted by hydraulic fluid in a tractor piston. You do not need to factor in the density, temperature, or viscosity of the fluid when you are simply converting the unit of measurement. The conversion ratios between psi, bar, and Pascals are fixed physical constants. A pressure unit just describes the force; it does not care what substance is doing the pushing.

How all units connect back to the Pascal

To see why these conversions work so reliably, look at the baseline. The Pascal is the standard SI unit for pressure. It is defined as one Newton of force applied evenly over one square meter. Every other pressure measurement you encounter is just a strict mathematical ratio of the Pascal.

When software or a calculator converts between two random units, it usually uses the Pascal as a bridge. For example, if you need to convert 2 atm to psi, the math first converts the atmospheres into Pascals: 2 × 101,325 = 202,650 Pa.

Then, it takes that baseline total and divides it by the psi conversion factor: 202,650 ÷ 6,894.757. The result is roughly 29.39 psi. Understanding this intermediate step clears up the mystery of how very different pressure scales relate to one another.

Ready to run your own numbers without doing the manual math? Check out our Pressure Converter to instantly switch between these units and more.