Pressure Converter

The quick presets (1 atm, 35 psi (tire), 1 bar, 101325 Pa, 120 mmHg (BP), 10 MPa) cover the most common real-world lookups. Click any preset and every output updates immediately. Use these when you know the context but need the number in a different unit.

What pressure actually measures

Pressure is force divided by area. Push 1 newton of force onto 1 square meter of surface and you have 1 pascal. That’s a tiny amount of pressure — a dollar bill sitting flat on a table exerts about 1 pascal on the surface beneath it.

That’s also why pascals are impractical for most human-scale applications. Atmospheric pressure at sea level is 101,325 Pa. Tire pressure is around 240,000 Pa. Engineers didn’t want to write those numbers constantly, so they invented kPa, MPa, bar, and psi to keep the working numbers manageable.

Everything in the converter is just that 101,325 Pa reference scaled differently. Bar is nearly atmospheric. PSI breaks it into pounds-force per square inch. Torr sets 760 as the atmospheric reference because mercury columns were the original pressure-measuring technology.

The 8 pressure units: what each one is and who uses it

Torr and mmHg are numerically almost identical (they differ by about 0.000014%) because Torr was originally defined as 1 mmHg. The formal definitions diverged slightly in the 20th century but the practical difference is invisible for any real-world measurement.

The pressure gauge: reading the logarithmic scale

The gauge in the calculator uses a logarithmic scale from 1 Pa to 10 MPa. This is deliberate. If it used a linear scale, a tire at 240 kPa and atmospheric pressure at 101 kPa would sit close together while a hydraulic press at 5 MPa would be off the right edge of the gauge. Logarithmic scaling compresses the huge range of real-world pressures into something visually readable.

The orange dot on the gauge marks 1 atm / 1 bar simultaneously. They’re close enough that on a logarithmic scale they sit on top of each other visually (1 atm is 101,325 Pa; 1 bar is 100,000 Pa, a 1.3% difference). The needle shows where your input pressure sits relative to those anchors.

The quick presets and what they represent

Conversion formulas: how each unit relates to Pascal

Pascal is the SI base unit, so every other unit converts through it. The converter uses Pascal as the internal pivot: any input converts to Pa first, then from Pa to the target unit.

The “all equivalent pressures” output panel

When you expand “All Equivalent Pressures” in the calculator, it shows all 8 units simultaneously. For 101,325 Pa (1 atm), the panel displays:

The 1.0132e-1 MPa display uses scientific notation because 0.101 MPa is below 1, and scientific notation is standard for values that small in engineering contexts. The same pressure in kPa is the more human-readable 101.325.

Where each unit actually lives in the real world

Gauge pressure vs. absolute pressure: the distinction that matters

The converter works with absolute pressure: the total pressure including atmospheric. But many instruments in the real world display gauge pressure: pressure above atmospheric.

This distinction matters when you’re working with calculations involving gas laws. Boyle’s Law and the ideal gas law require absolute pressure. Using gauge pressure in those formulas gives wrong answers.

Pressure at altitude: why your ears pop on planes

Atmospheric pressure drops with altitude because there’s less air above you pushing down. At 10,000 meters (cruising altitude for commercial aircraft), outside pressure is about 26,500 Pa: roughly 26% of sea-level pressure.

Aircraft cabins are pressurized to the equivalent of about 1,800-2,400 meters altitude (75-80 kPa), not sea level, because maintaining full atmospheric pressure in a metal tube at altitude would require a much heavier and more expensive airframe. Your ears pop when the cabin pressure adjusts because the air pressure in your middle ear equalized to a different value than the cabin air pressure surrounding you.

Common conversion errors and how to avoid them

Bar vs. atmosphere. They’re close (1 bar = 0.9869 atm) but not the same. Using bar when atm is required in a calculation introduces a 1.3% error. That’s fine for tire pressure. In a pressurized gas calculation for a storage vessel, it matters.

Gauge vs. absolute. Covered above, but worth repeating: mixing gauge and absolute pressure in gas law calculations produces results that can be off by 15% or more for low-pressure applications.

Hectopascals and millibars. Weather reports use hPa. Some older instruments use mbar. They’re identical numbers: 1 hPa = 1 mbar = 100 Pa. If you’re converting a weather pressure to compare with a different measurement, you can drop the hecto/milli prefix and just work in Pa by multiplying by 100.

The 1954 hurricane "Hazel" had a central pressure of 938 mbar. In modern units, that's 938 hPa, or 93,800 Pa, about 7.4% below standard atmospheric pressure. The pressure difference between the hurricane's eye and the surrounding atmosphere drives the wind. A deeper eye, lower pressure drop, faster winds.

Reading the formula line in the converter

Every conversion the calculator makes shows its work in a formula line directly below the input fields. For 101325 Pa to psi, it displays:

psi = Pa ÷ 6894.757 → 101325.000 Pa = 14.69595 psi

This line has 3 parts: the mathematical relationship (psi = Pa ÷ 6894.757), the arrow showing direction of conversion, and the specific numerical result. If you’re verifying a manual calculation or documenting a conversion in a report, copy the formula line. It shows the conversion factor used and the precise result to 5 decimal places.