Pressure Converter

Convert pressure units free online -- PSI, bar, pascals, atmospheres, and mmHg. Accurate standard conversion factors for tire pressure, weather, or engineering use.

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How to Use This Tool

Enter a pressure valueThe number you want to convert

Choose from and to unitsPSI, bar, pascals, atm, or mmHg

See the converted resultUsing exact standard conversion factors

Industry Convention Drives Which Pressure Unit You See

Tire gauges, weather reports, medical blood pressure readings, and engineering specifications each gravitated toward a different pressure unit based on historical convention and practical context, not because any one unit is technically superior -- which is exactly why converting between them comes up so often in practice.

The Mercury Column Behind mmHg

Blood pressure measurement in mmHg traces back to mercury column manometers, where pressure literally displaced mercury by a measurable height. Modern digital blood pressure monitors don't use actual mercury, but the unit name and convention persisted from that original measurement method.

Bar Versus Atmosphere -- Close But Not Identical

These two units are similar enough in magnitude (1 bar ≈ 0.9869 atm) that they're sometimes casually treated as interchangeable, but they are defined independently and aren't exactly equal -- a distinction that matters for precise engineering or scientific work even though it's negligible for casual purposes.

Frequently Asked Questions

This reflects regional and industry convention -- automotive contexts in the US and several other countries standardized on PSI, while meteorology commonly uses hectopascals or millibars, and medical blood pressure readings use mmHg specifically for historical reasons tied to mercury column measurement.

1 atmosphere is defined as 101,325 pascals, originally based on typical sea-level air pressure, now used as a standard reference unit across science and engineering regardless of actual current atmospheric conditions.

Historically, blood pressure was measured using a mercury column manometer, where pressure literally pushed a column of mercury up by a measurable height in millimeters -- the unit name and continued use reflects that original measurement method even though digital instruments don't use actual mercury today.

1 bar equals approximately 0.9869 atmospheres -- close to but not exactly equal to one atmosphere, a common point of confusion since the two units are similar in magnitude but not identical.

Tire pressure specifications, especially for vehicles or equipment manufactured in different countries with different unit conventions, often require converting between PSI and bar to match a gauge or specification sheet using a different unit than your tool displays.