Everyday Life · Unit Conversions · Energy & Power
Pressure Unit Converter
Converts a pressure value from one unit to another, including Pascal, bar, psi, atm, mmHg, and kPa.
Calculator
Formula
P_out is the converted pressure value. P_in is the input pressure value. F_in is the conversion factor of the input unit to Pascals. F_out is the conversion factor of the output unit to Pascals. All units are first normalised to Pascals as a common base, then converted to the target unit.
Source: NIST Special Publication 811 — Guide for the Use of the International System of Units (SI), 2008 Edition.
How it works
Pressure is defined as force per unit area. The SI base unit is the Pascal (Pa), equal to one Newton per square metre. Because pressure appears in so many fields — from weather forecasting to engine design to blood pressure measurement — dozens of units have evolved historically. Standard atmospheric pressure at sea level, for example, equals exactly 101,325 Pa, which is also 1 atm, approximately 14.696 psi, 1013.25 mbar, or 760 mmHg. Each of these units was developed independently in different scientific or industrial communities, which is why conversions between them are so frequently needed.
The conversion method used here is straightforward: every unit has a precisely defined conversion factor to Pascals. To convert from unit A to unit B, the input value is multiplied by A's factor and divided by B's factor. For example, to convert 1 bar to psi, we calculate 1 × 100,000 ÷ 6894.757 ≈ 14.5038 psi. This approach works for all unit pairs without needing a separate table for every possible combination. The conversion factors used in this calculator are sourced from NIST and are exact where international definitions permit.
Common applications include: checking tyre inflation (psi vs. bar), calibrating industrial pressure gauges (bar vs. MPa), reading weather maps (hPa/mbar), understanding blood pressure (mmHg), working with HVAC systems (inH2O or cmH2O), and designing hydraulic equipment (MPa or bar). Being comfortable with pressure unit conversions is essential for any technical discipline that crosses regional or industrial boundaries.
Worked example
Suppose you need to inflate a car tyre to 2.4 bar, but your tyre gauge reads in PSI. Here is how to convert:
Step 1 — Identify input and output units. Input: 2.4 bar. Output: PSI. Conversion factor for bar = 100,000 Pa. Conversion factor for PSI = 6,894.757 Pa.
Step 2 — Apply the formula. Result = 2.4 × (100,000 ÷ 6,894.757) = 2.4 × 14.5038 ≈ 34.81 PSI.
So a tyre inflated to 2.4 bar should read approximately 34.8 PSI on a gauge calibrated in imperial units. This is a very common conversion in everyday automotive maintenance across countries using different unit systems.
As a second example, convert standard atmospheric pressure of 101,325 Pa to atm: 101,325 × (1 ÷ 101,325) = 1.000 atm exactly, confirming the definition is self-consistent.
Limitations & notes
This converter assumes all input values represent absolute or gauge pressure in a consistent manner — it does not distinguish between absolute pressure (measured relative to a perfect vacuum) and gauge pressure (measured relative to local atmospheric pressure). If you are working with gauge pressure in one unit and need absolute pressure in another, you must add or subtract atmospheric pressure manually before converting. Additionally, some specialised fields (such as vacuum technology) use units like Torr or Pascal interchangeably with mmHg; while 1 Torr ≈ 1 mmHg for most practical purposes, they differ by about 0.000016%, which this calculator treats as identical using the 133.322 Pa factor. Very high precision scientific work may require more significant figures than those available here. Finally, cmH2O uses the standard gravity value of 9.80665 m/s² — if local gravity differs significantly (e.g. at high altitude or other planets), the cmH2O factor will not be exact.
Frequently asked questions
How many PSI is 1 bar?
1 bar equals approximately 14.5038 PSI. This is one of the most common pressure conversions in automotive and industrial work. To convert bar to PSI, multiply by 14.5038. To convert PSI to bar, divide by 14.5038 or multiply by 0.0689476.
What is the difference between Pa and kPa?
1 kPa (kilopascal) equals exactly 1,000 Pa (Pascal). The Pascal is the SI base unit of pressure. kPa is commonly used in weather reporting, tyre pressure (in some countries), and medical applications because the Pa is very small — standard atmospheric pressure is about 101,325 Pa or 101.325 kPa.
What is 1 atm in different pressure units?
1 standard atmosphere (atm) equals exactly 101,325 Pa, 101.325 kPa, 1013.25 mbar, approximately 14.6959 PSI, 760 mmHg (Torr), 29.9213 inHg, and approximately 1.01325 bar. It is defined as the mean atmospheric pressure at sea level and is a fundamental reference point in pressure work.
Why is mmHg used to measure blood pressure?
mmHg (millimetres of mercury) became the standard for blood pressure measurement because early sphygmomanometers used mercury columns to measure pressure. A column of mercury 1 mm tall exerts a pressure of approximately 133.322 Pa. Normal adult blood pressure of 120/80 mmHg equals about 16.0/10.7 kPa. Despite mercury manometers being phased out, the mmHg unit persists as the clinical standard worldwide.
When should I use MPa instead of bar or PSI?
MPa (megapascal) is preferred in high-pressure engineering applications such as hydraulic systems, structural material testing, and pressure vessel design, where pressures commonly reach hundreds of bar. 1 MPa equals exactly 10 bar or approximately 145.038 PSI. In automotive engineering, fuel injection pressures (200–2000 bar) are often expressed in MPa to avoid large numbers, while tyre pressures (2–3 bar) are more naturally expressed in bar or PSI.
Last updated: 2025-01-15 · Formula verified against primary sources.