The **key difference** between psi and bar is that **psi measures pressure as a one-pound force applied on an area of one square inch whereas bar measures pressure as a force applied perpendicularly on a unit area of a surface.**

Pressure, in general, is the force perpendicularly applied on a unit area of a surface. However, there are different systems that measure pressure differently. For example, the avoirdupois system uses pounds and ounces for measurements and its pressure is the one-pound force applied on an area of one square inch.

### CONTENTS

1. Overview and Key Difference

2. What is Psi

3. What is Bar

4. Side by Side Comparison – Psi vs Bar in Tabular Form

6. Summary

## What is Psi?

Psi is a unit of measurement of pressure which gives the pressure as the one-pound force applied on an area of one square inch of a surface. More accurately, the unit psi is useful to measure either pressure or stress. Moreover, this unit comes in the avoirdupois unit system. One psi is approximately equal to 6895 N/m^{2}. Further, thousand psi equals kilopounds per square inch; abbreviation ksi indicates this. The ksi unit is important in material sciences to measure tensile strength while Mpsi or mega pounds per square inch is important in mechanics as it measures elastic modulus (mainly in metals).

## What is Bar?

Bar is a unit of measurement which gives the pressure as the force applied perpendicularly on a unit area of a surface. One bar is exactly equal to 100,000 Pa. However, this value is slightly different from the average atmospheric pressure at sea level. Some other units derived from bar include Megabar, Kilobar, Decibar, Centibar and Millibar.

Moreover, one bar is approximately equal to 0.987 atm, 14.50 psi (absolute) and 750.06 mmHg. Often, we give the atmospheric air pressure in millibars. Here, the standard atmospheric pressure at sea level is 1013.25 millibar. Moreover, many engineers use the term bar rather than Pascal because, in the Pascal unit system, we have to work with large numbers.

## What is the Difference Between Psi and Bar?

Psi and bar are two common units we use to measure pressure. The key difference between psi and bar is that psi measures pressure as a one-pound force applied on an area of one square inch whereas bar measures pressure as a force applied perpendicularly on a unit area of a surface. Specifically, psi measures pressure or stress, whereas bar only measures pressure. When considering the relationship between two units, one psi equals 0.068 bar while one bar equals 14.50 psi. So, this is another difference between psi and bar.

## Summary – Psi vs Bar

Psi and bar are units of measurement of pressure. The key difference between psi and bar is that psi measures the pressure as the one-pound force applied on an area of one square inch whereas bar measures the pressure as a force applied perpendicularly on a unit area of a surface.

##### Reference:

1. “Pounds per Square Inch.” *Wikipedia*, Wikimedia Foundation, 9 Apr. 2019, Available here.

##### Image Courtesy:

1. “ReifendruckPruefen” By JEG14 – Own work (Public Domain) via Commons Wikimedia

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