Gigapascals, atmospheres, millimeters of mercury – when you read these common units for measuring pressure, your head might start spinning. It can feel especially overwhelming if you must convert between units. However, with a basic understanding of units and prefixes, the concept of pressure and unit conversion is straightforward and easy to master.

## What is Pressure?

When gas or fluid fills a container, the individual atoms and molecules in that substance don’t sit still. Instead, they move around inside the container, bouncing off its walls. This movement creates force or stress pushing against the walls of the container. This is pressure, and it's measured in units of force (or stress) per unit of square area.

The concept of physical pressure is all around you in the real world. You must understand pressure when checking or filling a bicycle or automobile tire to the manufacturer’s specifications. When it comes to weather, you hear about atmospheric pressure, or the pressure the atmosphere exerts on the planet. In terms of personal health, many people measure blood pressure every day; this is the measurement of the pressure your blood cells exert on the walls of your blood vessels during and between heartbeats.

## Units and Prefixes

Common units for measuring pressure include pounds per square inch (psi), atmospheres (atm), bars, millimeters of mercury (mmHg) and pascals (Pa). This last unit—pascals—is part of the International System of Units and therefore uses prefixes to indicate larger or smaller values. For example, a megapascal (MPa) comprises a million pascals because "mega" indicates "million" while a gigapascal (GPa) comprises a billion pascals because "giga" indicates "billion." One of the most important things to remember when it comes to pressure is that one pascal is equal to one newton per square meter (N/m^{2}).

## Converting from GPa to N/mm2

In order to convert from gigapascals (GPa) to newtons per square millimeter (N/mm^{2}), you must perform step-wise conversions. First, notice the prefix attached to GPa and convert to the base unit Pa. To do this, multiply the value—for instance, 3 GPa—by the value of the prefix “giga,” or 1 billion. 3 GPa is the same as 3 billion pascals.

Next, remember that one pascal equals one newton per square meter (N/m^{2}). This means you can directly substitute N/m^{2} so that your value now reads 3 billion N/m^{2}. Finally, notice the prefix attached to your goal unit, N/mm^{2}. When converting from m^{2} to mm^{2}, your prefix is “milli,” or 1 thousand. To move from the larger m^{2} to the smaller mm^{2}, divide your value by 1 thousand. In the example, you end up with 3 million N/mm^{2}. Therefore, 3 GPa is the same as 3 million N/mm^{2}.