Tire pressure is measured using a tire pressure gauge, with the most common measuring device being about the size of a pen. First, a bit about pressure: The atmosphere at the surface of our planet measures about 14.7 pounds (6.67 kilograms) per square inch, or a 1 inch (2.54 centimeters) square column of air weighs 14.7 pounds. This changes depending on the altitude. For example, at an altitude of 10,000 feet (3,048 meters), the air pressure decreases to 10.2 pounds (4.63 kilograms) per square inch.

But a car, truck, SUV, or bike tire needs more pressure in order to inflate. By increasing the number of atoms inside the tire, there are more collisions between the atoms and more pressure exerted on the sides of the tire. In other words, a pump stuffs more air into a constant volume (the confines of the tire), so the air pressure within the tire rises. A car tire’s pressure is typically at about 30 pounds (13.61 kilograms) per square inch; a bicycle tire’s pressure can be around 90 pounds (40.82 kilograms) per square inch.