Purchase a skateboard deck and identify what the width of the skateboard is. Standard skateboards range in width from of 6.5 to 8 inches and in length from 28 to 33 inches. The skateboard trucks will be the same width as the skateboard they are on. Meaning if you buy a complete skateboard that is 7.5-inches wide, the skateboard trucks will also be 7.5-inches wide.
Measure individual trucks that are not attached to a skateboard. To do this, simply measure the top of the trucks from the tip of the left axle to the tip of the right axle. The axles are the two end pieces of the trucks where the skateboard wheels will be placed. You will get a measurement between 6.5 to 8-inches wide.
Repeat step 2 to measure the second skateboard truck you are going to put on your skateboard. This should be the same measurement as you got in step 2. If the two trucks are not the same width your balance will be noticeably incorrect.
Convert the measurements that you got in steps 2 and 3 into millimeters. Most trucks are labeled in inches, however, some brands still use millimeters to label their truck's sizes. To convert inches into millimeters, divide the inch measurement you got in steps 2 and 3 by 0.03937007874.
Attach your trucks to the skateboard and bolt the wheels onto the axles of the trucks. Look down the side edges of the skateboard, if you have the right size trucks for your board, the wheels and the deck's edge will line up perfectly.