Measuring gas shocks requires two key numbers: the extended length and the collapsed (retracted) length, both taken center-to-center between the mounting points. These measurements let you find the correct replacement whether you’re shopping for vehicle shocks, hatch lift supports, or any other gas strut application. The process is straightforward, but getting it wrong by even half an inch can mean a shock that bottoms out, doesn’t hold weight, or fails to provide enough travel.
What You’ll Need
A standard tape measure works for most gas shock measurements. If you want more precision, especially on smaller struts like those on toolbox lids or tonneau covers, a digital caliper is a better choice. For coilover shocks with a spring wrapped around them, you’ll also need a spring compressor to safely remove the spring before measuring the shock body itself. A torque wrench is helpful if you plan to reinstall shocks after measuring.
Measuring Extended and Collapsed Length
Every gas shock has two critical dimensions. The extended length is the full length of the shock when it’s completely stretched out. The collapsed length is the full length when compressed as far as it will go. Both measurements are taken from the center of one mounting point to the center of the other.
The mounting points vary by design. Some shocks use ball sockets, others use eyelets (metal loops), and some attach with brackets and rivets. Regardless of the type, you always measure from the center of one attachment point to the center of the other, not from the tips or edges of the hardware. This center-to-center method ensures your replacement will fit the same mounting distance.
To get the extended length, pull the shock fully open and measure. For the collapsed length, push the shaft all the way in and measure again. If the shock is still functional and holds pressure, you may need to apply steady force to compress it fully. If it’s dead and no longer holds any resistance, it will collapse on its own, which actually makes measuring easier.
Measuring While Installed on a Vehicle
You don’t always need to remove a shock to get useful measurements. If you’re replacing suspension shocks on a car or truck, you can measure the distance between mounting points while they’re still bolted in place. This gives you the installed length at your current ride height, which is a useful starting point for choosing the right replacement.
To do this properly, set the vehicle to its desired ride height first. Use a jack or add the weight the vehicle normally carries (passengers, fuel, cargo) to simulate real driving conditions. If you have adjustable spring collars, finalize those settings before measuring. Then measure center-to-center between the upper and lower mounting points. For eyelet-to-eyelet shocks, that’s straightforward. For stud-mounted shocks, measure from the center of the lower mounting point to the lower washer at the upper stud.
This on-vehicle measurement tells you where the shock sits in its travel range at normal ride height, which matters more than most people realize.
Why Stroke Length and Travel Matter
Getting the right extended and collapsed length isn’t just about physical fit. It determines how much travel the shock has in both directions: compression (hitting a bump) and rebound (extending back out). A shock that’s too short for your application might run out of rebound travel. One that’s too long might bottom out under compression.
A good rule of thumb is that the shock piston should sit near the middle of its travel range at normal ride height, ideally within the middle third of the total stroke. Here’s a concrete example: if your center-to-center measurement at ride height is 14.5 inches, a shock with a 5-inch stroke works well. That shock would measure about 11.6 inches collapsed and 16.9 inches extended, placing your 14.5-inch ride height right in the sweet spot with room to move in both directions.
Getting this wrong can cause real problems. Consider a setup where the mounting points sit 10.75 inches apart at ride height. A shock that measures 7.88 inches collapsed and 11.00 inches extended would leave only a quarter inch of rebound travel, meaning the shock is nearly topped out just sitting still. A better match would be a shock measuring 8.63 inches collapsed and 12.88 inches extended, giving adequate room for both compression and rebound.
Measuring Gas Struts for Non-Vehicle Applications
Gas shocks on hatches, doors, lids, and cabinets follow the same center-to-center principle, but the process is simpler because you can usually remove the strut by hand. Most lift supports pop off their ball-socket mounts with a flathead screwdriver or a small pry tool.
Once removed, extend the strut fully and measure center-to-center between the ball sockets (or eyelets). Then compress it fully and measure again. Write down both numbers. You’ll also want to note the diameter of the shaft and the cylinder body, since replacement struts need to physically fit the same space. Some applications have tight clearances where a slightly wider cylinder won’t work even if the length is correct.
For lift supports specifically, force rating also matters. Two struts can share identical dimensions but have very different gas pressure inside. If you’re replacing a hatch strut that no longer holds the hatch open, you need both the correct measurements and the correct force rating, usually listed in newtons or pounds on the original strut’s label.
Common Measurement Mistakes
The most frequent error is measuring tip-to-tip instead of center-to-center. Measuring to the very end of a ball socket or the outside edge of an eyelet adds extra length that will throw off your replacement sizing. Always find the center of the mounting hardware on each end.
Another common mistake is measuring a gas strut that’s partially failed. If the shock has lost some pressure, it may not extend to its full original length, giving you a shorter extended measurement than the actual spec. If the strut feels weak or won’t stay open on its own, note that your extended measurement might be shorter than the original. Checking the part number printed on the shock body and looking up the manufacturer’s specs can confirm whether your measurement matches.
Finally, people sometimes forget to measure both lengths. Online retailers and manufacturers list gas shocks by both extended and collapsed dimensions. If you only have one number, you’re guessing on the other, and two shocks with the same extended length can have very different stroke lengths depending on their collapsed measurement.