If you're trying to figure out how to measure gas spring length, you've probably realized it's not as simple as just throwing a ruler against the metal tube and calling it a day. Whether you're replacing the struts on your car's hatchback, fixing a heavy toolbox lid, or working on some custom kitchen cabinetry, getting the dimensions wrong is a recipe for frustration. If the spring is too long, the door won't close; if it's too short, it won't open far enough, and you might even end up damaging the hinges.
The good news is that once you know what you're looking for, it only takes a minute or two to get the numbers you need. You don't need a degree in mechanical engineering, just a decent tape measure and a bit of focus on the right "points" of the spring.
Why the "Center-to-Center" Rule is Everything
When most people think about measuring something, they want to measure it from tip to tip. With gas springs, that's actually a mistake. The industry standard for how to measure gas spring length is almost always "center-to-center."
What does that mean? It means you aren't measuring the very ends of the plastic or metal connectors. Instead, you're looking for the middle of the mounting points. If your gas spring has ball sockets (the little cups that snap onto a round stud), you measure from the center of one ball to the center of the other. If it has eyelets (the flat ends with a hole through them), you measure from the center of one hole to the center of the other.
Why do we do it this way? Because the "total length" can change depending on how thick the plastic casing is on the ends, but the distance between the pivot points is what actually determines how the hardware moves. If you get the center-to-center measurement right, the spring will fit perfectly into the existing brackets.
Getting the Extended Length Right
To start, you really need to measure the gas spring when it is in its fully extended position. If the spring is still attached to a vehicle or a piece of furniture, open the lid or hatch as far as it will go. If the spring is "tired" and can't hold itself up, you might need a buddy to hold the lid open for you while you work.
Lay your tape measure at the center of the top mounting point and pull it down to the center of the bottom mounting point. This is your Extended Length. It's the most important number in the whole process. If you're holding a loose gas spring in your hands that's already off the machine, it should naturally be in its extended state unless it's locked or jammed. Just make sure it's pushed all the way out before you lock in that number.
Don't Forget the Stroke
The second piece of the puzzle is the "stroke." This is basically the travel distance—how far the skinny rod can actually move inside the thicker tube.
To measure the stroke, look at the shiny, thinner metal rod (the piston rod) that slides in and out of the black or silver cylinder (the pressure tube). You want to measure the length of that rod from the point where it exits the tube down to where it meets the end fitting.
Knowing the stroke is vital because it tells you how much "swing" your door or lid has. If you buy a replacement with the right total length but a stroke that's too short, your lid might only open halfway before the spring "bottoms out" internally. That puts a ton of stress on the mounting brackets and can actually rip them right out of the wood or metal.
Measuring the Diameters
While we're talking about how to measure gas spring length, we should probably mention the thickness of the parts, too. A spring's strength is often tied to its size. You'll want to measure two diameters:
- The Rod Diameter: This is the thickness of the skinny inner rod. Common sizes are usually around 6mm, 8mm, or 10mm.
- The Tube (Cylinder) Diameter: This is the thickness of the outer, fatter part. You'll often see these in pairs, like a "6/15" (6mm rod and 15mm tube) or an "8/18."
A simple pair of calipers is the best tool for this, but if you don't have those, you can wrap a piece of string around the tube, measure the string, and do a little "circumference to diameter" math (divide by 3.14). Or, honestly, just eye it against a metric ruler; usually, the differences are distinct enough that you can tell an 8mm rod from a 10mm one.
The Compressed Length: A Quick Double Check
You don't always have to measure the compressed length if you have the extended length and the stroke, but it's a great way to double-check your work. The math is simple:
Extended Length - Stroke = Compressed Length.
If you measure the physical spring while it's compressed (which is hard to do because these things are under a lot of pressure—never try to squash one by hand just to measure it!), you'll find it matches that math. The reason this matters is clearance. When your trunk or cabinet is closed, that gas spring has to fit into the available space. If the compressed length is too long, the door won't shut. It'll hit a "hard stop" because the spring physically can't get any shorter.
Dealing with Different End Fittings
Sometimes, the way a gas spring is measured can get confusing because the end fittings are different. You might have a ball joint on one end and an eyelet on the other. Don't let that trip you up. Stick to the "center" rule.
If you have a threaded end (where the fitting has been unscrewed), you measure from the base of the threads. However, most people are looking for a direct replacement for an existing assembly. If you plan on reusing your old end fittings, you'll need to measure the length of the spring without them, from the base of the threaded studs. But if you're buying a whole new unit, measuring center-to-center with the fittings attached is the way to go.
Check the Label Before You Work Too Hard
Before you get too deep into the weeds of tape measures and math, take a close look at the body of the gas spring. Most manufacturers print the specs right on the cylinder.
You're looking for a string of numbers and letters. Often, you'll see something like "0500N," which tells you the force (500 Newtons). You might also see numbers that look like "150/350." In many cases, those numbers represent the stroke and the extended length in millimeters.
If the label is still readable, it's a gold mine. It saves you the trouble of worrying if your tape measure slipped by a fraction of an inch. If the label is worn off or painted over, though, then the manual measurements we've talked about are your only path forward.
A Few Safety Tips
Since you're getting up close and personal with these parts, keep in mind that gas springs are essentially little bombs of pressurized nitrogen. They are very safe when handled normally, but you don't want to mess around with the cylinder.
- Don't try to compress them by hand. A standard car strut might have 100 pounds or more of pressure. You aren't going to budge it just by leaning on it, and if you slip, you could hurt yourself.
- Don't scratch the rod. When you're measuring the stroke, be gentle. If you nick or scratch that shiny rod with the metal tip of your tape measure, it can tear the internal seal next time it slides in. Once that seal is gone, the gas leaks out, and the spring becomes a paperweight.
- Measure twice, buy once. It sounds cliché, but being off by 5mm can be the difference between a smooth-operating hatch and one that binds up every time you use it.
Wrapping It Up
Learning how to measure gas spring length isn't exactly a thrilling Saturday afternoon activity, but it's one of those DIY skills that pays off immediately. By focusing on the center-to-center distance, identifying the stroke, and checking the diameters, you're ensuring that whatever you're fixing will work better than it did before.
Just remember: start with the spring fully extended, measure from the center of the pivot points, and always double-check the label for a part number. Do that, and you'll have the right replacement part ordered in no time.