How to Measure Lift Support Struts: Complete Technical Guide

Why Accurate Gas Strut Measurement Matters

Measuring a gas lift strut correctly is the foundation of successful replacement. An incorrectly sized strut will either fail to hold the lid or panel open, or it will hold it too aggressively, creating a safety hazard. Undersized struts will gradually sag, leading to repeated failures. Oversized struts can damage hinges, slam shut unexpectedly, or cause injury. The difference between a working installation and a failed one often comes down to a single measurement taken at the wrong extension point or a force rating misread by 50 Newtons.

Whether you’re replacing a car boot strut, a kitchen cabinet door support, a caravan storage locker lid, or an industrial machine guard, the measurement method remains the same. This guide walks you through every dimension you need and explains what each one tells you about strut specification.

The Three Core Measurements You Need

Every gas strut requires three fundamental measurements: extended length (also called open length), compressed length (or closed length), and stroke. These three dimensions define the strut’s geometry and determine whether a replacement will fit your application.

• Extended Length (Open Length): The total length of the strut when fully compressed by hand or in its natural resting state under minimal force.
• Compressed Length (Closed Length): The length of the strut when fully extended and cannot be pushed any further.
• Stroke: The difference between extended and compressed length. This is the distance the rod travels.

The fourth critical dimension—force rating in Newtons—must be read from the strut body itself or verified through the gas spring force calculator based on your application weight and mounting geometry.

Measuring Extended Length: The Resting State

Extended length is the distance from one end of the strut to the other when the strut is at rest with no external force applied. This is the easiest measurement to take, but precision matters.

Method: Place the strut on a flat workbench with both end fittings resting flush against the surface. Using a steel ruler or tape measure, measure from the centre of one end fitting to the centre of the other end fitting. Record this measurement in millimetres.

Common mistakes: Measuring from the outer edge of the end fitting (adds 5–15 mm depending on fitting type) or measuring at an angle rather than straight along the strut body. Always measure end-to-end through the centre point of each fitting.

Why this matters: Extended length tells you the space the strut will occupy when the lid or panel is closed. If you have a 600 mm cavity but your replacement strut is 650 mm extended, it will not fit.

Measuring Compressed Length: Full Compression

Compressed length is measured when the strut is pushed to its limit—the point where the rod cannot travel any further into the barrel. This requires careful handling because excessive force can damage the strut or cause sudden extension.

Method: Hold the strut vertically with the rod end pointing downward. Slowly apply steady hand pressure to the rod, pushing it into the barrel until you feel firm resistance. Do not force it; stop at the point of natural resistance. Measure from the centre of the top (barrel end) to the centre of the rod end fitting. Record in millimetres.

Alternative method (safer for delicate struts): Place the strut horizontally on a workbench. Position a steel block or thick book at each end. Gently slide the rod into the barrel by moving the end fittings closer together until you feel resistance. Measure the total length at this point.

Why precision is critical: Compressed length determines the footprint of the strut when the lid or panel is closed. If your compressed measurement is too large, the strut will protrude beyond the mounting surface and prevent the lid from closing flush.

Safety note: Gas struts store significant force. If a strut suddenly extends during measurement, it can cause hand injury. Always use steady, controlled pressure and never strike or drop a strut.

Calculating Stroke: The Critical Middle Step

Stroke is not a direct measurement—it is calculated by subtracting compressed length from extended length. Stroke tells you how far the rod travels and confirms the strut’s operating range.

Formula: Stroke (mm) = Extended Length (mm) − Compressed Length (mm)

Example: Extended length 600 mm, compressed length 350 mm = stroke of 250 mm.

Why stroke matters: Stroke determines how much vertical or horizontal movement the strut allows. A caravan storage locker lid might need a 150 mm stroke to open fully, while a car boot requires 300–400 mm. If you specify a strut with too little stroke, the lid will not open enough. Too much stroke means wasted space and potential over-extension.

Use the gas spring force calculator to verify that your stroke measurement aligns with the application requirements and mounting angle.

Reading Force Rating from the Strut Body

Every gas strut has a force rating stamped, etched, or printed on the barrel. This is measured in Newtons (N) and indicates the lifting force the strut provides across its stroke. This number is non-negotiable—you must match it exactly or within a tight tolerance.

Where to look: The force rating is typically stamped on the side of the barrel near one end fitting. It may be preceded by "N", "kN", or just the number followed by letters indicating the strut type.

Common formats:

• 100N (100 Newtons—typical for light cabinet doors)
• 500N (500 Newtons—typical for car boot struts)
• 1000N or 1.0kN (1,000 Newtons—heavy industrial or large lids)

What if the stamp is worn? If the force rating is illegible, do not guess. The strut cannot be safely reused. You must measure the application (lid weight, hinge distance, angle) and use the force calculator to determine the correct replacement rating.

Matching force rating to application: A 300 N strut under a 500 N load will fail within weeks. A 1000 N strut holding a 300 N lid will snap the hinges or prevent the lid from closing gently. Force must match the load.

End Fitting Type Identification

Gas struts use different end fittings for different mounting styles. Identifying your strut’s end fittings is essential because a replacement must use the same (or compatible) fittings.

Common end fitting types:

• Eye-to-eye: Both ends have a circular eye (hole) for pin or bolt mounting. Most common for automotive and furniture applications.
• Stud-to-eye: One end has a threaded stud, the other has an eye.
• Ball socket: One or both ends have a ball joint for swivel mounting. Common in industrial machinery.
• Clevis (clevis pin): A flat bracket end for side-mounting. Found on some machine guards and hatches.

Measure and note the diameter of each end fitting hole or stud. Record in millimetres. This ensures your replacement will accept the same mounting hardware.

Measuring Hinge Distance for Force Calculator Verification

Once you have your three measurements (extended, compressed, stroke), verify the force rating using the application specifics. You will need the hinge distance—the distance from the hinge pivot point to the strut mounting point.

Method: If the strut is still installed, measure from the hinge pivot centre to the centre of the strut’s mounting eye. This distance, combined with the lid weight and stroke angle, determines the force needed.

Why this matters: A 400 mm hinge distance with a 3 kg lid requires roughly 37 N per strut (if two struts are used). A 600 mm hinge distance with the same lid requires roughly 56 N per strut. The mounting geometry changes the force requirement dramatically.

Input your measurements into the gas spring force calculator to confirm your strut’s force rating is correct for your specific geometry.

Measuring Mounting Angles

Gas struts perform most efficiently at specific mounting angles. If the strut is mounted too vertically or too horizontally, its force output changes and can cause performance issues.

Ideal mounting angle: Most gas struts are designed to operate at 15–90 degrees from horizontal. Measure the angle between the strut body and the mounting surface (lid, panel, or door) when fully open.

How to measure angle: Use a digital angle finder or a standard protractor positioned against the strut barrel. Record the angle in degrees. If the angle is less than 15 degrees or greater than 90 degrees, the strut may not perform as rated and force requirements need recalculation.

Recording Your Measurements: The Data Sheet

Create a simple record of your measurements before purchasing a replacement:

• Extended length: _____ mm
• Compressed length: _____ mm
• Stroke: _____ mm
• Force rating (stamped on barrel): _____ N
• End fitting type: eye-to-eye / stud-to-eye / ball socket / clevis
• End fitting hole/stud diameter: _____ mm
• Hinge distance (if applicable): _____ mm
• Mounting angle: _____ degrees
• Application: (car boot / cabinet door / caravan locker / etc.)
• Lid or panel weight: _____ kg

This data sheet is your specification. When you search for a replacement or contact a supplier, you have exact dimensions—no guesswork.

Common Measurement Errors and How to Avoid Them

Error 1: Measuring at an angle — Always measure straight along the strut body, not diagonally. Use a ruler or tape measure placed flush against the barrel.

Error 2: Not accounting for end fitting thickness — Measure from the centre of each end fitting, not from the outer edge. This eliminates variance from different fitting styles.

Error 3: Compressing to the wrong point — Stop at natural resistance, not at maximum force. Over-compression can distort the strut and give you a false measurement.

Error 4: Forgetting to verify force rating — Never assume the force rating is correct just because it matches the original strut. Use the calculator to confirm it matches your application load and geometry.

Error 5: Measuring a damaged strut — If the strut shows signs of leaking (oil marks), dents, or corrosion, do not rely on its measurements. Strut performance may already be compromised.

When to Measure a Strut in Situ

If the strut is still installed on your lid, door, or panel, you can measure it without removal:

• Extended length: Close the lid fully and measure the strut end-to-end
• Compressed length: Open the lid fully and measure the strut end-to-end
• Stroke: Calculate the difference
• Force rating: Read from the barrel (may require removing dust or debris)

In situ measurement is faster but can be less precise if the mounting surface is not flat or if the strut is partly obscured. Removing the strut for measurement on a flat workbench is always more reliable.

Using Your Measurements to Find the Right Replacement

Once you have accurate measurements, use them to specify a replacement. Match all four dimensions:

• Extended length (±5 mm tolerance acceptable)
• Stroke (must be exact or within 10 mm)
• Force rating (must be exact for safety—no guessing)
• End fitting type (must be identical)

Search using your stroke and force together. For example: "300 mm stroke 500 N gas strut eye-to-eye" will yield far better results than generic searches. Visit a trusted supplier specialising in gas struts with your measurements and they can match you to the exact replacement.

Next Steps After Measuring

Armed with your three core measurements and force rating, you are ready to:

• Purchase an exact replacement strut for your vehicle or application
• Confirm compatibility before ordering
• Install the replacement with confidence

For vehicle-specific replacements, use our gas strut replacement finder by make and model to narrow results to your exact vehicle before comparing your measurements.