How Do I Determine What Size Gas Strut I Need? Complete Guide

Byrealshark64

Why Getting Gas Strut Size Right Matters

Selecting the wrong size gas strut is one of the most common mistakes in mechanical assembly. An undersized strut won’t hold your boot lid, bonnet, or cabinet door open. An oversized strut will slam shut dangerously or won’t compress properly. The financial cost is low—replacing a strut costs £15–£80—but the safety and functional cost of getting it wrong is significant.

Gas strut sizing is not guesswork. Three measurements determine the correct size: eye-to-eye length when extended, the stroke length (how far it compresses), and the force rating measured in Newtons. Measure once, order once, install once.

The Three Critical Measurements

Gas struts are defined by three technical parameters. Understanding each one prevents incorrect orders and installation failures.

1. Extended Length (Eye-to-Eye)

This is the distance from the centre of the top mounting eye to the centre of the bottom mounting eye when the strut is fully extended.

Extended length is measured in millimetres. It determines whether the strut physically fits the space between your mounting points. If your extended length is 500 mm and you order a 400 mm strut, it simply won’t reach. If you order 600 mm, it will protrude beyond the mounting points and bind.

To measure extended length:

  • Remove the old strut (or, if measuring a space without a strut, measure between mounting point centres)
  • Use a steel ruler or tape measure
  • Measure from the top eye centre to the bottom eye centre when the strut is pulled to its maximum length
  • Record in millimetres
  • Add no allowance—order the exact measurement

Common automotive extended lengths: 380 mm, 400 mm, 450 mm, 500 mm, 550 mm, 600 mm, 650 mm, 700 mm.

Common furniture extended lengths: 280 mm, 300 mm, 350 mm, 400 mm, 500 mm.

2. Stroke Length (Compressed Distance)

Stroke is how far the strut compresses from fully extended to fully compressed. It’s measured in millimetres and is the difference between extended length and compressed length.

Stroke determines the working range of your lid or panel. A boot lid with a 100 mm stroke will open 100 mm. If you select a strut with a 50 mm stroke, the boot won’t open far enough.

To measure stroke:

  • Measure the fully extended strut from eye centre to eye centre (this is your extended length)
  • Push the strut down by hand until it stops compressing (do not force it)
  • Measure from eye centre to eye centre in the fully compressed state
  • Subtract compressed length from extended length: this is your stroke
  • Example: 500 mm extended − 400 mm compressed = 100 mm stroke

Common strokes: 50 mm, 75 mm, 100 mm, 125 mm, 150 mm, 200 mm.

If you cannot access a physical strut to measure, consult your gas strut size finder tool by entering your vehicle or furniture make and model.

3. Force Rating in Newtons

Force rating is the weight-support capacity of the strut, measured in Newtons (N). This is the most critical dimension and the one most often misunderstood.

Force rating determines whether your strut can hold your lid or panel open. A 50 N strut supports roughly 5 kg. A 100 N strut supports roughly 10 kg. A 200 N strut supports roughly 20 kg. (These are rough conversions; 1 kilogram-force ≈ 9.81 Newtons.)

The force rating is printed on the strut body. Look for a number followed by “N” stamped or printed on the cylinder.

To determine the correct force rating, you must know the weight of the lid or panel and the mounting angle. Use the gas spring force calculator to calculate the exact Newton force needed for your application.

As a general rule:

  • Horizontal lids (kitchen cabinets, storage boxes): force ≈ weight in kg × 9.81 N/kg
  • Angled lids (car boots at 30° opening angle): force ≈ weight × 9.81 × sin(opening angle)
  • Vertical panels (machine guards): force ≈ weight × 9.81 (sometimes higher if panels are tall and need quick closing)

Common force ratings: 50 N, 80 N, 100 N, 150 N, 200 N, 300 N, 400 N, 500 N, 750 N, 1000 N.

How to Measure Your Current Gas Strut

If you have a failed or working strut to measure, this is the fastest method.

Step 1: Read the Force Rating

The force in Newtons is printed on the cylinder barrel. Common locations: the side of the rod tube (clearest), or near the base eye. It appears as a number with “N” suffix, often alongside a date code.

Example markings: “100 N”, “150N”, “STABILUS 200 N”.

Step 2: Measure Extended Length

Pull the strut to full extension by hand. Measure from the centre of the top eye to the centre of the bottom eye. Record in millimetres.

Step 3: Measure Compressed Length

Push the strut down until the rod is fully retracted (you will feel resistance). Measure eye-to-eye again. Record in millimetres.

Step 4: Calculate Stroke

Stroke = Extended length − Compressed length.

You now have three numbers: Extended (mm), Stroke (mm), Force (N). These are your ordering specifications.

How to Measure Without a Reference Strut

If the old strut is missing or you are specifying a new installation (e.g., a custom furniture build or classic car conversion), measure the space directly.

For Horizontal Applications (Cabinets, Storage Boxes, Loft Hatches)

  • Measure the distance between the two mounting points when the lid or hatch is fully open and the opening mechanism is relaxed
  • This is your extended length
  • Measure the distance when the lid is fully closed
  • This is your compressed length
  • Calculate stroke as the difference

For Automotive Applications (Boot Lids, Bonnets)

  • Open the boot or bonnet fully
  • Measure from the mounting point on the vehicle body to the mounting point on the lid
  • Close the lid gently and measure again
  • The difference is your stroke
  • The opened measurement is your extended length

For Furniture with Known Dimensions

If measuring a new build or drawing, measure the horizontal distance the lid travels. Typically:

  • Kitchen overhead cabinets: 200–250 mm stroke
  • Ottoman storage beds: 350–450 mm stroke
  • Motorhome storage lockers: 300–400 mm stroke

For force rating in new builds, weigh the lid or panel, then use the force calculator with your mounting angle to determine the correct Newton force.

Understanding Gas Strut End Fittings and Mounting

Beyond the three main dimensions, the mounting type must match your installation.

Common end fitting types:

  • Eye-to-eye: Both ends have circular eyes for bolt mounting. Most common for automotive and furniture. Order as “extended length, stroke, force, eye-eye” or “EE”.
  • Stud-to-eye: One end has a threaded stud (male); the other has an eye. Used in some automotive applications and machinery. Order as “extended length, stroke, force, stud-eye” or “SE”.
  • Clevis: A forked eye allowing side-to-side movement. Used in machinery and industrial guards. Requires separate clevis pin.
  • Ball socket: One or both ends have ball-and-socket connectors for articulation. Specialist applications.

Read your gas strut end fittings guide for detailed compatibility and installation notes.

Vehicle-Specific Sizing

For automotive applications, make and model matter. While generic sizing works, OEM fitment ensures perfect geometry and safety.

Common automotive sizing by model:

  • Ford Focus boot: Typically 500 mm extended, 100 mm stroke, 150 N per strut (two struts, so total 300 N system support)
  • VW Golf boot: Typically 430 mm extended, 120 mm stroke, 120 N per strut
  • BMW 3 Series boot: Typically 500 mm extended, 100 mm stroke, 150 N per strut
  • Toyota RAV4 bonnet: Typically 400 mm extended, 80 mm stroke, 100 N per strut (two struts)

Never rely on these as absolutes. Measure your vehicle or consult the gas strut replacement finder by make and model to confirm fitment before ordering.

Common Sizing Errors and How to Avoid Them

Error 1: Confusing Extended Length with Compressed Length

Extended length is always larger. If you measure 500 mm when the strut is open and 400 mm when closed, 500 mm is extended. Order 500 mm extended, not 400 mm.

Error 2: Ignoring Force Rating

A strut with the correct length but wrong force will not function. A 100 N strut on a 15 kg lid will not hold it open. Always measure or calculate force.

Error 3: Mixing Metric and Imperial

UK and European struts are specified in millimetres. US struts are sometimes listed in inches. 1 inch = 25.4 mm. Always confirm units when ordering.

Error 4: Assuming Symmetry

Not all lids open symmetrically. A boot lid that opens 100 mm at the back might open 80 mm at the front due to hinge geometry. Measure the actual mounting points, not assumptions.

Error 5: Ordering Single Strut When Two Are Required

Most automotive and large furniture applications use two struts. Always specify “pair” or “set of 2” when ordering.

Using the Gas Spring Force Calculator

For the most accurate force rating, use the gas spring force calculator. Input:

  • Lid or panel weight in kilograms
  • Distance from hinge to gas strut mounting point (lever arm) in millimetres
  • Opening angle in degrees (typically 30–90° for automotive, 60–120° for furniture)
  • Number of struts supporting the load

The calculator returns the recommended Newton force range. Order within that range.

Where to Confirm Your Measurements

Before placing an order, double-check your three measurements against these sources:

  • Original equipment manual: Most vehicles have strut specifications in the owner’s handbook or technical bulletins
  • OEM parts catalogue: Dealership parts lists show exact fitment by VIN or registration number
  • Aftermarket supplier fitment guides: Online retailers like those at aritechgassprings.com cross-reference make/model to compatible strut sizes
  • Replacement part label: If the original strut has failed in situ, its label shows the exact replacement spec

Summary: Your Three-Step Sizing Process

To determine the correct gas strut size:

Step 1: Measure extended length (eye-to-eye when fully open) in millimetres.

Step 2: Measure stroke (compressed distance) by subtracting compressed length from extended length, in millimetres.

Step 3: Determine force rating in Newtons by reading the old strut, weighing the load and calculating, or consulting a make/model fitment guide.

Order with these three specifications plus end fitting type (typically eye-eye for automotive and furniture), and you will receive the correct replacement. If you remain uncertain on any measurement, contact a supplier or use an online fitment tool before committing to an order.

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