How Do I Know What Size Gas Strut I Need? Complete Sizing Guide
Three Measurements Define Your Gas Strut Size
Every gas strut is defined by three critical dimensions: eye-to-eye length when fully extended, the distance it travels when compressed (stroke), and the force it delivers in Newtons. Get any one wrong and the strut will either fail to support your load, collapse under pressure, or worse—become a safety liability.
This guide covers how to measure each dimension accurately, interpret what you find, and avoid the most common sizing mistakes that lead to premature failure or unsafe installation.
What Is Extended Length and How Do I Measure It?
Extended length is the measurement from the center of the top eye (or mounting point) to the center of the bottom eye when the gas strut is fully extended and under no load.
To measure correctly:
- Lay the strut on a flat surface with the rod pointing away from you
- Measure from the inner edge of the top eye to the inner edge of the bottom eye
- Do not compress the strut during measurement—it must be fully relaxed
- Record in millimetres (mm)
For car boot struts, this is typically 400–600 mm. For furniture lids, 200–400 mm. For industrial guards, anywhere from 150 mm to over 1,000 mm. The extended length tells you whether a strut will physically fit your application and how much space it needs when open.
What Is Stroke and Why Does It Matter?
Stroke is the distance the rod travels when the gas strut compresses from fully extended to fully compressed. It is calculated by subtracting the compressed length from the extended length.
Example: If extended length is 500 mm and compressed length is 300 mm, the stroke is 200 mm.
Stroke directly affects how far your lid, boot, or panel can open. Too little stroke and the panel won’t open far enough. Too much stroke and the strut will extend beyond the space available, causing misalignment or mechanical damage.
To find the stroke you need:
- Open the lid or panel to its desired angle (usually 90 degrees for car boots)
- Measure the distance between the two mounting points when the lid is fully open
- Measure the distance between the same two points when the lid is fully closed
- The difference is your required stroke
Understanding Gas Spring Force in Newtons
Force rating in Newtons (N) is the upward push the gas strut delivers at any point in its stroke. This is the most misunderstood dimension, but it is also the most critical for safety.
A strut rated 100 N will push with approximately 100 Newtons of force at full extension and slightly less as it compresses (pressure decreases as volume increases). For reference, 100 N ≈ 10 kg-force, so a 100 N strut lifts roughly 10 kilograms.
To determine the force you need:
- Weigh the lid or panel you are supporting (in kilograms)
- Multiply by 10 to convert kg to Newtons—this is your baseline
- Add 20–30% safety margin for friction, hinge wear, and mounting angle inefficiency
- Use the Gas Spring Force Calculator for precise force recommendations based on hinge distance and mounting angle
Example: A car boot lid weighs 15 kg. Baseline force = 15 × 10 = 150 N. With 25% safety margin: 150 × 1.25 = 188 N. You would select a strut rated 150–200 N (manufacturers typically offer ratings in 50 N increments).
What Are Gas Strut End Fittings and Which Do I Need?
The mounting eyes at each end of the strut come in several types. Choosing the wrong fitting is a common reason replacements fail to fit.
Eye-to-Eye (most common): Both ends have circular loops. Fits most automotive and furniture applications. Typically uses 8 mm or 10 mm diameter pins.
Clevis (rod end): Bottom end has a clevis fitting (two-prong fork) instead of an eye. Common on industrial machinery and some German cars.
Stud (threaded rod): One or both ends terminate in a threaded stud instead of an eye. Used when bolted mounting is required rather than pinned.
Ball Socket: One end has a ball joint for angled mounting. Rare in consumer applications but standard in some machinery.
To identify your required fitting:
- Photograph the existing mounting points on the old strut
- Note the diameter of the pin holes (usually 8 mm, 10 mm, or 12 mm)
- Check if both ends use the same fitting type or different types
- Confirm whether the end must be pinned, bolted, or welded
How Mounting Angle Affects Force Sizing
The angle at which the gas strut is mounted significantly changes the force it actually delivers to your load. A strut mounted at 90 degrees (perpendicular to the panel) works at full efficiency. Mounting at 45 degrees reduces effectiveness by approximately 30%.
This is why guessing at force rating leads to failure. A 100 N strut mounted at 45 degrees effectively delivers only ~70 N to your load.
Most car boots mount struts at 60–80 degrees, furniture lids at 70–90 degrees, and industrial guards anywhere from 30–90 degrees depending on the design.
If your application has an angled mounting:
- Measure the angle between the strut and the panel when fully closed
- Use the angle and your panel weight in the Force Calculator
- The calculator adjusts the force recommendation automatically
Gas Strut Sizing by Application: Automotive
Car boot and bonnet struts are the most commonly replaced gas struts. Sizing rules differ slightly from furniture because automotive struts must support both weight and aerodynamic pressure at highway speeds.
Boot/Trunk Struts:
- Extended length: typically 400–550 mm
- Compressed length: typically 200–300 mm
- Force range: 120–250 N (depends on vehicle weight and lid construction)
- Most common sizes: 150 N, 180 N, 200 N, 250 N
Bonnet/Hood Struts:
- Extended length: typically 350–450 mm
- Compressed length: typically 200–250 mm
- Force range: 100–180 N
- Most common sizes: 120 N, 150 N, 180 N
If you are replacing a strut on a specific make and model (Ford Focus, BMW 3 Series, etc.), the easiest approach is to note the OEM part number from your existing strut and cross-reference it with the manufacturer specifications. Alternatively, consult the specific model replacement guide on this site.
Gas Strut Sizing by Application: Furniture
Ottoman beds, storage benches, and loft hatches are the primary furniture applications. Furniture struts are usually smaller and lighter-duty than automotive equivalents but must match the exact lid weight.
Ottoman Storage Beds:
- Extended length: typically 200–300 mm
- Compressed length: typically 100–150 mm
- Force range: 80–150 N (adjust for mattress weight plus lid)
- Most common sizes: 100 N, 120 N, 150 N
Loft Hatch Doors:
- Extended length: typically 250–400 mm
- Compressed length: typically 150–200 mm
- Force range: 120–200 N (depends on hatch material and insulation)
- Most common sizes: 120 N, 150 N, 180 N
Furniture applications often have non-standard fittings (welded studs, ball joints), so verify end fittings carefully before ordering.
Industrial and Specialist Applications
Machine guards, cleanroom doors, and agricultural equipment require precise force matching because these applications are subject to regulations and safety inspections.
Industrial gas struts typically offer:
- Extended lengths: 150–1,500 mm
- Force ratings: 50–1,000 N+
- Cycle life: 100,000+ cycles (vs. 50,000 for standard struts)
- Temperature range: -40°C to +100°C (wider than standard)
- Stainless steel options (316 grade for corrosive environments)
For industrial sizing, always involve the machine or equipment manufacturer. Incorrect force rating on a guard can result in regulatory non-compliance, warranty voidance, or injury.
Common Gas Strut Sizing Mistakes to Avoid
Mistake 1: Guessing force rating. Always calculate or measure; never assume. A 50 N error in either direction causes either collapse or excessive resistance.
Mistake 2: Measuring compressed length incorrectly. Many people measure with slight force applied, which gives a false compressed length. Compress firmly and measure consistently.
Mistake 3: Ignoring mounting angle. A horizontal mounting (0 degrees) requires roughly 2× the force of a vertical mounting (90 degrees) for the same load.
Mistake 4: Ordering by price instead of spec. A £8 strut that doesn’t fit is more expensive than a £15 strut that works for five years.
Mistake 5: Assuming all eye-to-eye struts are interchangeable. Pin diameter, stroke, force, and length all vary. Match all four.
How to Use Replacement Guides and Cross-Reference Tools
If you are replacing a strut on a specific vehicle or furniture brand, use the Gas Strut Replacement Finder to locate the exact OEM specifications. This tool cross-references make, model, and year to provide verified dimensions and force ratings.
For vehicles:
- Enter vehicle make and model (e.g., Ford Focus 2015)
- Select the application (boot, bonnet, tailgate)
- The tool returns OEM part number and exact specifications
- Use these to verify any aftermarket replacement before ordering
Many aftermarket struts are designed to meet or exceed OEM specs, which is acceptable. However, some budget struts undersized on force rating to cut costs—avoid these.
Measuring Your Current Gas Strut: A Step-by-Step Method
If you have a failing strut and want to know its exact specifications without removing it, follow this method:
- Photograph the strut from multiple angles, capturing the mounting points, any visible part numbers, and the end fittings
- Measure extended length: from inner edge of top eye to inner edge of bottom eye
- Gently compress the strut fully and measure compressed length
- Calculate stroke: extended minus compressed
- Search the body or rod for a part number or force rating label (often printed in small text)
- If no marking is visible, record the measurements and consult a replacement finder tool for your specific vehicle or application
Do not attempt to force a strut beyond its natural compression point—gas struts can rupture under excessive pressure.
Next Steps: Verify Compatibility Before Purchasing
Once you have your three measurements (extended length, stroke, force), use the Gas Strut Compatibility Checker to confirm that a replacement strut fits your specific make and model. This tool prevents the common error of ordering the correct dimensions but discovering the end fittings do not match your mounting hardware.
For automotive replacements, most suppliers now offer fitment verification at checkout. For furniture and industrial applications, contact the supplier to confirm mechanical compatibility before placing your order.