Gas Strut Calculator

Table of contents

• What is the Gas Strut Calculator?
• How lid weight, centre of gravity, mounting geometry, and opening angles are calculated
• Example Calculation: 20kg hinged lid with an 800mm length using a pair of struts mounted at 30% length
• Mounting geometry leverage: How moving mount points alters required Newton force and stroke length
• Gas strut sizing grades: Choosing between 6/15, 8/18, 10/22, and 14/28 rod/cylinder dimensions
• The Essential UK Gas Strut Installation & Mounting Geometry Checklist
• How to use the Gas Strut Calculator
• Frequently Asked Questions (FAQs)

Use this Gas Strut Calculator to determine required gas spring lifting force in Newtons and Pounds, estimate required stroke length, calculate open and extended dimensions, and identify ideal mounting geometry for hinged lids, engine bonnets, access hatches, and campervan pop-tops. Sizing gas struts accurately prevents heavy doors from slamming shut, reduces hinge strain, and ensures smooth, effortless lifting across domestic, automotive, and industrial applications.

What is the Gas Strut Calculator?

The Gas Strut Calculator is an engineering force and geometry estimation tool designed to determine the precise gas strut force rating required to lift and support hinged lids, doors, or hatches.

By selecting the number of gas struts used (2 Struts Standard Pair vs 1 Strut Single), entering lid mass weight in kilograms, specifying lid depth length from hinge to edge in millimetres, setting your planned mounting position on the lid (20%, 30%, or 40% length from hinge), and choosing maximum opening angle (45°, 85°–90°, or 110°), this calculator outputs required force per strut in Newtons (N) and Pounds (lbs), stroke length estimates, extended length dimensions, and recommended rod/cylinder sizing grades.

How lid weight, centre of gravity, mounting geometry, and opening angles are calculated

The calculator evaluates the required force for gas springs by analyzing rotational moment equilibrium around the lid hinge line.

To maintain complete transparency, the tool follows these logical mechanical engineering calculation steps:

• Gravitational Force Calculation:
  • Calculated by converting total lid mass into gravitational load in Newtons: Force (Newtons) = Mass (kg) x 9.81 m/s².
  • For example, a 20kg hinged access hatch exerts a downward force of 196.2 Newtons.
• Centre of Gravity (COG) Distance:
  • For uniform rectangular lids, the centre of gravity acts precisely at the midpoint of the lid length (50% distance from the hinge): COG Distance = Total Lid Length x 0.50.
• Effective Support Vector & Angle Mechanics:
  • • As the lid opens, the geometric mounting angle between the gas strut and the lid body shifts. The calculator evaluates the effective perpendicular force vector at the chosen mount position using the half-angle sine projection:

  Theoretical Strut Force (N) = (Weight x COG Distance) / (Number of Struts x Mount Distance x sin(Opening Angle / 2))

• Safety Margin & Temperature Buffer (+15% Factor):
  • The calculator applies a mandatory **15% safety buffer** to account for ambient temperature drops (gas pressure decreases by ~3.5% per 10°C drop in winter), seal friction, and wind loads on open lids.

Example Calculation: 20kg hinged lid with an 800mm length using a pair of struts mounted at 30% length

To observe how mounting distance affects the required gas strut lifting force, consider this standard workshop access lid installation.

Example: An engineer installs a pair of gas struts on a 20kg lid measuring 800mm from hinge to edge, mounting the struts at 30% length (240mm from hinge) with an 85-degree opening angle.

• Input Parameters:
  • Number of Struts: 2 Struts (Standard Pair)
  • Lid Weight: 20 kg (196.2 Newtons total downward force)
  • Total Lid Length: 800 mm (Centre of Gravity at 400 mm)
  • Mount Position: 30% of length = 240 mm from hinge
  • Maximum Opening Angle: 85 Degrees
• Step-by-Step Mechanical Breakdown:
  • Lid Rotational Moment: 196.2 N x 0.40 m = 78.48 Nm torque at hinge.
  • Theoretical Force Per Strut: (78.48 Nm) / (2 struts x 0.24 m x sin(42.5°)) = 317.3 Newtons.
  • Final Recommended Rating (+15% Buffer): 317.3 N x 1.15 = 365 Newtons (82 lbs) per strut.
  • Stroke & Extended Length Sizing: Recommended stroke length is ~240mm with an extended length of ~550mm (8/18 rod/cylinder grade).

The calculation demonstrates that choosing a pair of 365N gas struts mounted at 240mm provides effortless lifting, smooth damped opening, and a safe stay-open position.

Mounting geometry leverage: How moving mount points alters required Newton force and stroke length

Mounting position relative to the hinge line directly dictates mechanical advantage, strut extension, and required gas force rating:

• Mounting Closer to the Hinge (10% to 20% Lid Length):
  • Force Requirement: Requires significantly higher gas force (e.g. 600N to 900N) due to reduced lever arm distance.
  • Stroke Advantage: Requires a much shorter stroke length, keeping the gas strut compact and hidden near the back of the enclosure.
• Standard Mounting Position (25% to 35% Lid Length):
  • Sweet Spot Balance: Widely considered the optimal installation ratio across automotive bonnets, campervans, and storage boxes.
  • Performance: Balances manageable strut forces (~300N to 450N) with smooth, controlled damping action and standard off-the-shelf stroke dimensions.
• Mounting Further Down the Lid (40% to 50% Lid Length):
  • Force Requirement: Slashes required lifting force dramatically due to increased rotational torque leverage.
  • Space Trade-off: Requires a significantly longer gas strut and stroke length, which can obstruct internal box or boot storage space when fully extended.

Gas strut sizing grades: Choosing between 6/15, 8/18, 10/22, and 14/28 rod/cylinder dimensions

Gas struts are categorized by their piston rod outer diameter and main cylinder body outer diameter (measured in millimetres):

• 6/15 Size Grade (6mm Rod / 15mm Cylinder):
  • Force Capability Range: 50 Newtons to 400 Newtons.
  • Typical Applications: Lightweight access panels, small kitchen cabinet doors, furniture lids, and light equipment covers.
• 8/18 Size Grade (8mm Rod / 18mm Cylinder):
  • Force Capability Range: 100 Newtons to 800 Newtons.
  • Typical Applications: Standard automotive bonnets, tailgates, utility trailer hatches, toolboxes, and campervan pop-top beds.
• 10/22 Size Grade (10mm Rod / 22mm Cylinder):
  • Force Capability Range: 150 Newtons to 1,200 Newtons.
  • Typical Applications: Heavy commercial vehicle hatches, horsebox ramps, boat engine bay covers, and large steel floor access hatches.
• 14/28 Size Grade (14mm Rod / 28mm Cylinder – Heavy Duty):
  • Force Capability Range: 500 Newtons to 2,500+ Newtons.
  • Typical Applications: Heavy industrial machinery, architectural roof lights, and large commercial steel doors.

The Essential UK Gas Strut Installation & Mounting Geometry Checklist

Adhering to correct mechanical orientation and mounting guidelines ensures maximum strut seal longevity and smooth operation:

✅ Orientation & Rod Downward Position

• Mount Piston Rod Downward when Closed: Always install gas struts with the thinner piston rod pointing downward when the lid is in its resting closed position. This ensures internal oil bathes and lubricates the main rubber shaft seal, preventing gas leakage and seal drying.
• Verify Ball Joint & Bracket Clearance: Ensure ball studs and mounting brackets allow 360-degree articulation without binding or contacting frame edges during the full opening sweep.

✅ Stroke Length & Bottoming-Out Protection

• Prevent Mechanical Bottoming Out: Ensure the gas strut leaves at least 10mm of unused stroke when the lid is completely closed. Allowing the piston to hit internal hard stops under compression damages internal valves and tears mounting brackets off.
• Use External Closed Stops: Never rely on a compressed gas strut to act as the primary physical stop for a heavy lid; always use frame rubber stops or mechanical hinges to absorb final closing force.

✅ Force Tuning & Safety Isolation

• Utilize Vari-Pressure Valve Struts for Exact Tuning: If exact lid weight or center of gravity is uncertain, order gas struts equipped with an adjustable de-gassing valve to bleed off small amounts of nitrogen gas until perfect equilibrium is achieved.
• Replace Gas Struts in Matching Pairs: Always replace both left and right gas struts at the same time. Running a new strut alongside an old, de-pressurized unit causes uneven twisting force, warping hinges and damaging seals.

How to use the Gas Strut Calculator

1. Number of Gas Struts Used: Choose your installation setup (“2 Struts (Standard Pair)” or “1 Strut (Single)”) to divide lifting force requirements across strut mount points.
2. Lid / Hatch Weight (kg): Input the total mass weight of the door, lid, or bonnet in kilograms (for reference, 1 kg equals approximately 2.2 lbs).
3. Lid Length from Hinge to Edge (mm): Enter the total depth of the opening lid moving perpendicular to the hinge line in millimetres.
4. Mount Position Ratio (% from Hinge): Select your planned mounting point on the lid (“20% Length (High Force)”, “30% Length (Standard)”, or “40% Length (Lower Force)”) to establish mechanical leverage.
5. Maximum Opening Angle (Degrees): Choose your target open position (“45° Angle”, “85°–90° Angle”, or “110° Wide Open”) to factor in angular force vectors.
6. Review Gas Strut Specification Summary: Examine the results panel to view required force per strut in Newtons and Pounds, recommended stroke length, approximate extended length, and suggested rod/cylinder sizing grade.

Frequently Asked Questions (FAQs)

How do I calculate the Newton force required for a gas strut?
Gas strut force is calculated by multiplying total lid weight by gravitational acceleration (9.81 m/s²) and the distance to the lid centre of gravity, then dividing that figure by the product of the number of struts, the mounting distance from the hinge, and the sine of the effective mounting angle. A safety margin of 15% is added to account for friction and temperature drops.

Why should gas struts be installed with the rod pointing downwards?
Installing gas struts with the piston rod pointing downward when the lid is closed ensures that internal lubrication oil bathes the main rubber shaft seal. This keeps the seal moist, prevents gas pressure leakage, and provides smooth end-of-stroke hydraulic damping.

What happens if a gas strut is too strong or too weak for a lid?
If a gas strut is too strong, the lid will spring open violently, putting severe strain on hinges and making it extremely difficult to pull closed. If a gas strut is too weak, the lid will fail to stay open independently and may slam shut unexpectedly, posing a safety hazard.

Can I adjust the pressure in a gas strut after buying it?
Standard fixed-pressure gas struts cannot be adjusted once sealed. However, adjustable “vari-pressure” gas struts come over-pressurized with a release valve, allowing you to bleed off small amounts of nitrogen gas using an Allen key until your lid achieves perfect balance.

Sources

• SGS Engineering UK – Official technical sizing charts, mounting geometry rules, and Newton force formulas
• Dictator Gas Springs – Industrial engineering standards for gas strut stroke lengths, mounting kinematics, and temperature variance factors
• Industrial Gas Springs (IGS) – Mechanical guidelines on piston rod diameter grades, mounting brackets, and seal maintenance

This calculator provides estimated Newton force ratings, stroke lengths, and mounting geometry specs based on standard mechanical moment equilibrium formulas. Final strut selection should always account for non-uniform mass distribution, heavy handles, and extreme wind load conditions.