Size Your Gas Strut By Length Stroke Force Thread In Four Steps

What size gas strut do I need?

Sizing a gas strut comes down to four numbers: how long it is, how far it travels, how hard it pushes, and what thread is on each end. Get those four right and the strut will run for years. Get one wrong and the lid either will not stay up, or it flies open and tears its own mounting out.

This page gives you the arithmetic, the specification tables for every series we carry, and a calculator that does the sum for you. It applies to canopy doors, toolbox lids, machine guards, cabinet doors, trailer lids, tractor bonnets and anything else that needs a hand staying open.

The calculator

You need a tape measure and a rough weight. Measure the door from the hinge line to the far edge, weigh it (there is a tip in the box below if you cannot get it onto a scale), and set the slider to where the strut will bolt onto the door. The calculator works out the centre of gravity for you.

If you would rather see the arithmetic, it is set out in the next section.

Gas Strut Force Calculator

Two measurements and a weight. Everything else has a sensible default.

1. Door or lid length (mm) From the hinge line to the far edge.

2. Door or lid weight (kg) See the weighing tip below if unsure.

3. Strut mounting point How far down the door the strut attaches: 192 mm

4. Number of struts Two for anything wider than 700mm.

5. Weight distribution Where the mass sits on the door.

6. Where it works Sets the safety margin.

How to weigh a door you cannot lift onto a scale

Open the door until it is roughly horizontal, rest the far edge on a bathroom scale, and take the reading. For an evenly weighted door the true weight is about twice the scale reading. Support the door while you do this - do not rely on the old struts.

This is an estimate for a top-hinged door or lid. Rubber seal friction, wind loading and anything bolted to the door all add load. Confirm the specification with us before you order.

Step 1: The four dimensions

Three lengths define every gas strut. Measure the strut fully extended, from the centre of one mounting point to the centre of the other.

L1 — extended length. Eye centre to eye centre with the rod all the way out. This is the primary ordering dimension.
L2 — stroke. How far the rod moves. On our standard series stroke runs at roughly 45% of extended length. Stroke sets how far your lid opens.
L3 — cylinder length. The body. L1 minus L2.
Q1 and Q2 — cylinder and rod diameter. Written as a pair, for example 18*8, meaning an 18mm cylinder on an 8mm rod. This sets the maximum force the strut can carry and how much side load it will tolerate.

The most common measuring error by a wide margin is measuring a dead strut while it is compressed. A failed strut will not push itself out. Pull the rod out by hand until it stops, then measure. If you measure it as it sits you will order something roughly half the length you need.

Step 2: The force formula

For a top-hinged lid or door, the force each strut has to provide is:

Force per strut (N) = (W × 9.81 × D_cg) ÷ (n × D_m) × SF

W = weight of the lid in kilograms
D_cg = distance from the hinge line to the centre of gravity of the lid, in mm. For an evenly weighted door this is half the door length, which is what the calculator assumes unless you tell it otherwise
D_m = distance from the hinge line to where the strut mounts on the lid, in mm
n = number of struts
SF = safety factor, normally 1.2

For a uniform flat lid, the centre of gravity sits at half the lid’s length from the hinge. If the lid has a heavy edge — a glass panel, a lock assembly, a spare wheel carrier — the centre of gravity shifts toward it and you should measure rather than assume.

Why the safety factor matters. A gas strut loses a small percentage of its charge every year past the seal, and gas pressure falls with temperature at about 0.3% per degree Celsius. A strut sized with no margin will hold on the day it is fitted and sag on the first cold morning of its second winter. The 1.2 factor buys you that margin. Use 1.3 for anything that lives on gravel roads or works in the cold.

Step 3: Worked examples

Example 1 — canopy side door

Glass side door, 10kg, centre of gravity 240mm below the hinge, struts mounted 190mm below the hinge, two struts, standard vehicle safety factor.

(10 × 9.81 × 240) ÷ (2 × 190) × 1.2 = 74N per strut. Round up to the 80N band. Series: SA4000, 18×8, M6 thread.

Example 2 — steel toolbox lid

Chequer-plate lid, 18kg, centre of gravity 300mm from the hinge, struts mounted 220mm from the hinge, two struts, dusty environment so a 1.3 factor.

(18 × 9.81 × 300) ÷ (2 × 220) × 1.3 = 157N per strut. Round up to 200N. Series: SA4000, or SA4100 if the box lives on a gravel road.

Example 3 — tractor bonnet

Bonnet, 45kg, centre of gravity 620mm from the hinge, struts mounted 340mm from the hinge, two struts, 1.3 factor for farm conditions.

(45 × 9.81 × 620) ÷ (2 × 340) × 1.3 = 523N per strut. Round to 600N. Series: SA4100 at minimum, SA6000 preferred for rod diameter and side-load tolerance at this weight.

Example 4 — single strut on a light hatch

Aluminium access hatch, 4kg, centre of gravity 180mm from the hinge, strut mounted 150mm from the hinge, one strut, indoor use so 1.1.

(4 × 9.81 × 180) ÷ (1 × 150) × 1.1 = 52N. Round to 80N, the smallest practical band. Series: SA4000.

Step 4: Check the stroke

Force decides whether the lid holds. Stroke decides how far it opens, and it is the dimension most people forget.

A strut can only move the lid through the arc its stroke allows. If you need a canopy door to swing through 80 degrees and the strut only has 155mm of stroke mounted 190mm from the hinge, it will not get there — the strut runs out of travel and becomes a mechanical stop, which bends rods and tears brackets.

A workable rule for a top-hinged door: the stroke needs to be at least the straight-line distance between the strut’s mounting points measured closed, subtracted from that same distance measured at your target open angle. In practice, if you are replacing like for like, matching the original stroke is correct. If you are designing from scratch, mount the strut further from the hinge to get more opening angle from the same stroke, and accept that this raises the force you need.

Always leave free travel at both ends. Five to ten millimetres at full open and full closed. A strut that bottoms out is being used as a door stop, and that is the fastest way to destroy one.

Step 5: Choose the series

Once you know the force and the length, the series follows. Each of our series is defined by its end thread and its cylinder and rod diameter. Diameter matters as much as force rating: an 18×8 strut technically rated to 700N will not survive long on a 30kg door, because the 8mm rod cannot take the side loading that a heavy door imposes as it swings.

As a rule, if you land near the top of a series’ force range, step up to the next series. The extra cost is small and the strut will last several times longer.

Series	End thread	Cylinder x Rod	Force range	Extended length range	Typical applications
SA4000	M6	18 x 8mm	50 - 700N	150 - 800mm	Canopy doors, toolbox lids, cabinet and hatch lids, light machine covers
SA4100	M8	22 x 10mm	50 - 1300N	150 - 1000mm	Heavy canopy and service doors, caravan lockers, seating, medium machine guards
SA6000	M10	25 x 12mm	100 - 2000N	350 - 1000mm	Trailer lids, bakkie canopies with roof loads, industrial access panels
SA6100	M12	28 x 14mm	100 - 2500N	350 - 1500mm	Agricultural bonnets and cab windows, large machine guarding, heavy hatches
SA6200	M14	28 x 16mm	On request	350 - 2000mm	Long-stroke industrial applications, large covers and doors
SA7000	M18	40 x 20mm	200 - 4500N	350 - 2000mm	Truck and bus panels, heavy plant, large industrial covers and platforms

Full part-number tables with every length, stroke and cylinder length in each series are on our lift gas springs page. If you need a locking strut — one that holds at any point in its travel rather than only fully open — see locking gas springs.

Force by application

If you do not have a scale handy, this table gives typical weights and force requirements by application. Use it as a sanity check on your calculation rather than as a substitute for it — two canopy doors of the same size can differ by 4kg depending on whether the glass is fixed or sliding.

Application	Typical lid or door weight	Total force required	Struts fitted	Suggested series
Kitchen or office cabinet lift-up door	2 - 5 kg	60 - 150N	1 - 2	SA4000
Bakkie canopy side door	8 - 12 kg	300 - 400N	2	SA4000
Bakkie canopy rear door	14 - 20 kg	500 - 700N	2	SA4000 / SA4100
Trailer or toolbox lid	10 - 25 kg	400 - 900N	2	SA4100
Caravan or camper luggage door	30 - 45 kg	1000 - 1600N	2	SA4100 / SA6000
Machine guard or access panel	20 - 60 kg	700 - 2200N	2	SA6000 / SA6100
Tractor bonnet or cab window	25 - 70 kg	900 - 2500N	2	SA6100
Bus or truck luggage panel	60 - 150 kg	2200 - 5500N	2	SA7000
Heavy industrial cover or platform	100 - 250 kg	3500 - 9000N	2 - 4	SA7000

Step 6: Confirm the end thread

The last number you need is the end thread, because it determines which fittings will screw on. Our series run M6 (SA4000), M8 (SA4100), M10 (SA6000), M12 (SA6100), M14 (SA6200) and M18 (SA7000).

If you are reusing your existing ball studs, eyelets or clevises, the thread must match exactly. If you are starting fresh, choose the fitting to suit the mounting rather than the other way round:

Ball and socket — tolerates a few degrees of misalignment and unclips for removal. The default choice for vehicle doors and canopy lids.
Eyelet on a clevis pin — rigid and cheap, correct only where the geometry is true and stays true.
Spherical eyelet or rose joint — maximum angular tolerance, for heavy doors that rack as they open.
Blade or fork fittings — for machine guarding and industrial frames where a bolted, serviceable joint is wanted.

Browse the full range on our end fittings and mounting brackets pages.

Sizing mistakes to avoid

Nearly every warranty return we handle traces back to one of the items below. None of them are the strut’s fault.

Mistake	What you will see	The fix
Measured the strut compressed instead of extended	Ordered strut is roughly half the length needed and will not reach the mountings	Always measure fully extended, eye to eye
Force too low	Lid creeps down, will not hold in the cold, needs a hand to finish opening	Recalculate with the 1.2 safety factor and round up a band
Force too high	Lid flies open, slams against its stop, hinges work loose, corners crack	Drop one force band or move the mounting closer to the hinge
Stroke too short	Lid does not open far enough to be useful	Choose a longer strut, or move the mounting points further from the hinge
Stroke too long	Strut bottoms out and becomes the door stop, bending the rod	Leave 5 - 10mm free travel at each end of the arc
Mounted rod-up	Works for a season, then loses force rapidly and starts squeaking	Remount cylinder-up, rod-down so oil sits against the seal
Ignored side loading	Rod becomes scored, strut leaks within months	Use ball joints or spherical eyelets to absorb misalignment
Mixed a new strut with an old one	Lid opens crooked, seals leak, frame twists	Always replace in matched pairs
Wrong end thread ordered	New strut will not accept the existing ball studs or eyelets	Confirm M6, M8, M10, M12, M14 or M18 before ordering

Gas strut sizing FAQs

What size gas strut do I need for a canopy door?

For a standard bakkie canopy side door of 8 to 12kg, fitted with two struts, you are usually looking at 150N to 200N per strut in a 350mm to 450mm extended length, SA4000 series with M6 threads. Measure your old strut fully extended to confirm the length, and use the calculator above to confirm the force.

Is a higher newton rating better?

No. A strut rated too high will fling the lid open, load the hinges, slam against the stop and can crack a fibreglass or composite door. Size it properly and round up by one standard band, no more.

How do I measure a gas strut that has failed completely?

Pull the rod out by hand until it stops, then measure eye centre to eye centre. A dead strut has no gas left to extend itself, so measuring it as it sits will give you roughly half the true length.

Can I use one strut instead of two?

On small hatches and light cabinet doors, yes. On anything wider than about 700mm, two struts are strongly preferred – a single strut lets the lid flex and twist, and a single failure drops the lid completely.

What does 18*8 or 22*10 mean?

It is the cylinder diameter followed by the rod diameter, in millimetres. 18*8 is an 18mm body on an 8mm rod. Larger numbers mean more force capacity and much better tolerance of side loading.

Does temperature affect gas strut force?

Yes. Force falls roughly 0.3% for every degree Celsius drop. A strut that is marginal at 25 degrees will not hold at 2 degrees. This is why we build a safety factor into the calculation.

Can gas struts be mounted horizontally or upside down?

Horizontally is fine. Rod-up is not recommended – the internal oil drains away from the seal and the strut fails early. Mount cylinder up, rod down wherever the geometry allows.

Do you make gas struts to a custom specification?

Yes. We can supply struts charged to a specific force within the range of each series, and we build to length for industrial and OEM applications. Send us your requirement and we will quote.

Can a gas strut be adjusted or recharged?

Ours can be recharged, provided the rod and seal are sound. See our gas strut recharging page for when recharging makes more sense than replacement.

Still not certain? Send us the extended length, the stroke, the thread size and the weight of what you are lifting, and we will specify it for you. We have been supplying gas springs into the South African canopy, trailer, agricultural and industrial trade since 2009.

Gas Strut Size Calculator