Before ordering a window actuator, you should know how much force it needs. If the actuator is too weak, the window will not open or the motor runs constantly at its limit. Three steps give you a good estimate: determine the sash weight, apply the approximation formula, plan a reserve.
In brief: You estimate the force a window actuator needs in three steps: determine the sash weight, apply the approximation formula, plan a reserve. For rooflights F ≈ 0.54 × G applies, for bottom-hung and top-hung sashes in the façade F ≈ 0.54 × G × stroke / H. Snow, wind and seals require additional reserve.
Step 1: Determine the sash weight
The glass weighs the most. As a rule of thumb: 1 m² of glass weighs about 2.5 kg per millimetre of glass thickness. Double glazing with 2 × 4 mm glass therefore weighs around 20 kg per m². Add the sash frame. If you want to know exactly, ask the window manufacturer or weigh the removed sash.
Step 2: Apply the approximation formula
Window actuator manufacturers use simple formulas for a first estimate. Here G is the weight force of the moving sash in newtons (weight in kg × 9.81), Stroke the Stroke length and H the sash height:
- Rooflight or horizontal sash: F ≈ 0.54 × G
- Tilt or top-hung sash in the façade: F ≈ 0.54 × G × stroke / H
The difference is large: a vertical tilt window needs only a fraction of the force because the actuator hardly has to lift any weight. A rooflight, on the other hand, has to be lifted almost completely.
Why 0.54 and not 0.5? Physically, an actuator opposite the hinge side gives a factor of 0.5, because the centre of gravity lies in the middle of the sash. Manufacturers add a safety margin, for example 0.54 (Aumüller) or 0.68 (GEZE for the E 740 chain actuator). In this guide we use 0.54. The binding sizing follows the documentation of the actuator and window manufacturers.
Step 3: Plan a reserve
The formulas take into account neither wind nor snow, seals or stiff fittings. Therefore always plan a clear force reserve; correspondingly more for roof windows and rooflights with snow and wind load. An actuator with reserve runs more quietly and lasts longer. More on the influences: Technical design of window drives.
Calculation examples
| Window | Weight | Invoice | Result without reserve |
|---|---|---|---|
| Tilt window 1.2 × 0.8 m, stroke 200 mm | approx. 25 kg → 245 N | 0.54 × 245 N × 200/800 | approx. 33 N |
| Rooflight 1.2 × 1.2 m | approx. 40 kg → 392 N | 0.54 × 392 N | approx. 212 N |
Example: four fanlights in a workshop hall
Four identical top-hung sashes measuring 1,400 × 800 mm, with 4/16/4 mm double glazing and aluminium frames, are to ventilate via a shared switch.
- Weight: 1.12 m² of glass × 20 kg/m² gives around 22 kg, plus about 8 kg for the frame, around 30 kg in total (294 N).
- Stroke: For an opening of about 20°, 2 × 800 mm × sin 10° ≈ 278 mm, so 300 mm is chosen.
- Force: 0.54 × 294 N × 300 / 800 ≈ 60 N. A VS-KA with 250 N is sufficient, with plenty of reserve for seals and wind.
- Control: Four VS-KA in 24 V draw around 3.2 A together, and the VS-C24-7A supplies up to 7 A.
- Width: With a sash width of 1,400 mm, check with the profile manufacturer whether one actuator in the middle is enough, otherwise use two synchronised actuators or a double chain actuator.
For the tilt window, even the smallest VS-KA chain drive with 250 N is easily enough. For the rooflight, with reserve and snow load, a VS-LA linear actuator (600 to 5000 N) is the safe choice. The values are examples; calculate with the data of your window.
And the chain length?
The second important variable is the stroke, i.e. how far the actuator extends. Calculate it with our Chain Length Calculator.. Force and stroke are related: a larger stroke on a tilt window also increases the force required.
Which actuator for which force?
| Required force (with reserve) | Suitable actuator |
|---|---|
| up to 1500 N, tilt, top-hung and roof windows | VS-KA chain drive (250 to 1500 N) |
| up to 2500 N, heavy sashes | VS-KA 2500 or VS-SA 2500 spindle drive |
| 600 to 5000 N, rooflights and heavy roof sashes | VS-LA linear actuator |
| wide sashes | two actuators in synchronous operation, For example,. VS-KA-i |
How snow and wind change the force required is explained in our guide Snow load and wind load.
Frequently asked questions
Is more force always better?
Not necessarily. A much too powerful actuator also pushes harder in the event of a fault. On windows within reach, the closing force plays a role in protection against trapping, see Main and secondary closing edge.
Does the formula also apply to SHEV?
For SHEV windows, the actuator and window are designed and verified for each project. The approximation is only suitable for initial orientation, see NRWG.
Consultation and offer
Send us dimensions, weight and a photo of your window, we will check the force and suggest the right actuator. Phone 040 50694631 (Mon–Fri 8 am–5 pm), Contact form or measurement sheet.
Further reading: Actuators for roof windows · Actuators for rooflights