Window Actuator Calculator: Thrust Force, Stroke and Cable Cross-Section

Use the free window actuator calculator to work out the required thrust force, the right stroke and the minimum cross-section of the 24 V supply cable in seconds – for top-hung, bottom-hung and fanlight windows as well as roof windows and rooflights.

In brief: The thrust force depends on sash weight, stroke and sash height, and for roof windows also on pitch and snow load. The calculator gives guide values and suggests a suitable actuator. The final design is based on the manufacturer’s data.

How the calculation works

1. Sash weight

If the weight is not known, the calculator estimates it from the glass area: m = width × height × glass thickness × 2.5 kg/(m²·mm) × 1.1. Glass weighs about 2.5 kg per square metre and millimetre of thickness; the factor 1.1 accounts for the frame.

2. Thrust force for vertical windows

For bottom-hung, top-hung and fanlight windows the common industry approximation applies: F = 0.54 × m × g × stroke / sash height (g = 9.81 m/s²). The larger the stroke in relation to the sash height, the more force the actuator needs.

3. Thrust force for roof windows and rooflights

For inclined or horizontal sashes the actuator carries about half the weight: F = m × g / 2 × cos(pitch). A snow load is added as q × area / 2 × cos(pitch) . These formulas are a simplified lever model; wind load and seal forces are not included.

4. Stroke from opening angle

The required stroke follows from sash height and opening angle α: Stroke = 2 × sash height × sin(α/2). Example: 800 mm sash height and 20° give about 278 mm – you then choose the next available stroke length, e.g. 300 mm. Explained in detail in the guide Calculate chain length.

5. Cable cross-section for 24 V

With extra-low voltage the voltage drop is decisive: A = 2 × L × I / (56 × ΔU) with L = one-way cable length in m, I = current in A, ΔU = permissible voltage drop in V and 56 = conductivity of copper. Example: 25 m, 2 A, 1.2 V give 1.49 mm² – choose 1.5 mm².

Worked example: bottom-hung window

Sash 1000 × 800 mm with 2 × 4 mm glass: weight ≈ 1.0 × 0.8 × 8 × 2.5 × 1.1 = 17.6 kg. With 300 mm stroke: F = 0.54 × 17.6 × 9.81 × 0.3 / 0.8 ≈ 35 N. A chain actuator such as the VS-KA with 250 N therefore has plenty of reserve.

Frequently asked questions

How much reserve should the thrust force have?

Choose a force rating well above the calculated result. Seals, fittings and wind increase the actual load; the manufacturer of the actuator specifies the exact allowances.

When do I need two actuators?

For wide or heavy sashes, two synchronised actuators distribute the force evenly (tandem operation). This prevents the sash from twisting.

Does the calculator also apply to smoke ventilation (SHEV)?

The force calculation is the same. For smoke and heat exhaust, however, the complete system must be designed for the specific project and operated with an approved smoke control panel. Smoke control panels are not part of the Vent Systems product range.

Which voltage is better, 24 V or 230 V?

24 V is the standard for ventilation and smoke ventilation, can be combined with sensors and switches and synchronised. 230 V saves the control unit for single windows. More on this in the window actuator comparison.

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<a href="https://ventsystems.de/en/window-actuator-calculator/">Window Actuator Calculator: Thrust Force, Stroke and Cable Cross-Section</a>

Note: All results are guide values and do not replace design by the manufacturer or a specialist planner. No liability is accepted.