Selecting a motor for a roller shutter looks deceptively simple. You measure the opening, order a motor, and install it. But get the torque wrong — even by a modest margin — and you will face a motor that strains under load, overheats prematurely, fails its thermal cutout on every third cycle, or wears out in a fraction of its rated service life.
The correct motor torque for a rolling shutter is determined by three variables: the total curtain weight, the drum winding radius at maximum load, and an applied safety factor. The core formula is: Required Torque (Nm) = Curtain Weight (kg) × Gravitational Constant (9.81 m/s²) × Winding Radius (m) × Safety Factor. Everything else in motor selection flows from getting this number right.
This guide walks through the complete calculation method, explains each variable in plain terms, provides two worked examples, and gives practical guidance for common shutter types — so you can specify with confidence.
Key Takeaways
- Torque (Nm) is the rotational force a motor must produce to lift the curtain. It is the single most important specification when sizing a shutter motor.
- The torque requirement is determined by curtain weight × winding radius, not by shutter area alone.
- Always apply a safety factor of at least 1.3 to 1.5 to your calculated torque to ensure reliable long-term operation.
- As the curtain winds onto the drum, the effective winding radius increases, which means torque demand changes throughout the travel. Motor selection must account for the worst-case (maximum radius) condition.
- Counterbalance springs, when correctly set, can significantly reduce the net torque the motor must produce — but should not be relied upon to compensate for an undersized motor.
- Common tubular motor ranges run from 6 Nm for lightweight residential shutters up to 100 Nm or more for heavy industrial curtains.
- When in doubt, size up — not down. A motor operating comfortably within its torque range will outlast an overloaded motor by years.
Why Motor Torque Selection Matters More Than Most People Think
Torque is not just a technical specification to satisfy on a datasheet. It is the direct expression of whether your motor can do its job reliably, day after day, across thousands of operating cycles.
An undersized motor will operate at or near its torque limit on every cycle. This generates excess heat in the motor windings, accelerates wear on internal components, and repeatedly trips the thermal overload protection. Over time, this shortens motor life dramatically — and in commercial environments where doors cycle dozens of times per day, it can mean motor failure within months rather than years.
What Happens When Torque Is Too Low
A motor with insufficient torque will struggle visibly on the way up — moving slowly, pausing, or stopping mid-travel and triggering the thermal cutout. In some cases it will appear to work initially but fail progressively as the motor heats up during a busy operational period. These are not installation problems. They are sizing problems, and the only real solution is replacing the motor with an appropriately rated one.
What Happens When Torque Is Too High
Oversizing a motor is a less critical error, but it is still an error. A motor with significantly more torque than the application requires will cost more, may not fit the available installation space, and — most importantly — can generate curtain speeds and forces that exceed what the guide rails and end stops are designed to handle. An extreme torque mismatch can damage the shutter structure itself. Some torque overhead is desirable; excessive torque is not.
Understanding Torque: What Does Nm Actually Mean?
Torque is the measure of rotational force — the twisting power a motor applies to the shaft to wind the curtain upward. It is expressed in Newton-meters (Nm).
One Newton-meter means a force of one Newton applied at a distance of one meter from the axis of rotation. In practical shutter terms: a heavier curtain requires more force to lift, and a larger drum radius means that force must act over a greater distance — both factors increase the torque demand.
This is why two shutters of identical area can require completely different motor torques. A wide, short shutter with heavy steel slats on a large-diameter drum is a fundamentally different mechanical challenge from a narrow, tall shutter with lightweight aluminum slats on a compact drum.
The Three Variables That Determine Required Torque
Variable 1 — Curtain Weight (kg)
The total weight of the shutter curtain is the most obvious input. It includes the weight of all slats, the bottom bar (T-bar), any reinforcing elements, and — where relevant — any weatherstripping or sealing components that move with the curtain.
Slat weight is typically expressed by manufacturers as kilograms per square meter (kg/m²). Common values:
| Slat Material | Typical Weight (kg/m²) |
|---|---|
| Lightweight aluminum | 3.5 – 5.0 kg/m² |
| Standard aluminum | 5.0 – 7.5 kg/m² |
| Galvanized steel | 8.0 – 12.0 kg/m² |
| Heavy steel / perforated steel | 10.0 – 16.0 kg/m² |
| Polycarbonate / vision slat | 2.5 – 4.5 kg/m² |
Total curtain weight = Width (m) × Height (m) × Weight per m² (kg/m²) + Bottom bar weight (kg)
Always request the slat weight specification from your profile supplier. Do not estimate — even a 1 kg/m² error on a large curtain translates to a meaningful torque miscalculation.
Variable 2 — Drum or Winding Radius (m)
The winding radius is the distance from the center of the drum shaft to the outer surface of the wound curtain at the point of maximum winding — which occurs when the shutter is fully open and the maximum number of slats are coiled around the drum.
This is where many people make a critical error. They measure or estimate the drum radius when the curtain is fully unwound (at its smallest), but the motor must work hardest when the curtain is fully wound (at its largest radius). At maximum winding, the radius may be 40 to 80% larger than the bare drum radius, depending on curtain thickness and total curtain height.
To calculate maximum winding radius:
- Start with the bare drum radius (r₀)
- Calculate the total thickness of wound curtain: number of slat layers × individual slat thickness
- Maximum radius = r₀ + (number of layers × slat thickness)
For practical motor sizing, use the maximum winding radius — this represents the worst-case torque demand on the motor.
Variable 3 — Safety Factor and Mechanical Efficiency
No mechanical system is 100% efficient. Guide rail friction, bearing resistance, curtain alignment drag, and temperature effects all reduce effective torque delivery. A safety factor (SF) of 1.3 to 1.5 is standard practice for most roller shutter applications. For heavier commercial or industrial shutters, or where high-cycle operation is expected, using SF = 1.5 is the more conservative and recommended approach.
Some engineers also apply a separate mechanical efficiency factor (η), typically 0.85 to 0.90, to account for drivetrain losses. This can be incorporated directly into the safety factor for simplified calculations.
The Motor Torque Calculation Formula (Step by Step)
The Core Formula
Required Torque (Nm) = [Curtain Weight (kg) × 9.81 (m/s²) × Maximum Winding Radius (m)] × Safety Factor
This formula calculates the torque needed at the drum shaft to lift the curtain against gravity, accounting for real-world inefficiencies.
Step 1 — Calculate Total Curtain Weight
Multiply the shutter width by the curtain height by the slat weight per m², then add the bottom bar weight.
Example: 3.0 m wide × 2.5 m high, aluminum slats at 6.0 kg/m², bottom bar at 4 kg
- Slat curtain weight = 3.0 × 2.5 × 6.0 = 45.0 kg
- Total curtain weight = 45.0 + 4.0 = 49.0 kg
Step 2 — Determine the Maximum Winding Radius
Measure the bare drum radius and add the thickness of the fully wound curtain.
Example: Bare drum radius = 0.060 m, slat thickness = 8 mm = 0.008 m
- Total curtain height = 2.5 m, slat height = 0.040 m → approximately 63 slats
- Curtain layers on drum (assuming drum circumference manages approximately 8 slats per layer) ≈ 8 layers
- Additional winding radius = 8 × 0.008 = 0.064 m
- Maximum winding radius = 0.060 + 0.064 = 0.124 m
Note: The precise number of winding layers depends on drum circumference and slat geometry. For simplified sizing, many experienced installers use an estimated maximum winding radius based on the drum assembly specification provided by the shutter system manufacturer.
Step 3 — Apply the Torque Formula
Required Torque = 49.0 kg × 9.81 m/s² × 0.124 m = 59.6 Nm (before safety factor)
Step 4 — Apply the Safety Factor
Required Torque (with SF 1.4) = 59.6 × 1.4 = 83.4 Nm
In this example, you would select a tubular motor rated at a minimum of 85 Nm, and ideally from the next standard rating above that — typically 100 Nm in most manufacturer ranges.
Worked Examples
Example 1 — Residential Aluminum Slat Shutter
Shutter specification:
- Width: 2.2 m | Height: 2.0 m
- Slat material: Standard aluminum at 5.5 kg/m²
- Bottom bar: 3.0 kg
- Bare drum radius: 0.055 m | Estimated maximum winding radius: 0.095 m
- Safety factor: 1.3 (residential, moderate cycle rate)
Calculation:
- Curtain weight = 2.2 × 2.0 × 5.5 + 3.0 = 24.2 + 3.0 = 27.2 kg
- Base torque = 27.2 × 9.81 × 0.095 = 25.3 Nm
- With safety factor = 25.3 × 1.3 = 32.9 Nm
Motor selection: A 35 Nm tubular motor is appropriate. A 30 Nm motor sits too close to the calculated requirement and provides insufficient margin.
Example 2 — Commercial Steel Curtain Rolling Door
Shutter specification:
- Width: 4.0 m | Height: 3.5 m
- Slat material: Galvanized steel at 10.0 kg/m²
- Bottom bar (heavy T-bar): 12.0 kg
- Bare drum radius: 0.080 m | Estimated maximum winding radius: 0.175 m
- Safety factor: 1.5 (commercial, high-cycle operation)
Calculation:
- Curtain weight = 4.0 × 3.5 × 10.0 + 12.0 = 140.0 + 12.0 = 152.0 kg
- Base torque = 152.0 × 9.81 × 0.175 = 260.9 Nm
- With safety factor = 260.9 × 1.5 = 391.4 Nm
Motor selection: This application is well beyond standard tubular motor ranges and requires a three-phase industrial door operator or a high-torque geared motor assembly. This also highlights why a counterbalance spring system is typically essential for large commercial shutters — it can offset a significant portion of the curtain weight, reducing the effective motor torque requirement to a manageable level.
Tubular Motor Torque Ratings: Common Nm Ranges and Their Applications
Most tubular motor manufacturers — including Somfy, Nice, and Cherubini — offer motors across a range of standard torque ratings. The table below provides practical guidance for common shutter types.
| Torque Rating | Typical Application |
|---|---|
| 6 – 10 Nm | Small residential aluminum shutters, up to approx. 15 kg curtain weight |
| 15 – 20 Nm | Standard residential shutters, aluminum slats, up to approx. 30 kg |
| 30 – 40 Nm | Larger residential or light commercial, up to approx. 55 kg |
| 50 – 60 Nm | Medium commercial shutters, aluminum or light steel, up to approx. 80 kg |
| 80 – 100 Nm | Heavy commercial, steel slats, up to approx. 130 kg |
| 120 Nm + | Industrial applications; consider three-phase operators above this threshold |
These ranges are indicative only. Always complete the torque calculation for your specific installation. Do not select a motor based solely on application type.
How Drum Diameter Affects Your Torque Requirement Over Time
This is one of the least-discussed aspects of shutter motor sizing — and one of the most practically important.
When the shutter is fully closed, the curtain is entirely unwound from the drum. The motor starts lifting at the bare drum radius — the smallest, most favorable condition. As the curtain winds upward, each layer of slats adds to the effective radius. By the time the shutter reaches full open, the winding radius may be 60 to 100% larger than the bare drum, depending on curtain height and slat thickness.
This means the torque demand on the motor is not constant throughout travel — it increases progressively as the shutter opens. The motor must be sized for the worst case: maximum radius at full open.
Some installers incorrectly calculate torque using the bare drum radius, arrive at a comfortably sized motor, and then find it struggles in the final portion of the opening travel — precisely because torque demand is highest there.
Additional Factors That Influence Motor Selection
Motor RPM and Operating Speed
Torque and speed are inversely related in motor design. Higher-torque motors tend to operate at lower RPM, which translates to slower curtain travel speed. For large commercial doors, a slower travel speed is often preferable — it reduces dynamic loads on the structure and gives people more time to clear the opening. For residential shutters, faster operation is typically preferred for user convenience.
Standard tubular motor operating speeds range from approximately 12 to 20 RPM, producing curtain travel speeds of roughly 8 to 16 cm per second depending on drum diameter.
Duty Cycle and Cycle Frequency
A motor's duty cycle defines the ratio of operating time to rest time it can sustain without overheating. Residential motors are typically rated for intermittent operation — perhaps 4 minutes of running time per 20-minute period. Commercial and industrial motors are rated for higher duty cycles, reflecting the reality that a warehouse door may cycle 50 or more times per day.
Always match the motor's duty cycle rating to the expected operational frequency of the installation. An underrated duty cycle is functionally equivalent to an undersized torque rating in terms of real-world motor life.
Counterbalance Spring Assistance
A correctly tensioned counterbalance spring offsets a portion of the curtain's dead weight, reducing the effective load the motor must lift. In large installations, a well-set spring can reduce motor torque requirements by 30 to 50%, enabling the use of a smaller, more cost-effective tubular motor.
However, spring assistance must be calculated and set precisely. A spring that is under-tensioned provides less help than expected. A spring that is over-tensioned will assist the motor on the way up but actively resist it on the way down — causing a different set of operational problems. Spring tension should always be set by a qualified technician as part of commissioning.
Torque vs. Motor Power (Watts): Understanding the Difference
Motor power (expressed in watts) and motor torque (expressed in Nm) are related but distinct specifications. Power describes the rate at which the motor can do work; torque describes the rotational force it can produce.
The relationship is: Power (W) = Torque (Nm) × Angular Velocity (rad/s)
In practical terms, a high-wattage motor is not automatically a high-torque motor. Motor wattage tells you about energy consumption and general motor size — it does not directly tell you how much rotational force is available at the shaft. Two motors with the same wattage but different gear ratios can have very different torque outputs.
Always size a shutter motor by its rated torque (Nm), not by its wattage. Wattage is useful context; Nm is the specification that matters.
Common Mistakes to Avoid When Sizing a Shutter Motor
1. Using the bare drum radius instead of the maximum winding radius
The motor works hardest when the curtain is fully wound. Always calculate torque at maximum winding radius.
2. Ignoring the bottom bar weight
The bottom T-bar on commercial shutters can weigh 8 to 15 kg or more. Omitting it from the curtain weight calculation creates a meaningful error.
3. Applying a safety factor below 1.3
Real-world installations always have friction, misalignment, and aging effects. A safety factor below 1.3 leaves no margin for the inevitable.
4. Confusing motor wattage with motor torque
A 200W motor is not necessarily more capable than a 150W motor. Compare Nm ratings, not watt ratings.
5. Not accounting for operational cycle frequency
A motor correctly sized for torque will still fail prematurely if its duty cycle is insufficient for the number of daily operations required.
6. Assuming the counterbalance spring compensates for an undersized motor
Springs change tension over time. Never use spring assistance to justify a smaller motor than the calculation demands.
Conclusion: Get the Numbers Right Before You Order
Torque calculation is not complicated — but it does require accuracy at every step. A few minutes spent on the correct formula, with verified input data, will save hours of troubleshooting, costly motor replacements, and frustrated clients.
The method is consistent regardless of shutter type: calculate total curtain weight, determine the maximum winding radius, apply the torque formula, and add a proper safety factor. Everything after that — motor brand, speed, duty cycle, control system — is secondary to getting that core Nm number right.
At MFP Automatismos, we work with installers and specifiers across the full range of rolling shutter applications — from small residential aluminum shutters to heavy industrial steel curtain systems. If you need support selecting the right tubular motor or door operator for your project, our technical team can walk through the calculation with you and recommend the appropriate solution.
Specify the torque correctly the first time. Your motor — and your client — will thank you for it.
FAQ
Q1: What is the basic formula for calculating roller shutter motor torque?
The standard formula is: Required Torque (Nm) = Curtain Weight (kg) × 9.81 (m/s²) × Maximum Winding Radius (m) × Safety Factor. The safety factor should be a minimum of 1.3 for residential applications and 1.5 for commercial or high-cycle installations. Always use the maximum winding radius — measured when the curtain is fully open and maximum layers are wound on the drum — not the bare drum radius.
Q2: What happens if I install a motor with too little torque for my roller shutter?
An undersized motor will operate at or near its torque limit on every cycle. This causes excessive heat buildup, frequent thermal cutout trips, accelerated wear on internal components, and significantly reduced motor lifespan. In commercial applications with high cycle rates, an undersized motor can fail within months. The curtain may also move slowly or stop mid-travel, particularly in the final stages of opening when the winding radius — and therefore torque demand — is at its highest.
Q3: Why does the winding radius matter so much in torque calculations?
Because torque demand increases as the curtain winds onto the drum. Each layer of slats adds to the effective radius, and since torque = force × radius, a larger radius means the motor must produce more torque to lift the same weight. By the time a shutter reaches full open position, the effective winding radius may be 60 to 100% larger than the bare drum radius. Sizing a motor based on the bare drum radius will systematically underestimate the torque required.
Q4: Can a counterbalance spring replace the need for accurate torque calculation?
No. A counterbalance spring can reduce the effective load on the motor, which may allow a lower Nm motor to be used — but only if the spring tension is correctly calculated and set by a qualified technician. Spring tension changes over time, and a motor that relies on spring assistance to function within its rated torque will eventually struggle as the spring loses tension or if the spring is incorrectly set during installation. Torque calculation should always be completed independently of spring assistance, with spring offset treated as a bonus margin rather than a design dependency.
Q5: What is the difference between motor torque (Nm) and motor power (W) for roller shutters?
Torque (Nm) measures the rotational force the motor produces at its output shaft — this is what lifts the curtain. Power (W) measures the rate of energy consumption and gives a general indication of motor size, but does not directly indicate available torque. Two motors with the same wattage but different internal gear ratios can produce very different torque outputs. When selecting a shutter motor, always specify and compare by Nm rating. Wattage is secondary information.
Q6: At what torque level should I move from a tubular motor to an industrial door operator?
Standard tubular motors are typically available up to approximately 100 to 120 Nm, depending on the manufacturer. Above this threshold — or where three-phase power is required, high duty cycle operation is necessary, or curtain weights exceed approximately 150 kg — the application moves into the territory of industrial door operators or geared motor assemblies. These systems offer significantly higher torque capacity, better thermal management, and more robust construction suited to demanding commercial and industrial environments.
