How to Calculate Garage Door Spring Torque
The formula for garage door torsion spring torque, a worked example, and how drum diameter and spring count change the result.
Torsion spring torque is the twisting force a spring must supply to counterbalance a garage door's weight through the cable drum it winds onto. Getting this number right is the foundation of safe, correctly balanced spring sizing.
The formula
Torque per spring (in-lbs) = (door weight ÷ number of springs) × drum radius (half the drum diameter). Splitting the weight across two springs — the most common residential setup — halves the torque each individual spring needs to supply.
Worked example
| Input | Value |
|---|---|
| Door weight | 175 lbs |
| Number of springs | 2 |
| Weight per spring | 87.5 lbs |
| Drum diameter | 3 in (1.5 in radius) |
| Torque per spring | 87.5 × 1.5 ≈ 131 in-lbs |
Why drum diameter changes the answer so much
A larger drum (common on high-lift or vertical-lift door setups) increases the torque needed for the same door weight, since torque scales directly with radius. This is why a door that switches from a standard low-headroom track to a high-lift track often needs a different spring even though the door itself hasn't changed — the leverage point moved.
Run your own numbers, including single-spring setups on lighter doors, in the Garage Door Spring Calculator.