Torsion Spring Calculator

How the calculator works

Everything a spring supplier's chart does comes from four relationships, and the tool above uses the same ones. First, the torque needed to hold the door is its weight times the drum radius: a 180 lb door on a 4" drum needs 180 × 2 = 360 inch-pounds. Second, the turns the spring winds through is the cable travel divided by the drum circumference, plus about three-quarters of a turn so the door stays snug at the floor: an 84" door on a 4" drum is 84 ÷ 12.57 + 0.75, which rounds to 7.5 turns. Third, IPPT (inch-pounds per turn) is torque divided by turns, split across the number of springs: 360 ÷ 7.5 ÷ 2 = 24 IPPT per spring. Fourth, a spring's IPPT is set by its wire diameter to the fifth power, divided by its mean diameter and its length, using the standard torsion-spring rate formula; that is why a small step in wire size is such a large step in lift. The calculator holds the wire-size list and works the formula backwards: for each inside diameter it finds the smallest standard wire that reaches your cycle target and the length that spring has to be to give the IPPT your door needs.

The rate constant is calibrated against common field pairings (for instance, a 16' × 7' door around 200 lb on 4" drums ending up on a pair of .250" × 2" springs in the low-to-mid 30s of inches), so the results line up with what a supplier would quote rather than a textbook ideal. Our wire size page shows the stress and deflection formulas in full.

Why the cycle life changes with wire size

Springs fail by fatigue. Each cycle bends the wire; the harder the wire is worked, the fewer cycles it survives. The stress in a torsion spring at full wind goes with the torque divided by the wire diameter cubed, so stepping from .243" to .250" wire at the same torque cuts the stress by about eight percent, and from .243" to .273" by about a third. The calculator turns that stress into an estimated cycle count, with 10,000 cycles as the builder-grade reference. The estimate is a planning number: the real life of a spring also depends on the garage being damp, the door being heavier than it was sized for, and whether the bearings are turning freely. The spring lifespan page has the day-to-day arithmetic.

Why most doors want two springs

Two springs each carry half the torque. At the same wire size that halves the stress in each, and a spring at half stress lasts many times longer, not twice as long. Two springs are also shorter than one spring doing the same job, which matters on a 16' shaft with a centre bracket in the middle. And when one breaks the other still holds part of the door, so it comes down slowly rather than dropping. The tool defaults to two; switch to one to see how long a single spring has to be for the same door. On a light single door one spring is fine and normal.

Why the drum matters

The drum sets both the torque (weight × radius) and the number of turns (travel ÷ circumference). A 5.25" drum needs 31 percent more torque per turn than a 4" drum but takes fewer turns to lift the same door, so the IPPT required comes out similar and the spring is wound less. High-lift and vertical-lift drums are tapered, so the effective radius changes as the door rises; the tool uses the drum's flat diameter and flags the result as a first pass. Those doors are always sized from the drum maker's chart on site. The cables and drums page has the drum families and what each is for.

Getting the weight right

Every number above hangs off the door weight, and guesses are usually wrong. A single-skin steel 16' × 7' door can be under 140 lb; the same size with a 2" polyurethane core, windows and a strut can pass 220 lb. The estimate menu in the tool uses typical pounds-per-square-foot for each construction, and it says so on the result. The right way is to weigh it: disconnect the opener, lower the door, slide a bathroom scale under one bottom corner with the other corner on a block of the same height, and double the reading. If the door has old springs that are still holding, we weigh it with the springs unwound on the day. Doors that have been painted, had trim or glass added, or had a strut fitted since the springs were sized are covered on added weight and springs.

Using the measuring tool

The lower tool is for reading the spring that is on your shaft now. Measure ten or twenty coils tight with a tape and enter the length; the measurement guide shows exactly where to put the tape. Twenty coils is better because a 10-coil reading only separates neighbouring wire sizes by a sixteenth of an inch. Add the inside diameter (stamped on the cone, or measure across the inside of the coil) and the coil-to-coil length, and the tool reports the spring's IPPT and the door weight it was sized for on the drum and spring count you chose above. If that weight is a long way from what your door actually weighs, you have found the reason the door has been heavy or the opener has been straining. The colour code page gives a second check from the paint on the end coil.

What the tool will not do

It will not tell you how to wind a spring, and neither will we. A wound torsion spring on a double door is holding a couple of hundred pounds through two set screws and a winding cone, and the tools and sequence to release that safely are not something to learn on your own door. What the tool is for: understanding a quote, checking whether the spring on your shaft matches your door, seeing what a high-cycle upgrade involves, and deciding whether a one-spring door should become a two-spring door. When you are ready, broken spring repair is a same-day visit with both springs, a bearing check, and a balance test before we leave.