How to Calculate Garage Door Spring IPPT (With a Free Calculator)

Ask three techs what IPPT actually means and you'll get three different answers. It's the single most important number in torsion spring work — and the most misunderstood. This guide explains it in plain English, gives you the formula, and shows you how to skip the math entirely with DoorBot's free IPPT calculator.

What is IPPT?

IPPT stands for inch-pounds per turn. It's the amount of torque a torsion spring delivers for every full turn you wind it. Think of it as the spring's strength rating: a 38 IPPT spring wound 8 turns is pushing about 304 inch-pounds of torque into the shaft.

A door is balanced when the torque the springs deliver matches the torque the door demands:

IPPT × turns × number of springs = door weight × drum moment arm

The drum moment arm is half the drum diameter — 2 inches on a standard 4-inch residential drum. That's why drum size matters: a bigger drum demands more torque for the same door.

The IPPT formula

IPPT comes from three measurements you can take in the field:

IPPT = E × d⁴ / (10.2 × D × active coils)

• E — modulus of elasticity of spring steel, 30,000,000 psi for oil-tempered wire. A constant.
• d — wire diameter in inches. Note the fourth power: wire size dominates everything. Going from 0.250" to 0.273" wire raises IPPT by over 40%.
• D — mean coil diameter: inside diameter plus wire diameter.
• Active coils — total coils (length ÷ wire diameter) minus the 5 dead end coils that don't deflect.

Two practical consequences fall out of that formula. First, a longer spring has a lower IPPT — more coils to share the twist — but lives longer at the same load because each coil works less. Second, small wire-measurement errors blow up fast because of the d⁴ term, so measure carefully: calipers, or the 20-coil method if the paint's worn off.

How to measure a spring for IPPT

1. Wire diameter: measure 20 coils with a tape and divide by 20, or use calipers on a single wire. Our wire size reference snaps your measurement to the nearest standard gauge.
2. Inside diameter: usually stamped on the cone (P200 = 2", P175 = 1-3/4") or measure across the inside of the coil.
3. Length: unwound coil length, cone to cone, not including the cones.

Punch those three numbers into the IPPT quick reference and you get the spring's IPPT and its maximum safe turns.

What IPPT does my door need?

Work it from the door side:

1. Door weight — weigh it with a scale under the center of the bottom section (spring tension released), or estimate it from material and size.
2. Turns — door height ÷ drum circumference, plus 1 turn for cable tension. A 7-foot door on 4" drums needs about 7.7 turns.
3. Required IPPT — (door weight × drum moment arm) ÷ (turns × number of springs).

A 200 lb door on standard residential drums works out to about 26 IPPT per spring in a two-spring setup — or 52 IPPT for a single spring. That divide-by-two is the whole trick behind converting one spring to two: each spring in a pair needs exactly half the IPPT, which is why conversions are more about finding a matched pair than doing hard math. The conversion flow in our spring calculator handles it.

Skip the math

All of this is built into DoorBot, free and working offline:

• IPPT quick reference — spring measurements → IPPT, or door specs → required IPPT.
• Spring identifier — full identification with replacement options ranked by cycle life.
• Spring sizer — door specs → ranked spring options.

The formulas match the SSC Spring Engineering convention used across the industry, so DoorBot's numbers cross-reference with the supplier charts you already know.