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# Garage Door Spring Calculator

This garage door spring calculator estimates the appropriate garage door weight for a given torsion spring. It also estimates the spring service life in openings.

This calculator works with all torsion spring, drum and door combinations – also non-standard ones. For standard springs and doors, see my SPRING SIZING CHART.

NB. The calculator is for regular-lift doors whose tracks are horizontal just above the door opening. High-lift doors which rise vertical some distance above the opening are balanced different.

## Use instructions

1. Coil wire diameter d: Enter the thickness of the spring wire (.207″… .261″ are common). Measure with a caliper (single wire) or a tape measure (10…20 loops).
2. Coil inner diameter D: Enter the internal diameter (D) of the spring coil (1-3/4″ or 2″ residential standards). If the cones are installed, measure coil outer circumference (OC) with flexible tape and calculate: D = OC/3.14 – 2*d. Make sure the spring is not under tension.
3. Drum diameter: Enter diameter of the cable drum (measured at cable tracks; 4″ is standard). This drum is at the end of the torsion shaft. Measure the track circumference (TC) with a flexible tape and calculate: DD = TC/3.14.
4. Door height: Enter the height of the door opening in feet (7…8 ft. common). This is the the height of the vertical part of your door.

## How precise is the weight rating?

Garage door springs should be matched to the door weight to within ±5%. The reason is that the springs should assist the lift by a correct amount through the range of motion.

## What if there are two springs?

If your door will have two springs, use the weight indicated for two springs – everything else stays the same.

## What is the Spring rate and IPPT?

Spring rate basically means spring stiffness. For torsion springs it is measured in inch-pounds per turn (IPPT) or newton-meters per radian (Nm/rad).

Torsion springs with the same IPPT behave the same. Different spring sizes may have practically identical IPPT and be almost totally interchangeable. (The difference is in service life: a short thin-wire spring will fail much sooner than a longer thick-wire one with the same stiffness.)

## How is the service life estimated?

The service life indicated by the calculator is based on estimated spring material stress and typical spring steel resistance to failure through fatigue.

The actual service life may vary depending on the specific spring model, coating, corrosion, rubbing, and so on.