How to measure a garage door torsion spring (without guessing)

Every wrong-spring trip starts the same way: somebody eyeballed it. The spring looked like a .250, the truck had a .250, and two days later the door is sagging at half-open and the customer is on the phone. A torsion spring order needs four numbers. Wire size, inside diameter, length, and wind direction, and every one of them is measurable in under two minutes with a tape. Here's how techs who never eat a second trip do it.

First, the safety line, and it's not boilerplate: a wound torsion spring stores enough energy to break an arm. Everything below happens on a spring that is unwound or broken. Never put a tape measure near coils that are still under tension. If you're a homeowner reading this, use it to understand what your tech is doing, not to replace them.

Wire size: the 10-and-20-coil method

Calipers lie on an installed spring. Rust, paint, and grime add a few thousandths, and a few thousandths is the difference between two different springs. So the field method is to measure a run of coils and divide:

1. Push 20 coils tightly together so there are no gaps.
2. Measure that stack in inches. Divide by 20. That's your wire diameter.
3. Cross-check with 10 coils ÷ 10. Both numbers should agree. If they're more than about 0.010" apart, you miscounted. Recount and remeasure.

Twenty coils of .250 wire measure 5 inches on the nose. Twenty coils of .2187 measure about 4-3/8. The 20-coil stack spreads your reading error across twenty coils instead of one, which is why it beats a caliper on anything that's been hanging in a garage for a decade.

Two sizes are sometimes too close for any tape to split. The classic pair is .2950 and .2970. That's what the paint is for: DASMA's color standard puts white on one and red on the other. When the math lands between two sizes, read the color instead of guessing.

What the painted color does (and doesn't) tell you

The color on the end coil encodes wire size only, per DASMA's standard chart. It does not tell you inside diameter, length, or wind direction, and the colors repeat as sizes climb, so a color alone doesn't even pin the wire size down to one answer. Treat it as a cross-check on your measurement, not a substitute for it.

And don't confuse it with the winding-cone color. Red and black cones are a different code entirely. They mark wind direction, not wire size.

The other three numbers

Inside diameter is usually stamped on the cone (P175 = 1-3/4", P200 = 2"). If the stamp is gone, measure across the inside of the coil. Residential is almost always 1-3/4" or 2".

Length is the coiled body measured at rest, cones excluded. On a broken spring, measure both pieces and add them, but push the coils together first, because the break lets the halves relax apart.

Wind direction trips up new techs constantly. Standing inside the garage facing the door on a standard center-mount setup: the left-wound spring has a red cone and mounts to the right of center; the right-wound spring has a black cone and mounts to the left. Yes, it's backwards from what you'd expect. That's why the callback happens.

Do the math once, not in your head

We put the whole flow into a free tool: enter the 10- or 20-coil measurement (or the color, or a known wire size), plus whatever else you have, and it returns the DASMA wire size, flags ambiguous near-misses, and builds the order string. It also handles extension springs, which are a different game entirely. Those are picked by door weight, not coil measurement, and you always replace them in pairs.

Try it: the garage door spring size calculator. No signup, and it runs the same logic Audrey uses in the truck.

Why this matters to the shop, not just the tech

A misread spring isn't a $60 part mistake. It's a second truck roll, an hour of labor, a burned slot in tomorrow's schedule, and a customer who now watches your tech work with their arms crossed. Across a five-truck shop, spring misreads alone are worth a few hundred dollars a month, and they're the single most preventable callback in the book. Measure twice, order once.