How to Measure a Garage Door Spring Without Calipers (The 20-Coil Method)
Three measurements identify any torsion spring: wire diameter, inside diameter, and length. Get all three right and you can order an exact replacement — or something better. Here's how to nail each one with a tape measure.
Wire diameter: count coils, don't guess
Wire size is the measurement that matters most — spring strength rises with the fourth power of wire diameter, so the difference between .250" and .243" wire is real money and real cycle life. It's also the easiest to get wrong, because adjacent sizes differ by a hundredth of an inch.
The field fix is the 20-coil method:
- Pick a section of tight, undamaged coils away from the cones.
- Mark off 20 coils and measure the stack with your tape.
- Divide by 20. A 20-coil stack measuring 5" = .250" wire. 4-7/8" = .243".
Use 10 coils on short springs, but 20 averages out small gaps and reads more accurately. The Wire Size tool has a Count Coils mode that does the division and snaps the result to the nearest standard gauge — and if the end coils still have paint, the DASMA color codes confirm your answer.
Inside diameter: read the cone
Most of the time you don't have to measure — the cone is stamped. P175 means 1-3/4". P200 means 2". Commercial springs run 2-5/8", 3-3/4", and up. No stamp? Measure across the inside of the coil at the end of the spring, not the middle (a worn spring can belly slightly).
Length: cone to cone, not including the cones
Measure the coiled body only, with the spring unwound. On a broken spring, measure both pieces and add them — but push the coils together first; the break leaves a gap. On a wound spring, remember it grew when it was wound: subtract roughly one wire diameter per turn (7-8 turns on a 7' door) or just measure after the tension is off.
Put the three numbers to work
Feed wire size, ID, and length into the spring identifier and you get the spring's IPPT, the door weight it balances, and longer-life replacement options ranked by cycle life. The full step-by-step with photos of where to measure is in our free Spring Measurement & Identification procedure.
Calipers are still the gold standard — a $20 pair lives in many spring techs' pockets. But a tape measure and 20 coils will get you the same answer, every time.