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Coil spring rate calculator. Works offline, calibrates to springs you measure.
Spring Loaded works out the rate of a coil compression spring from three measurements you can take with a vernier caliper. Enter the material diameter, the number of active coils and the inside diameter, and it gives you the spring rate straight away — in N/mm, kg/mm and lb/in at the same time, so you don't have to convert anything by hand. WHAT YOU GET • Spring rate in N/mm, kg/mm and lb/in • Mean and outside diameter • Solid height and total coil count • Spring index (C) • Available travel and the load at that travel • Shear stress at load, with the Wahl correction applied • Eight selectable materials — spring steel / music wire, chrome silicon (SAE 9254), chrome vanadium (SAE 6150), stainless steel 302/304, stainless steel 316, phosphor bronze, titanium 6Al-4V and Inconel X-750 Two optional fields sharpen the answer. Enter the free height and you get the exact coil-bind travel instead of an estimate. Enter the travel you need and it tells you the free height required to deliver it. IT TELLS YOU WHEN THE SPRING WON'T WORK Most calculators hand you a number and stay quiet. Spring Loaded flags the things that actually bite: • A spring index outside the 4–12 band that springs are practical to wind in • A working stress past what the chosen material will take • A free height long enough that the spring will buckle rather than compress • A required travel the spring simply cannot deliver before it binds CALIBRATE IT TO YOUR OWN SPRINGS The textbook formula assumes an ideal spring. Real ones come out a few percent different because of end treatment, presetting, wire tolerance and the actual shear modulus of the batch. Rather than guess at that, measure it. Tap Calibrate, enter a spring you already know the rate of — one with the rate stamped on it, or one you have put on a scale — and the app solves the formula backwards for that spring's real shear modulus. The difference becomes a correction applied to every calculation after it, so the numbers match the springs actually in your hands. The correction is saved on the device and survives a restart. A CALIBRATED badge appears on the result and the correction is shown as a percentage, so a tuned reading is never mistaken for a nominal one. Remove it at any time. A correction beyond ±10% is flagged, and beyond ±40% treated as a probable input error — usually the wrong rate unit or a miscounted active coil. You can calibrate the travel estimate the same way, against the measured travel of that reference spring. THE MATHS k = G·d⁴ / (8·D³·n) where D is the mean coil diameter (inside diameter plus material diameter), n is the active coil count and G is the shear modulus of the material. Stress uses the Wahl correction factor to account for the higher stress on the inside of the coil. WHO IT IS FOR Motorcycle and car suspension work, valve springs, clutch and brake springs, machine building, small engine repair, model engineering, restoration, and anyone who has a box of unmarked springs and needs to know what is in it. NO ACCOUNT, NO INTERNET, NO ADS Spring Loaded is a calculator, not a service. It works fully offline — try it in aeroplane mode. There is no sign-in, no advertising, no analytics and no tracking of any kind. Nothing you type leaves your phone. The only thing saved on the device is your calibration, and only if you create one. A NOTE ON ACCURACY The rates reported are theoretical, derived from the dimensions you enter. Real springs vary. Calibrate against a known spring to absorb that difference, and verify any safety-critical spring against a physical test or the manufacturer's specification.