Spring upgrade,no chrono surprises.
Plug in your current spring, real chrono reading and target spring. The calculator preserves your gearbox's actual efficiency and shows what to expect — velocity, FPS and joules.
Ratio between your real chrono reading and the spring's theoretical value. 100% = ideal setup.
/ How it works
The math, in plain English
Efficiency carries over
If your M100 reads 95 m/s instead of the theoretical 100, your gearbox is running at 95% efficiency. Swap the spring while keeping cylinder, barrel and seal — efficiency stays roughly the same.
Typical efficiency
Decent stock: 90–95% · Well tuned: 95–100% · Below 85% you have an air leak; check seal and cylinder before upgrading.
When math isn't enough
Big jumps (M90 → M150) often need a wider cylinder, full metal teeth piston, reinforced air nozzle, higher-torque motor. The calc gives expected velocity — not the mechanical health of your gearbox.
/ Frequently asked questions
Quick answers about airsoft spring upgrades
How much FPS does an M120 spring give?▾
An M120 spring is rated for around 120 m/s — roughly 375 FPS and about 1.39 J with a 0.20 g BB. Your real reading depends on gearbox efficiency, cylinder, barrel and air seal, so a healthy setup usually lands a little below the rated figure.
What FPS does an M100 spring give?▾
An M100 spring targets about 100 m/s, near 328 FPS and roughly 1.0 J with a 0.20 g BB. M90 sits around 295 FPS / 0.81 J and M110 around 345 FPS / 1.18 J — the M-rating is the spring's theoretical velocity in m/s.
Will upgrading my spring increase joules?▾
Yes. A stiffer spring stores more energy, so muzzle energy in joules rises. Going from M100 (~1.0 J) to M120 (~1.39 J) is a real power jump — check it stays within your field's joule limit before fitting it.
Do I need a new motor when I upgrade the spring?▾
Not always. Small steps like M100 to M110 are usually fine on a stock motor. Bigger jumps (M120 and up, or speed gears) put more load on the motor, so a higher-torque motor and a stronger battery help keep rate of fire and trigger response healthy.
Why is my real FPS lower than the spring rating?▾
The M-rating is a theoretical value. Air leaks, a worn piston head, a loose cylinder seal or a barrel-to-cylinder volume mismatch all cost FPS. That gap is your gearbox efficiency — this calculator carries it over so the prediction stays realistic.
What spring do I need for 350 FPS?▾
Around 350 FPS with a 0.20 g BB is roughly 1.2 J, which lines up with an M110–M115 spring in an efficient gearbox. Because efficiency varies, enter your current chrono reading above and the calculator picks the spring that lands on your target.
/ Related reading
Related reading
Theoretical estimate. Real-world variables: air seal, cylinder length, barrel volume, hop-up, temperature, wear. Always re-chrono after the upgrade.