Airsoft joulecalculator.
Plug in your spring, pick your BB weight and get joules, m/s and FPS in real time. Compare every BB weight, check field limits and pick the right ammo for the job.
/ BB weight comparison
Fixed energy of 0.81 J (M90 spring) — varying BB weight
| BB (g) | Velocity (m/s) | Velocity (FPS) | Energy (J) |
|---|---|---|---|
| 0.12 g | 116.2 | 381 | 0.81 |
| 0.2 g | 90.0 | 295 | 0.81 |
| 0.25 g | 80.5 | 264 | 0.81 |
| 0.28 g | 76.1 | 250 | 0.81 |
| 0.3 g | 73.5 | 241 | 0.81 |
| 0.32 g | 71.2 | 233 | 0.81 |
| 0.36 g | 67.1 | 220 | 0.81 |
| 0.4 g | 63.6 | 209 | 0.81 |
| 0.43 g | 61.4 | 201 | 0.81 |
| 0.45 g | 60.0 | 197 | 0.81 |
| 0.48 g | 58.1 | 191 | 0.81 |
/ M-rating spring chart
Quick reference: spring → energy and velocity with the most common BB weights
| Spring | Energy | 0.20 g (m/s) | 0.25 g (m/s) | 0.30 g (m/s) | 0.40 g (m/s) | FPS @ 0.20 g |
|---|---|---|---|---|---|---|
| M80 | 0.64 J | 80.0 | 71.6 | 65.3 | 56.6 | 262 |
| M90 | 0.81 J | 90.0 | 80.5 | 73.5 | 63.6 | 295 |
| M100 | 1.00 J | 100.0 | 89.4 | 81.6 | 70.7 | 328 |
| M110 | 1.21 J | 110.0 | 98.4 | 89.8 | 77.8 | 361 |
| M120 | 1.44 J | 120.0 | 107.3 | 98.0 | 84.9 | 394 |
| M130 | 1.69 J | 130.0 | 116.3 | 106.1 | 91.9 | 427 |
| M140 | 1.96 J | 140.0 | 125.2 | 114.3 | 99.0 | 459 |
| M150 | 2.25 J | 150.0 | 134.2 | 122.5 | 106.1 | 492 |
| M170 | 2.89 J | 170.0 | 152.1 | 138.8 | 120.2 | 558 |
| M190 | 3.61 J | 190.0 | 169.9 | 155.1 | 134.4 | 623 |
/ How it works
The key concepts behind the numbers
Spring M-rating
An "M90" spring is rated for ~90 m/s with a 0.20 g BB in stock conditions. Higher number = more energy.
Joules are what matters
Energy in joules stays the same when you change BB weight (with the same spring). What changes is velocity: heavier BBs leave slower but carry the same energy and fly straighter at long range.
Typical field limits
Sport rules (not law): CQB up to 1.14 J · Outdoor AEG up to 1.5 J · DMR 1.5–2.3 J · Bolt-action sniper 2.3–3.0 J. Every site has its own rulebook — always check before you show up.
Velocity conversion
1 m/s = 3.2808 FPS · 1 FPS = 0.3048 m/s. The physics: E = ½ · m · v². Mass in kg, velocity in m/s, energy in joules.
/ Legal limits by country
Field rules are one thing — the law is another. Here's the legal joule limit in major European countries.
🇮🇹 Italy · 1 joule, no exceptions
In Italy airsoft replicas are classified as "toy weapons" only if muzzle energy stays at or below 1 J. Above that threshold they become regulated firearms, sold only to license holders. Every prototype must be certified by the Banco nazionale di prova. Reference: Law no. 110 of 18 April 1975, art. 2, as amended by Legislative Decree no. 204 of 26 October 2010.
D.Lgs. 204/2010 — Gazzetta Ufficiale →| Country | Legal limit | Notes | Official source |
|---|---|---|---|
| 🇫🇷 France | ≤ 2 J / ≤ 20 J | ≤ 2 J: free sale (toy). 2–20 J: category D, sold to 18+, no license. >20 J: category B, prefectural authorization. | Code de la sécurité intérieure R311-2 ↗ |
| 🇩🇪 Germany | ≤ 0,5 J / ≤ 7,5 J | ≤ 0.5 J: toy (14+). 0.5–7.5 J: "F" mark required, sold to 18+. >7.5 J: requires Waffenbesitzkarte license. | Waffengesetz — Anlage 1 ↗ |
| 🇬🇧 United Kingdom | ≤ 1.3 J / ≤ 2.5 J | ≤ 1.3 J full-auto · ≤ 2.5 J single-shot. Above: classified as firearm. | Policing and Crime Act 2017, s.125 ↗ |
| 🇪🇸 Spain | ≤ 3,5 J | ≤ 3.5 J with BB ≤ 0.45 g · 8 mm. Category 4 (recreational/sport use). | Orden INT/2860/2012 ↗ |
| 🇵🇱 Poland | ≤ 17 J | ≤ 17 J: no permit required (free purchase). Above: weapons license required. | Ustawa o broni i amunicji ↗ |
| 🇨🇿 Czech Republic | ≤ 16 J | ≤ 16 J: category D, free purchase 18+. | Zákon č. 119/2002 Sb. ↗ |
Limits current as of 2026. Laws change: before buying or using a replica, always verify the official source for your country and the field's rulebook.
/ Frequently asked questions
Quick answers to the most common questions
How many joules does an M90 spring make?▾
An M90 spring delivers around 0.81 J with a 0.20 g BB — that's ~90 m/s or ~295 FPS. The exact number varies slightly with cylinder, barrel and hop-up tuning.
Does changing BB weight change the energy in joules?▾
No. Energy stays essentially the same — what changes is velocity. A heavier BB leaves slower but carries the same energy with better ballistic stability.
What's the legal joule limit?▾
It varies by country. Many EU countries cap retail airsoft at 1 J. Most outdoor fields allow AEGs up to 1.5 J and DMR/Sniper higher with minimum engagement distances. Always check your local law and field rulebook.
How do I convert FPS to joules?▾
Use E = ½ · m · v² with mass in kg and velocity in m/s. To go from FPS to m/s, divide by 3.2808. The calculator above does it for you in real time.
What BB weight should I use?▾
CQB and indoor: 0.20–0.25 g. Outdoor with stock AEG: 0.25–0.30 g. DMR and marksmen: 0.32–0.40 g. Bolt-action snipers: 0.40–0.48 g. Heavier BBs = flatter trajectory, longer effective range.
Theoretical calculation based on E = ½ · m · v². Real chrono readings vary with barrel losses, hop-up, temperature and wear. Always wear ANSI Z87.1 (or equivalent) eye protection.