The Ultimate Airsoft FPS & Joule Guide
In the world of Airsoft, understanding the relationship between FPS (Feet Per Second), BB Weight, and Joules (Energy) is critical. It determines not only the performance and range of your replica but also whether you are legally allowed to play.
Our FPS Calculator (Airsoft) is designed for players, technicians, and site marshals. It instantly converts your chronograph readings into Joules and predicts how your gun will perform with different BB weights.
FPS vs. Joules: What Matters More?
For years, players focused on FPS. "My gun shoots 400 FPS!" sounds impressive. However, FPS changes depending on the weight of the BB used.
- FPS (Velocity): How fast the BB travels. A lighter BB travels faster.
- Joules (Energy): How much force the BB hits with. This is the true measure of power.
Example: A gun shooting 350 FPS with a 0.20g BB has an energy of 1.13 Joules. If you put a heavier 0.25g BB in the same gun, the FPS will drop to roughly 313 FPS, but the energy (Joules) remains roughly the same (1.13 J). This is why sites now limit by Joules, not just FPS.
Understanding BB Weights
| Weight | Best For | Typical Usage |
|---|---|---|
| 0.20g | Chronographing | Standard for testing. Rarely used in outdoor games as they drift in wind. |
| 0.25g | Indoor / CQB | Good balance of speed and stability for short ranges. |
| 0.28g - 0.32g | Outdoor AEG | Standard for rifles (M4/AK). Punches through brush and wind better. |
| 0.40g+ | Snipers / DMR | Heavyweight for maximum range and accuracy. Requires high power. |
Legal Limits: UK vs US
Safety limits vary strictly by region. Our calculator helps you check compliance.
🇬🇧 UK Limits (UKARA / Policing and Crime Act)
- Full Auto (AEG/GBB): Max 1.3 Joules (approx 370 FPS @ 0.20g). Most sites strictly cap at 1.14J (350 FPS).
- DMR (Locked Semi): Max 1.88 Joules (approx 450 FPS @ 0.20g). Minimum engagement distance applies.
- Sniper (Bolt Action): Max 2.5 Joules (approx 500 FPS @ 0.20g).
🇺🇸 US Limits (Typical Field Rules)
- Indoor (CQB): Often 350 FPS @ 0.20g (1.14J) or sometimes 400 FPS with no minimum engagement distance (MED).
- Outdoor: 400 FPS @ 0.20g (1.49J) for rifles.
- Snipers: often up to 500-550 FPS (2.8J) with a significant MED (e.g., 100ft).
What is Joule Creep?
Joule Creep is a phenomenon where a gun produces more energy with a heavier BB than with a light one.
Example: An HPA gun might shoot 1.1J with 0.20g but 1.5J with 0.40g.
Why? Heavy BBs stay in the barrel longer, allowing expanding gas to push them for a greater duration. This is why marshals now chrono with the weight you intend to use (Game Weight).
Frequently Asked Questions (FAQ)
Does higher FPS mean more range? ▼
Not necessarily. Range comes from the Hop-Up unit and BB quality. A 300 FPS gun with a great hop-up lifting 0.28g BBs will out-range a 400 FPS gun with a bad hop-up.
What happens if I am over the limit? ▼
You will not be allowed to play. In the UK, possessing a full-auto airsoft gun over 1.3J classifies it as a Section 5 Firearm, which carries a mandatory 5-year prison sentence. Always check your power!
Can I use 0.12g BBs? ▼
Avoid them. They are cheap, unstable, and can shatter inside your gun, damaging the internals. Stick to high-quality polished BBs of 0.20g or heavier.
Check your power now
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