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Easy FPS Conversion Chart
What is FPS?
FPS (Feet Per Second or ft/sec) is an imperial unit of speed and velocity that represents the distance travelled by an object in feet in one second.
In airsoft, FPS usually refers to the speed of a BB as it leaves the barrel of an airsoft gun (usually measured with a chronograph), and how many feet it will travel in one second.
In countries that use the metric system (rather than imperial measurements, like the US and the UK), companies and airsoft enthusiasts may refer to MPS, or meters per second.
Although the unit of measurement is different, the overall idea is the same – how far will a BB travel in meters over one second of time.
If ever pressed to covert FPS to MPS, remember that one foot = 0.3048 meters.
Conversely, one meter = 3.28084 feet (3.28 feet roughly).
If math was never your thing, you can refer back to this chart:
|I need to convert||So…|
|FPS to MPS`||Multiply the FPS by 0.3048|
|MPS to FPS||Multiply the MPS by 3.28|
I like FPS. What’s the problem with FPS?
A big problem with just relying on FPS is that it depends on a lot of factors.
A big one is that FPS can change radically depending on the weight of the BBs used.
All things being equal (spring, compression, barrel, etc), using a heavier BB will result in lower FPS, while using a lighter one will result in higher FPS.
After all, a lighter object will travel more distance in a shorter time than a heavier one when pushed with the same amount of force.
Relying solely on FPS, an unscrupulous player may try to skirt the rules by loading up on heavy BBs, which will hit harder and can cause injuries to other players, despite being under any FPS limits.
Much like how flicking a grape into someone’s face at 50 MPH feels a lot different (and does a lot less damage) than hitting them in the face with a baseball at 50 MPH, getting hit with a 0.20g BB at 300 FPS feels a lot different than being hit with a 0.50g or even a 0.70g BB at 300FPS.
So using FPS alone is not such a brilliant idea and not really an option.
Smarter and more responsible fields, therefore, add riders or caveats to any FPS limits, stipulating FPS given a particular weight of BB, and they use that weight when doing chrono tests.
For example, a field may say they limit players to 300 FPS at 0.20g, meaning that players are limited to 300 FPS when using 0.20g BBs (which you better bet they’ll insert for a test).
Simple enough, right?
The problem (aside from the fact that some shady owners don’t bother adding the relevant BB weights) is that airsoft players like using different BBs and tend to show up using a variety of different weights.
Figuring out what the limits should be for each and every weight, and then creating a chart for it to post at the entrance of an airsoft field, is cumbersome to say the least.
Relying on random airsofters to do their own quick thinking and mental math before walking in the door is even less of an option and more likely to result in loud arguments and poor reviews on the internet.
All this is an even bigger problem if you’re a lawmaker looking to set firm rules as to how powerful airsoft guns can legally be so that people don’t get hurt.
What was needed was a standard unit of measurement that everyone could use to figure out (and limit) how much energy will be transferred when a BB hits its target, regardless of the weight and FPS.
Which is why those concerned with airsoft weapons and their power began to consider the joule.
What are Joules?
A joule is a standard unit of measurement used all over the world by the International System of Units or SI (note, by the way, that we’re not talking about imperial feet or metric meters anymore).
Very simply, a joule is a unit of energy.
It is equal to the kinetic energy of one kilogram moving at a speed of one meter per second (or the kinetic energy of a mass of 2.2 lbs moving at 3.28 feet per second).
In terms of airsoft, this can represent roughly how much kinetic energy is going to transfer into your target, or (in layman’s terms) how hard is a BB going to hit the person you’re shooting it at.
It’s important to notice here that a measurement in joules intrinsically takes mass and speed into account.
By determining what a “safe” limit for airsoft weapons could be in terms of joules (restricting them to 1.3J for example) it no longer matters what weight of BB an airsofter chooses to use that day (or what the measured FPS is) as long as users stay under this standard energy limit.
This makes it very easy for people all around the world to understand, and limit, the output of any airsoft gun.
Why is it important to keep track of FPS or Joules?
It is extremely important to keep the maximum output of an airsoft replica in mind when playing.
For one thing, high-powered airsoft guns can be very dangerous to those around you.
Airsoft weapons that shoot “hot,” that is at higher FPS or Joules that are typically allowed/sensible, can easily cause other players to bleed and bruise and even (in the worst case) penetrate goggles and cause ocular injury and/or blindness.
As a result, people take these things seriously and airsoft fields can (and will) remove and ban players who exceed the limits, as they should.
More importantly, there are laws as to how powerful airsoft guns can be before they fall under firearms legislation.
These laws vary by jurisdiction, but they tend to place restrictions on the maximum power of an airsoft replica in terms of Joules or FPS (or both).
As you can imagine, falling afoul of these laws with an overpowered airsoft rifle is typically a lot more serious than irritating your friends and getting banned from the local airsoft field, with punishments such as significant fines or even prison time.
Just something to keep in mind when building that custom 800 FPS sniper rifle to hunt your friends with.
How do I calculate Joules?
Figuring out your airsoft replica’s output in Joules is actually a fairly simple physics equation that has to do with kinetic energy. This is:
Kinetic Energy (Joules) = ½ X Mass X (Velocity)²
Where Mass = kilograms and Velocity = meters per second.
For mass, we use the weight of the BB over a kilogram. So, with a 0.25g BB it would be 0.25g / 1000g or 0.25/1000.
For velocity, we use the replica’s MPS or meters per second.
[Note: for those in the US, UK or any country that uses FPS, just multiply your FPS by 0.3048 to get MPS.]
Let’s do a little airsoft math
Airsoft Johnny has an airsoft M4 that fires at 340 FPS when using 0.25g BBs.
His local airsoft field has set a limit to 1.6J for assault rifles.
Airsoft Johnny wants to know if the local field is going to let him in or not and needs to figure out how many Joules his airsoft M4 is going to put out.
Living in the US, Airsoft Johnny doesn’t care much for the metric system and prefers his freedom units.
Unfortunately, this means he has to convert FPS to MPS before he can do anything else.
Accordingly, he first figures out that
340FPS = 103.632 MPS (340 X 0.3048).
He then dusts off the old calculator and plugs all his data into our equation.
Joules = ½ X (0.25/1000) X (103.632)²
He hits the enter key and gets:
Joules = 1.342448928 or 1.34J
Using his preferred BBs and replica, at 1.34J Airsoft Johnny is well below the Joule limit set by the field he wants to go to (1.6J) and other players are certain to soon feel his wrath.
What is Joule Creep in Airsoft?
If you’ve been paying attention so far, or are a fan of math and physics, you might have noticed something.
A heavier BB can change the Joule output calculated in our above equation significantly, with the amount of Joules rising significantly.
This means that an airsoft replica that is fine with 0.25g BBs for example can be banned if a player switches to heavier BBs.
This change in the total muzzle energy of an airsoft gun due to using heavier BBs is referred to in airsoft as Joule creep.
Most airsoft replicas use a certain volume of air to push BBs out of their barrel.
In this system, lightweight BBs are subject to what is called “over volume,” which is where the airsoft gun produces more volume of compressed air than is necessary to propel this light BB out.
The excess air is released harmlessly and without adding extra energy.
Heavier BBs, on the other hand, have more inertia behind them, that is they resist being pushed forward by this volume of air more than lighter BBs.
As a result, it takes longer for them to leave the barrel, and in that time they absorb more energy.
This energy is then transferred to the target or to a measurement device, giving the airsoft gun in question a higher energy output than perhaps the user anticipated.
Now, all airsoft guns are subject to joule creep to one degree or another.
In general, though, gas blowback and HPA replicas do suffer from it a little more due to using a greater volume of compressed gas than AEGs and springers (which usually have a smaller, fixed volume of air in their cylinders).
Interestingly, and perhaps counterintuitively, long barreled airsoft replicas are actually less affected by Joule creep as short barreled ones, as they use more air volume to get any BBs out of the longer barrel and, as a result, it takes far more compressed air to over volume them.
In other words, stock long barrels use more air volume to push BBs out and there isn’t as much left over to let heavier BBs build up that extra oomph of energy.
It is the duty of every responsible airsoft enthusiast to adhere to FPS and Joule limits and to make sure that their replica’s firepower is kept reasonable and appropriate for the game they’re playing.
Keeping an eye on your airsoft replica’s FPS and your Joules isn’t just a matter of the law or a way to keep the owner of your local airsoft field off your back – it can be a matter of safety and the difference between having a fun day and someone now needing to order eyepatches off of Amazon.
Will Martin – Will has been into airsoft and paintball for well over 10 years, and has done it all – from upgrading and fixing gearboxes as a tech to building custom airsoft loadouts for his friends to supporting off those friends as a DM.