In order to calculate your RC helicopter head speed, there are 4 things you need to know:

1. The number of Teeth (T) of your motor pinion. If you set up your RC helicopter yourself, you should know that by heart. If you bought the RC Helicopter RTF (ready to fly) or someone set it up for you, just count the teeth of your motor pinion. It is a lot easier than counting the teeth of your main gear.
2. The number of T of the main gear. This is easy to find – just count the teeth on your main gear. Well, not that easy – just try it, you’ll know what I mean. I get a different number every time I count it. Again, you may find the number of your main gear T on the owner manual.
3. Motor kv – the rpm (head Speed) produced by a motor per volt applied. You should be able to get the motor kv from the motor specification sheet.
4. Voltage: This is the voltage of your battery. You may get this information from your battery specification sheet or label.
5. Efficiency rate: This is the percentage of your RC helicopter motor efficiency when you are flying with your set up. It is the norm to use 80% (.8) or 90% (.9) depending many factors though you run 100% on your throttle curve.

As it was said earlier, the result would only give you a rough estimate of the head speed. A better way is to use a tachometer to measure your RC Helicopter head speed.

Estimate Head Speed = (Motor kv * Battery Voltage /(Main Gear /Teeth of Pinion)) * Efficiency Rate