There are many risks to flying. Some might assume turbulence is a major risk, but in reality, there’s little need to be afraid of with turbulence. Ask any pilot, and they’ll tell you the two times where flight riskiness is at its highest is during takeoff and landing. For takeoff in particular, there are so many variables — from wind to birds to engine functionality — that pilots need to be extra vigilant when they are preparing for takeoff. One good rule that pilots should always keep in mind is the “50/70 Rule.”

This isn’t an official rule, but the FAA recommends it as a good rule of thumb. The 50/70 Rule is simple: When taking off, you should reach 70% of your plane’s rotation speed by the time you reach the halfway point of your takeoff distance. This comes from the physics principal that distance correlates with the square of takeoff velocity. 0.7² is 0.49, or 49%, hence the more digestible 50% rule of thumb.

Not every runway is marked the same, so it can be a little tricky to know exactly when you hit that halfway point. Some runways have markers telling you how many thousands of feet are left on the runway, like a “3” signifying 3,000 feet remaining. For others, it’s best to pick a landmark that’s at roughly the halfway point, such as a tower a perpendicular cross path. Keep in mind, you’re looking for the halfway point of your takeoff distance, not of the runway. If you haven’t reached 70% of your rotation speed when you reach that point, you should abort the takeoff to assess what the issue is, be it mechanical, weight, wind, or something else.

There are many different speed markers a pilot needs to know when flying. They’re known as V-speeds, indicated by a capital V followed by a number or lower-case letter. For instance, V1 is the speed at which the pilot must decide whether takeoff is feasible. If you haven’t reached that speed, you can abort a takeoff if the conditions don’t seem right, but if you reach or go beyond V1, takeoff must happen.

Rotation speed — or Vr — is the speed in which the pilot starts applying the control inputs to get the nose up and create vortexes at the wing tips. Vr is not a universal number. Every single plane is going to have its own unique rotation speed based on its weight and flap setting, but elements like wind and weather can affect it as well. Some fighter jets are even built to reach these speeds without the need for a runway at all. This is different from liftoff speed, designated by the symbol Vlof.

Importantly, these are airspeeds and not ground speeds. You aren’t measuring your Vr in miles or kilometers per hour. These are in knots. To put the 50/70 Rule into effect, you just need to do a little bit of math with your Vr. If the plane you’re flying has a Vr of 100 knots, that means you need to reach 70 knots by the time you reach that halfway marker. If it’s 155 knots, you need to reach 109 knots. Because you still have 50% of tune runway remaining, you don’t need to worry about aborting a takeoff because you have plenty of runway to stop.