Let Fly Ballistics Calculator

Welcome. Let Fly is a ballistics calculator for modeling the flight of a rifle bullet. Let Fly is useful for adjusting aim when making long shots in all weathers. Let Fly is simple to use in the field. We hope you enjoy it.


The simplest way to use Let Fly select a cartridge from the list, provide the target and weather details of the shot, and tap Go. Let Fly displays the drop and drift for the bullet at the target. Drop and drift are expressed in inches and in scope turret clicks.

For best results, do some preparation at home before taking to the field. The preparation is this: create a new cartridge in Let Fly for your rifle. This includes measuring the height of your scope above the bore and gathering the ballistics information for the ammunition you use.


The Shot screen, and other ballistic input screens display the results at the bottom.

When inputs change, the results display in gray as shown in the lefthand example above. To display the current results, tap the Go button. The ballistic simulation usually takes less than a second. While computing the results the Go button is replaced by a Let Fly graphic. When the results are current, they are displayed in black as shown on the righthand example above.

Ballistic Inputs

There are more than a dozen different inputs to the Let Fly ballistic simulation. They are summarized on The Shot screen. The inputs can be adjusted on the four input screens. The details of each of these inputs is discussed in later sections. The entries in the Cartridge List include a profile containing the contents of the Bullet screen and the Sights screen.

Let Fly uses slider controls with auxillary fine tuing buttons marked plus and minus.

Use the slider to get close to the desired input, then tap the plus and minus buttons to fine tune the slider.

Bullet Details

To see the Bullet Details screen tap the Bullet button on The Shot screen.

There is a wealth of bullet ballistics information available from ammunition makers and reloading suppliers. Ammuntion packaging often has the bullet weight and muzzle velocity.

More detailed information including the ballistics coefficients are available from reloading manuals published by several manufacturers. The internet also has a wealth of information on rifle shooting.

New ballistic chronograph technology makes it possible to directly measure the ballistic coefficient for a bullet in flight. The flight of a bullet is modeled by three of its characteristics: muzzle velocity, weight and ballistic coefficient.

This slider allows the Muzzle Velocity for the bullet to be adjusted up or down.

The Bullet Weight is in grains. The grain is a traditional unit of measure used for bullets. There are seven thousand grains to the pound.

The Ballistic Coefficeint is used in conjunction with one of the G tables to find the drag on the bullet during the simulation. The bullets in the Cartridge List provided by Let Fly all use the G1 table which has been the standard since the late nineteenth century. The G5-G8 tables are specialized drag models of the newest bullet designs.

Sight Details

To see the Sight Details screen tap the Sights button on The Shot screen.

The sights are defined by the zero in range and the height of the sight above the bore centerline. The denomination of the scope adjustment click is also needed to provide clicks to the results.

The Zero in Range is in yards or meters. The slider has a 25 yard/meter granularity.

The Sight Above Bore distance is in inches, and is measured from the center of the scope or sights to the center of the bore.

Many rifles have telescopic sights with adjustment turrets. The turrets have detents which make turns denominated in clicks. Adjusting the turrets makes an angular adjustment to the sight, either vertical or horizontal, depending on the turret. This angle is expressed either in minutes of arc or milliradians.

NONE indicates a rifle without turret click adjustable telescopic sights.

MOA indicates minute of arc. There are 21,600 minutes in a circle.

MIL indicates milliradian. There are about 6283 milliradians in a circle. A MIL is about three times as large an angle as a MOA.

One, 1/2, ... 1/10 is the fraction of a MOA or MIL. A very popular click size is one quarter MOA.


To see the Weather screen tap the Weather button on The Shot screen.

Weather is described in Let Fly with four inputs: elevation, barometric pressure, humidity and temperature. The default weather settings are the Standard adopted when the original G1 table was developed, a rather cool damp sea level.

Elevation affects the air pressure and density. Bullets fly more efficiently at higher altitudes. When the elevation is adjusted, the barometric pressure slider is automatically adjusted to standard.

Lower air pressure improves bullet flight. Set the Let Fly elevation before setting the local barometric pressure slider. If the local pressure is not known, use the standard pressure provided by the elevation slider.

Humidity improves bullet fight, but is less important at cooler temperatures. In general humidity is a small factor, although at high temperatures high humidity can be significant. If the humidity is not known, use 50%. If it feels wet, use 90%. If it feels dry, use 20%.

Temperature is important for modeling. Higher temperatures improve bullet flight. The input range is from -40F to 120F.


To see the Target screen tap the Target button on The Shot screen.

The Target inputs are distance, incline and crosswind.

The Range is in yards or meters. The slider has a 25 yard/meter granularity. The yards/meters setting is controlled on the Sights screen.

The Incline ranges from -50 degrees downhill to +50 degrees uphill.

Crosswind is the component of the wind which blows at 90 degrees to the flight of the bullet. The crosswind is in mph and ranges from 0 to 30 mph. If the wind blows directly across the flight of the bullet, 100% of the wind is crosswind. If the wind blows at 60 degrees to the flight path, the crosswind component is 86% of the wind's speed. At 45 degrees thge crosswind component is 70% and at 30 degrees 50%. So a 10 mph wind blowing at 30 degrees from the flight path has a 5 mph crosswind component.

Drift is derived from the crosswind. When the crosswind is zero, the drift will be zero.

Creating a New Cartridge

Once the settings for Bullet Details and Sight Details are as desired they may be saved as a new cartridge.

The Save as button on The Shot screen creates the new cartridge and displays a text box to enter the name for the new cartridge. The default value is the same name as its predecessor with "new" appended. Tap in the text box and the keyboard appears. Edit the name using the keyboard then tap "Done" to finish editing the name, then tap "The Shot" to continue.

If you get the name wrong, create another new bullet with the correct name. Then delete the incorrectly spelled entries using the process described in Editing the Cartridge List. Use this same process to make other changes in a cartirdge profile.

Editing the Cartridge List

As new cartridges are defined, they appear at the top of the Cartirdge List. To remove unwanted bullets from the list, tap the Edit button.

Tap the No Entry icon to unlock the bullet to be deleted. The lock icon will turn to the vertical orientation, and a red Delete button appears. Tap the Delete button to confirm the bullet delete. Tap the Done button to finish editing the list.

Help Screen

The About button at the top of the Bullets list summons the Help screen, which is this web page.