Different From The Desktop

Using your mouse to aim in a first person shooter is completely different from using your mouse to move your cursor on the desktop and games with a 2D perspective such as LoL and StarCraft. When you use your mouse on the desktop or in a game with an isometric view where you move a mouse cursor and point and click your cursor's precision is limited by your monitor's resolution. When you use your mouse in a first person shooter it turns your view in a 3D space and there is no limit based on your monitor's resolution. Instead your precision is limited by the game engine's turning sensitivity precision which typically allows much more precision. This allows you to take advantage of higher DPI settings.

Use Raw Input

Many games have an option to turn raw mouse ipnut on or off. Mouse movements are typically much more accurate using raw mouse input, especially in first person shooters. It varies game to game, but in some games with raw mouse input off the mouse aiming will be very inconsistent, especially with higher DPI settings. The raw setting takes mouse input directly from the mouse instead of from the operating system which will change the input based on things like cursor position on the desktop which can limit the maximum turning speed and create inconsistent turning speed.

Use Lower In-Game Sensitivity and Higher Mouse DPI

In-game sensitivity controls how fast you turn in the game. This value is set inside your game, usually in the options menu under input. Sometimes only a slider with no values is shown in the menu, but you can usually set an exact value by using the game's console or configuration files. Each game handles sensitivity values in its own way but a lower sensitivity allows more precise movements, so it is usually best to set the in-game sensitivity to 1.0 or as low as possible and increase your mouse DPI to get the turning speed you want.

A 1.0 in-game sensitivity will typically yield the most precise mouse movement the game engine can handle. I recommend you set your in-game sensitivity to 1.0 then adjust your DPI until you get a speed you are comfortable with. If you still need faster turning speed after maxing out your DPI increase your in-game sensitivity.

Why is lower in-game sensitivity better?

Lower sensitivity allows for higher precision. For example, in Counter-Strike a setting of 1 sensitivity turns your character 0.02199 degrees for each dot sent from the mouse, so at 1 sensitivity you have a precision of .02199 degrees. If you set your in-game sensitivity to 10 your character would turn in increments 10 times as much and you would only have a precision of .2199 degrees. You want this number to be as low as possible. If you are trying to shoot at small target from a far away distance and your sensitivity is too high your crosshairs will skip over it and you won't be able to hit it because you can't make precise enough movements.

How fast should I set it?

Just like with desktop speed there is no perfect number. Your ideal number will depend on your play style, body, and equipment. The further you have to move the easier it is to make small precise movements which can be good for sniping but will make it take longer to turn. The best thing to do is experiment around and find what feels most comfortable to you. I found my ideal setting for Counter-Strike to be 800 DPI and 1 in-game sensitivity. This equates to just over 20 inches of mouse movement for a full 360 degree turn.

Most people have their sensitivity too high to be able to make snappy headshots. You want your sensitivity high enough to quickly turn without lifting your mouse, but you also want it as low as possible for maximum precision. In a worst case scenario you would need to do a 180 degree turn in half the width of your mouse pad. It's typical for professional gamers to use large mouse pads so they can use very low sensitivity.

Keep your turning speed consistent between games

Find your perfect turning speed in one game then configure your other games so it is the exact same. Consistent mouse looking between games makes it so your body does not need to adjust when switching games. Consistent mouse aiming will improve your accuracy, reaction time, and confidence in every game you play. Not every game uses the same turning speed sensitivity settings so you will have to adjust each in-game sensitivity or your mouse DPI when you play different games. You can use the sensitivity calculators here or here to figure out which settings you need to use for each game.