Kevlar body armor costs $650 and reduces damage taken by bullets and grenades, but headshots by most weapons will still be fatal.
Players can also buy kevlar armor with a helmet for $1000 that saves players from one-shot kills to the head by any weapon except sniper rifles, close range shotguns, the Desert Eagle, the AK-47, and the SG 553. All rifles will deal lethal damage to the head in Counter Strike: Global Offensive.
Damage from rifle rounds and the armor piercing SS190 FN57 ammunition (used in the Five-seveN and P90) is reduced only by about 25%, although it should be noted that the FN57 ammunition does little damage in the first place.
As long as you have any armor at all, the exact armor value does not matter.
Once the armor value reached to zero, the armor and helmet will be discarded and the damage will no longer be reduced.
Kevlar will not protect you from attacks that hit anywhere other than on the chest, stomach, and head (with a helmet) hitboxes, so sometimes you will see your health is very low, but the armor is still 100%.
There is no visual difference when having armor equipped thus making it easy to fool other players. Enemies can only tell if a target has armor by damaging them and seeing white sparks (and the lack of blood) which indicates that the player is wearing armor.
The world model file for kevlar is named "w_kevlar" while the kevlar and helmet is named "w_assault".
Kevlar and Helmet is referred as "assaultsuit" in the gamefiles while the kevlar is still "kevlar".
If a mapper has decided to use content from Half-Life and place the battery used specifically for the H.E.V. suit, players will gain 15 points of armor. In some custom servers, if a Half-Life NPC is used, the Kevlar has the same properties as the H.E.V. suit.
Moreover, the H.E.V. recharge port is usable in the Goldsrc Counter-Strike games.
Although the world model of the Kevlar does not appear in official maps (except the training mission from Counter-Strike 1.0), some custom maps feature the MDL file that can be obtained by players.
It is impossible to purchase the helmet separately without obtaining Kevlar.
The Kevlar and Helmet is less effective in the Source and Global Offensive version, as most weapon's damage is increased. Also, in Global Offensive, the Knife's damage is now unaffected with the kevlar, which is adding realism.
The helmet in real life is not bullet proof, even the 9mm bullet can still be lethal. The Kevlar vest however, can only protect most pistol and shotgun caliber rounds as well as explosive sharpnel but not rifle bullets. In-game, this was implemented, most likely as a balance reason.