Armor

Armor is an attribute of characters which reduces incoming damage towards the character's hit points. Armor does not apply to shields, which take 200% damage from ice damage and 100% from everything else.

Damage through Armor is calculated using the following formula:
 * $$Lost\ hit\ points = Damage * \frac{100}{Armor+100}$$

For example, 50 damage passing through 100 armor calculates as (50 * 100/200) = (50 * 1/2) = 25, resulting in a loss of 25 hit points.

A simpler interpretation of armor is that it increases effective health by 1% for every point of armor. For instance, having 100 armor reduces damage by half, effectively doubling health. 200 points of armor reduces damage by two thirds, effectively tripling health(a 200% increase).

A frame with 250 health and 150 armor has an effective health point total of 250 * (1 + 150/100) = 625. That same frame with 10 armor has an effective health total is 250 * (1 + 10/100) = 275; these two armor values are the extremes of what warframes may have for their base stats. The amount of armor a frame has is significant and varies strongly between frames.

The Steel Fiber mod increases a Warframe's base armor by a percentage. For example, 100 base armor with a 50% Steel Fiber mod gives the frame 150 armor; when receiving a hit for 50 damage, this calculates as (50 * 100/250) = 20 lost hit points.

The following table shows the percentage damage reduction benefit gained by using a given Steel Fiber mod. For example, using the 50% Steel Fiber mod with a base armor value of 100 results in 20% less damage being taken than if the Warframe did not use the Steel Fiber mod.

For frames with 100 base armor, the cost effectiveness of Steel Fiber peaks at 80%; for frames with 150 base armor, it peaks at 70%. However, even after the peak the damage reduction is still considerable. For frames with 50 base armor, the cost effectiveness of the mod is highest at the basic 10% Steel Fiber mod; the higher ranked mods are less cost effective.