What are all these small numbers next to my hero portrait? Why are they in the way of Terrorblade's intimidating features? Why is my hero so dang squishy to right-clicks, especially in the late game (or, if you're feeding like me, for the entire game)? Trigger warning: This post will contain math. As our good friend Cheeks would say, Dota is just one giant math equation, so buckle up buckaroos.
Pictured below is a wonderful Terrorblade, prancing about in a practice match. The two red arrows point us towards his armor. What does armor do? Armor decreases the amount of damage you take from physical attacks (enemy right clicks, as well as lots of abilities like Anchor Smash, Quill Spray, Shadow Wave, Slithereen Crush, etc).
The number in grey is his main armor. Main armor is defined as a hero's base armor plus armor gained by agility. Base armor is different for each hero - Terrorblade starts with 7. Don't worry too much about base armor - IceFrog assigns each hero their base armor and there's nothing you can do to change it. But to understand main armor, you need this component. So once you take into consideration Terrorbale's armor gain from agility, and you wind up with 10.14, which gets rounded up to 11. Here's a fun math equation for all of you keeping score at home:
main armor = base armor + (agility * (1 / 7)).
10.14 = 7 + (22 * (1/7))
Again, that 10.14 gets rounded up to 11, and represents Terrorblade's main armor, which you see in grey. Each point in agility gets you a whopping 1/7th of a unit of armor. Is it super important that you remember that 7 points in agility equals 1 unit of armor? Probably not, but we're doing a deep dive into armor, so this is what you get.
The green number is the amount of armor gained from items. The armor bonus received from items does not transfer to illusions. So here's the best way to think about armor - it makes your hit point pool bigger. This idea is called Effective HP - as you increase armor, it takes more physical damage to bring you down. So here's another fun math equation:
Once you get over the shock of high school algebra, that handy-dandy equation allows you to understand that armor stacks linearly. Check out the table below. Assume you have a hero who has 1000 hit points. If my squishy, make believe hero has 0 armor, his Effective HP is also 1000. Now let's say we get a gift-card for 1400 gold coins and buy ourselves a platemail. Our platemail gives 10 armor, so Effective HP jumps up to 1600. In this example, our 10 armor gave us 600 additional hit points. Now say there was a 2 for 1 special going on at the Secret Shop, so we have two platemails in our inventory. Now our armor is 20. Our fine hero now has an effective HP of 2,200! On the margins, we gained another 600 hit points. That's what we mean when we say armor stacks linearly.
The key take-away from all this - armor will help you from getting right clicked down, but not from getting nuked. Armor will help you tank up against physical attacks, even if you already have an armor item in your inventory. I find myself doing this frequently when I'm playing Undying. I'll buy a Blade Mail in the early mid-game, and then pat myself on the back and never consider another armor item. The earlier acquisition of Blade Mail shouldn't affect my decision later in the game - if I'm against a physical-heavy lineup, I should definitely consider a Plate Mail to build into a Lotus Orb or a Shiva's Guard. So - buy armor! It'll help. I promise.