Combat Mechanics
So, you have a character and want to have them to hit things. Or you're following my test character thing and want to see how to fill in the rest of the info on the Character Sheet. Either way, this post will cover how to do all that stuff.
Armor and Armor Class
First let's make sure your character has protection in place. Armor Class is sort of a misnomer because it isn't just about having thick armor. It's a general measurement of how hard a character is to hurt with an attack, and that can be anything from heavy armor or hide to just being really good at dodging stuff or both. It's a general measurement of how hard someone is to hit.
Figuring that out for your class will be a bit different if you have a class feature that gives you incentive to not wear armor (like Barbarians and Monks). They have their own formulas listed in their class info to note how to calculate that AC. For everyone else, it works the same way. If you're not wearing armor, your AC is 10 + your Dexterity modifier. If you are wearing armor, the AC info for the equipment will tell you what to do: light armors have you start at a different number and add DEX, medium armors start at a different number of DEX but only up to a maximum or two from DEX, and then heavy armors just give you a straight number to use for your AC without any DEX bonus because they're too heavy to dodge around in. It's also worth noting that some armors (mostly heavy and noisy ones) also give you disadvantage on Stealth checks, and some heavy ones require a certain Strength score to be worn. On top of that, if you have a shield and are proficient in using shields, you add the shield's value to your AC as well.
So, let's get back to our Dwarf Fighter example guy. We picked up chain mail for him from his class equipment choices, and that is a heavy armor that gives a flat AC of 16 and disadvantage on Stealth checks. He also has a shield, which gives 2 more AC. That brings him to a total of 18, which I'm writing into his Armor Class box at the top middle of the Character Sheet. That means anyone who wants to hit him with a sword or targeted spell attack will need to roll their d20, add their modifiers, and get a total of 18 or higher to actually hit this Dwarf. That's a pretty solid number to start with, so our test character is looking good on the defensive side of things. Now for offense.
Weapons
For weapons, you'll have to take a look at the equipment info to see what exactly it has going on. There are a lot of special keywords used in the Player's Handbook that you'll want to check before settling on a weapon if you're looking to select the perfect weapon. You'll want to pick a weapon that's suited to your class and stats (and that you are proficient with) if you want to do any good damage with it.
Melee weapons as a default use Strength for attack and damage rolls, but if they have the finesse property you can choose to use Dexterity instead. Ranged weapons use Dexterity for their rolls. Thrown weapons use Strength, unless it has finesse (like a dagger) which allows you to use Dexterity. Anything that's thrown or is a ranged weapon will note two distances for range: the first is its ideal operating range, and the second is its maximum range. If you're firing or throwing at something beyond the ideal range, your attack roll will have disadvantage.
For all the other special properties, I'll leave you to read the book to see what they're about. The other key information is how much damage the weapon does, and what kind of damage it deals: bludgeoning, piercing, or slashing. The type is important because some creatures will resist certain types (say creatures with thick hides resisting slashing damage) or be vulnerable to them (like skeletal undead creatures being vulnerable to bludgeoning).
Our test Dwarf has two weapons: a longsword and a light crossbow. I'll be putting the information for actually using those into the Attacks & Spellcasting box in the middle of the Character Sheet. First, for the Longsword, I'll write Longsword into the name slot of that first line. The ATK Bonus column is for writing what you add to any attack roll made with the weapon. That'll be the modifier for the ability score it uses + your proficiency bonus IF you are proficient with the weapon. Test Dwarf has +3 STR and +2 proficiency bonus, so he has a +5 attack bonus. The Damage/Type column is where you put down what you'll roll to determine damage and what type it does. A Longsword used in one hand (because we have a shield) does 1d8 slashing damage, and you also add the same ability modifier you used for the attack roll; you do NOT at proficiency bonus to damage. So, for that column, I'm putting in 1d8+3 slashing.
Putting in the Light Crossbow info follows the same steps. It's a ranged weapon, so we use Dexterity instead of Strength. Test Dwarf has +2 there and is proficient with this weapon, so the attack bonus is a +4. Light crossbows deal 1d8 piercing damage, so with the ability modifier added on that makes 1d8+2 piercing damage. It also has a range note of (80/320), which means it works like normal on targets up to 80 feet away or can be fired at disadvantage at things up to 320 feet away, but that's too much to fit into the spot so I'm writing it down below in the same box.
This all fits together pretty simply. When you attack something, you make your attack roll. The DM will compare the result with the the target's AC. If your roll is lower than that AC or you roll a 1, you miss. If your roll is equal to or greater than the AC, you hit them so you do your damage roll and tell the DM how much it equals. If you roll a 20 on the d20 that's considered a critical hit, which gets extra damage: you roll your normal attack roll and add the modifier as normal, then you roll the weapon die/dice again and add that result (but not the modifier again), and that's your extra damage on a critical hit. Some DMs prefer to instead have you just double the number on the dice for the damage roll and add the modifier to that, so be sure to clarify how they want it done before you go nuts with extra rolls.
Okay, once you have your AC and weapon rolls figured out, you're almost ready for combat. Now you just need to, you know, learn how combat works.
Initiative, Turns, and Actions
Combat in D&D is done in turns, rather than everyone going at once. It can get mildly weird because it's
technically supposed to be everyone going at once, it just... doesn't mechanically work that way. A single round of combat is 6 seconds long, which is good to remember for the sake of things like the duration of spells and class features. Within that 6 seconds, EVERYONE in combat does stuff at sort of the same time but not exactly, and then if fighting is still required it moves on to the next round.
Determining who goes in what order is done by rolling initiative. This is simply a d20 roll with your Dexterity modifier added to it, unless you have a feature that adds some extra bonus to it as well. All the player characters roll, the DM rolls for their NPCs, and then the DM is in charge of making a turn order list and keeping track of whose turn it is. For our test dwarf, I'm writing a +2 in the Initiative box at the top center of the Character Sheet because that's the modifier he'll get to initiative rolls with his +2 DEX modifier.
On each turn, the person/creature doing stuff has a limited number of actions they can perform, with that limit intended to represent how much they can do in 6 seconds. Yes, this will get nutty at higher levels where a class like Monk can sprint 60 feet and then punch something like 4 times all within supposedly 6 seconds, but it's fine, you're playing in a world with magic and shit so a little superhuman speed is nothing. The actions each entity has on their turn is as follows: movement, one action, one bonus action, and one interaction/free action. However, if they have a feature that allows them to bypass this limitation, the feature overrides the limits of the action economy.
Movement is exactly what it sounds like. Remember that speed stat you got from your race? That's how many feet you can move in a turn without needing to spend any of your other actions to run. You can split up your movement between other actions as much as you like, so you can run a bit, hit someone with your action, move again, use a bonus action, move again, use your interaction, and then move again if you really feel like it. It's worth noting that if your DM uses a grid/map for combat, each space on it will generally represent 5 feet of space, so moving from one square/hex/whatever to the next will cost 5 feet of movement.
Your action is the big deal of your turn, the thing that's supposed to take the most effort/time of your turn. There are a limited number of actions that are presented in the Player's Handbook as combat options: Attack is making an attack with your weapon, Cast a Spell is casting a spell with a casting time of 1 action, Dash is turning your action into more movement (equal to your speed, so doubling your speed), Disengage is letting you move without getting hit by opportunity attacks (more on that in the Reactions section), Dodge is making attackers have disadvantage when they try to hit you until your next turn, Help is helping someone else with an action or attack and giving them advantage, Hide is trying to hide by making a Stealth roll, Ready is to prepare an action to be done later with a trigger (like "I ready my bow and I'll shoot if the goblin tries to run away"), Search is to spend your turn looking at/for something, and Use an Object is for using objects that can't be done quickly (like opening a heavy door or loading a ballista). You don't really NEED to remember all of those, but those are the options. Generally you'll be attacking or casting a spell most of your turns in combat, and anything else not covered by the other kinds of actions will use your main one.
Bonus actions are quickie extra things that you don't always use. Some classes start off with nothing that is a bonus action, and that's fine. You need a spell that has a casting time of a bonus action or a feature that says you can use it as a bonus action, otherwise you can't use it. Lots of people make the mistake of thinking it's just action #2, but that is not the case. Think of the main action as something that takes like 4-5 seconds to do and the bonus action is filling that last 1-2 seconds, so definitely not enough time to squeeze in another full attack under normal circumstances. You get one bonus action at most per turn, and you can do it before or after the main action as you see fit.
The interaction or free action is for simple things that could be done super quick or as part of another action. Stuff like opening a door that isn't locked or handing an item to someone else could be done as part of your movement, or drawing your weapon could smoothly tie into making an attack. There's a nice table of options for this, titled Interacting with Objects Around You on page 190 of the Player's Handbook that provides a lot more of these small actions that you can do for free once per turn.
So, for example, for our level 1 Warrior test character, he only has one bonus action: the class feature Second Wind. All in one turn he could move somewhere, draw his sword or crossbow, attack something with it, and use Second Wind to heal a bit. That's it, nothing left to do, turn over. However, you can do some things (other than getting hit by enemies) when it's not your turn as well.
Reactions
The final component of the action economy of D&D 5e is the reaction. This is, just as it says on the tin, something you do in reaction to something else. You get one reaction per round, and then you regain your ability to react at the start of your turn.
The most common reaction is the opportunity attack. This is a game mechanic way of showing someone has lowered their guard (or done something stupid) and their enemy takes advantage of it. Opportunity attacks happen when someone tries to leave the reach of an enemy near them. To turn and walk away from an armed enemy leaves you open, so that enemy takes the opportunity to attack. Player characters get opportunity attacks, but so do
all creatures and entities that can fight. That's why the Disengage action exists: if you want to run away and not get stabbed in the back, Disengage before running.
There are also a variety of spells and class features that give reactions. Wizards and Sorcerers can get the Shield spell, which costs a reaction (and spell slot) to use when they get attacked to raise their AC until their next turn (and hopefully make the attack not hit). Monks have a class feature for using their reaction to try to catch arrows and bolts fired at them. Your spells or abilities will make it very clear if something is a reaction, and if you don't have anything that explicitly says it can be done as a reaction then you just don't have a way to use your reaction (unless an enemy is stupid enough to provoke an opportunity attack).
Hit Points, Conditions, and Death
The ultimate goal of combat is to beat the baddies and not die in the process. That all revolves heavily around hit points. Hit points are your measure of health, and if your hit points drop to 0 then you fall unconscious. In D&D 5e you don't die for losing all your hit points, but it DOES make you a lot more likely to die, which I'll get into at the end of this section. I'll go over healing options briefly here.
Healing hit points only happens one of three ways: resting, being healed by a spell/feature, or using an item that heals you (like a potion of healing). The latter two are pretty obvious and are the only ways to heal in combat, but resting can be done two ways: short rests and long rest. Long resting is getting a good night's sleep, and in D&D 5e that means you heal almost all that ails you: all hit points are restored, so you could go from being on death's door to full health overnight. Sleep is powerful, clearly.
However, short rests can also heal you, but it's more limited. Back in the class selection part I told you to skip over the stuff about hit dice, but here's where they come into play. Each class gets things called hit dice for each level you have in that class, with different classes getting dice with different numbers of sides, and when you take a short rest you can choose to use some of these hit dice to heal some hit points (specifically the number rolled + your Constitution modifier for each die used). Hit dice only get restored on a long rest, and you only get back at most half of your total hit dice per long rest, so if you've got a high level character who had to used all their hit dice for healing on short rests it's going to take them a couple long rests to get them all back.
For test Dwarf, since he's a Fighter his hit dice are d10s. He's a level 1 Fighter, so he only has 1 hit dice. On the Character sheet, near the very center of it, there's a small box that says Hit Dice. I'm putting a 1 in the Total line, and a d10 to note the kind of hit dice he has. While I'm at it, I realized I also forgot to add in the Current Hit Points field up above that. He's got all his hit points because he just got a good night's rest after a long day of Chaotic Evil dragonchess playing, so I'm throwing a 12 in there.
Temporary hit points can be gained from a variety of sources. They work sort of like a shield over your normal hit points: if you take damage, you lose them from the temporary ones before your normal hit points go down. Once you lose temporary hit points they are gone forever and you'll need to get new ones from a new spell or feature or whatnot. You can't heal temporary hit points like you do with normal hit points. You also can't stack them on top of each other: whatever the highest number of temporary hit points you have from a single source is, that's all you've got. If you get 5 temporary hit points from an ally's spell and then use a class feature to gain 10, you only have the 10 temporary hit points, not 15. Test Dwarf doesn't have any way to gain temporary hit points, so I'm leaving that field blank.
Conditions are effects (usually but not always negative) that can be applied to player characters and enemies that do a wide variety of things. I'm not going to go over them all here, but you can find them in Appendix A of the Player's Handbook or probably somewhere online. Each condition has its own rules for how it works, and some quick examples are blinded (they can't see, their attack rolls have disadvantage, and attack rolls against them have advantage) and incapacitated (they can't take actions or reactions). Using these on your enemies will usually make them easier to kill, but getting hit with a condition also makes your character easier to kill, so try to roll well to avoid them!
Death in D&D 5e is a very forgiving mistress. As mentioned before, losing all your hit points doesn't equal death. You can just be instantly killed by some high level spells or if you take so much damage that it would take you down to negative hit points equal to or greater than your maximum hit points (like if our test Dwarf stumbled into an angry Ancient Red Dragon with his measly 12 hit point maximum and got blasted for more than 24 damage by its fire breath), but those are special occasions. There's another teeny part of the Character Sheet that I have not talked about at all: the Death Saves box near the hit point info. This is the thing you'll use most often to see if your character dies in a fight.
When your character drops to 0 hit points they go unconscious (which is a specific condition with special rules listed in Appendix A of the PHB). They stop being unconscious if they regain hit points, but while they remain unconscious they have to make a death saving throw every time their turn comes around in the combat rounds again. Death saves are a straight d20 roll with no modifiers (unless you have a feature or item that says otherwise), and you need to get a 10 or higher to succeed. If you succeed you mark off one of the successes circles in the Death Saves box; if you fail you mark of one of the failures. Getting a 1 on the d20 counts as
two failures, but getting a 20 makes you regain 1 hit point and regain consciousness. If you take any damage while at 0 hit points it counts as a death save failure, and if it's a critical hit it counts as 2 failures (also, fun fact, part of the condition included in being unconscious is that attacks against you have advantage and any melee range attack that hits is an automatic crit, so all it takes is an extra two close range hits after you drop to kill you). Oh, and remember that bit about going to negative hit points equal to your maximum? If one hit you take while unconscious does enough damage to take you from 0 down to negative points equal to your max, you die instantly (but you don't actually keep track of negative hit points, you go back to 0 after every hit... not that it will take many to kill you if you're already unconscious anyway).
With all that fun stuff explained, the simple mechanical basis for death saves is this: if you rack up 3 failures, you die. If you rack up 3 successes, you stabilize. Being stable means you're still at 0 hit points but you stop needing to make death saves, and you regain 1 hit point after 1d4 hours (either the DM will make the roll or tell you to make it). Someone can also be stabilized with some spells, or by using an action to make a Medicine check on the unconscious person and beating a DC of 10. Oh, and just to round the subject off, it's up to the DM whether monsters and NPCs and such get death saves or just instantly die at 0 hit points, so maybe be careful if you're just trying to beat someone up to capture them alive.
You Are Now Ready To Play
And it only took reading over 13000 words to do it, at an estimated average reading time of over an hour based on some website I checked. Wow!
But for real, you have read through a condensed version of the key info that fills a couple hundred pages of the Player's Handbook (which reaches over 300 pages including the ~90 pages of spell information that I barely touched). There's going to be some stuff missing here, but you have all the information you need to dive into a game and learn hands on, which is generally the best way to learn all the gritty details. As stated previously, please do ask questions if something was confusing or you feel like something vital was missing! This guide is worth existing if it's actually useful in serving its purpose, so don't be shy in pointing out any issues.
Oh, and for anyone who was following along with the test Dwarf, let's check in on what he looks like after the additions made beyond the Making A Character post.
Here's an image of the Character Sheet page. Not much different from the first look, but all this needs is a character name chosen and those personality info boxes filled out to be ready to throw into a game.
The final post of this guide is stuff that you might be interested to know but probably won't be necessary for starter games, so no need to rush into reading it unless you have a ravenous thirst for more knowledge.