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I have been playing 3.5 for 6 years now and am trying out 5e. Does size affect AC in 5e? In 3.5 a tiny creature would get a bonus to their AC for being smaller, while a huge creature would get a penalty to their AC for being larger. Is this rule still around in this edition?

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No, Size does not govern Armor Class

The default way AC is calculated does not involve a creature's size at all.

PHB pg 14, Armor Class

Without armor or a shield, your character's AC equals 10 + his or her Dexterity modifier.

If you do wear armor, your AC is not 10 + Dex mod, but instead what the armor specifies you may add. Shields give +2 AC. The different types of armor in the PHB give you the following general types of AC. Note that as you get heavier armor, the base AC increases as well:

  • Light Armor: Base AC plus Dex mod

  • Medium Armor: Base AC + Dex mod (max 2)

  • Heavy Armor: Base AC

Features and Spells may modify AC

There are some class features, feats, and spells that change your AC as well. Below are some of them.

  • Barbarian's Unarmored Defense: AC = 10 + Dex mod + Con mod

  • Defensive Duelist Feat: +Prof. Mod to AC as a reaction against 1 attack

  • Mage Armor Spell: AC = 13 + Dex mod (better than all light armor)

If you are the DM, this leaves open the possibility of creating monsters with special traits that does take into account size for their AC.

What does Size govern?

From the DMG, on the section of creating monsters:

DMG pg 275, Step 2 Size

Make your monster whatever size you want: Tiny, Small, Medium, Large, Huge, or Gargantuan.

A monster's size determines which die is used to calculate its hit points in step 8. Size also determines how much space the monster occupies, as discussed in the Player's Handbook.

And the following are the Hit Die for different sizes of monsters:

\begin{array}{|c|c|} \hline \text{Monster Size} & \text{Hit Die} & \text{Average HP per Die} \\ \hline \text{Tiny} & d4 & 2\frac{1}{2} \\ \hline \text{Small} & d6 & 3\frac{1}{2} \\ \hline \text{Medium} & d8 & 4\frac{1}{2} \\ \hline \text{Large} & d10 & 5\frac{1}{2} \\ \hline \text{Huge} & d12 & 6\frac{1}{2} \\ \hline \text{Gargantuan} & d20 & 10\frac{1}{2} \\ \hline \end{array}

Size also determines if hostile creatures can move through your space, or vice versa:

PHB 191, Moving Around Other Creatures

[Y]ou can move through a hostile creature's space only if the creature is at least two sizes larger or smaller than you. Remember that another creature’s space is difficult terrain for you.

However, the designers do hint in places that Size is supposed to influence AC

In spells such as Animate Objects, Size does matter. With this spell, you can animate objects of a certain size of your choosing, but you can only animate up to a Huge sized object. Notice how they trade in AC for HP as Size increases.

\begin{array}{|c|c|} \hline \text{Size} & \text{HP} & \text{AC} \\ \hline \text{Tiny} & 20 & 18 \\ \hline \text{Small} & 25 & 16 \\ \hline \text{Medium} & 40 & 13 \\ \hline \text{Large} & 50 & 10 \\ \hline \text{Huge} & 80 & 10 \\ \hline \end{array}

The idea for this spell is, the larger you are, the easier you are to hit, but also the harder you go down since you're beefier.

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No size no longer effects ac in 5e. The only stated effect of size in the players handbook is under the description of strength. Any creature who is Large, or bigger has Double carry weight for each step bigger they are. So a Huge creature has 4 times it's calculated carrying capacity. And this is reversed on tiny creatures. So if you're tiny you have half your carry capacity, and each step below tiny you divide by 2 again.

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    \$\begingroup\$ @Timi You cannot grapple a creature 2 sizes larger than you \$\endgroup\$ – user27327 Jun 6 '17 at 0:41
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    \$\begingroup\$ And just to clarify, there are no penalties for grappling a creature any size smaller than you. (page 195 PHB 5e) \$\endgroup\$ – C. A. Jones Jun 6 '17 at 1:13

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