Pretty new to being a DM but one of my players asked if it was possible to inflict damage on a specified area such as the ankle.

I asked them to roll 2d20 dices. 1 dice to see if they generally hit the enemy, and 1 dice to see if they succeed in hitting that specific area, where I judged the DC on the AC in both dice rolls. They sadly didn't succeed on hitting that specific area, so it just turned into a normal dmg attack, but it got me wondering. How would you rule a specific damage inflicting area` such as the ankle?

Would you give them a condition that best suited their hit, such as them losing a bit of their walking speed if the ankle attack had been successful, or would you do something else?

Again, I'm pretty new to being a DM, so sorry if this question may seem dumb, but I would like to be prepared for the next time my player tries anything like this again :-D

The short answer is: Don’t allow it.

The longer version is: D&D’s combat system is an abstraction and quickly falls apart when you add in specific body parts as targets. Largely because everyone just starts going for the head. Secondarily, because players try and do things like your player did, which would basically allow adding conditions to basic attacks.

If a player wants their character to slow someone down, there are options like shoving or grappling which do it, or feats like slasher. The rules update also includes weapon properties which will add effects. You shouldn’t give away those sort of things for free.

As Xalthus notes, that isn't how D&D combat works, and the easiest way to demonstrate the impracticality of it is to have the bad guys do it to the players.

The basics are "if a PC can do it, a monster can do it" -- only it does not work the the other way around. Monsters are supposed to be able to do things PCs cannot.

But the Combat system in D&D is not about striking a specific area and inflicting a condition, it is about a kind of imagined series of swift parries, feints, and the use of all the skills thus far that individual knows against others who are also quite good at it. There is always a moment in a six second period wher ehtey can break throught he defenses of the opponent and land something that causes them to tire or bleed or be wounded -- and this is what hit points reflect.

There are no hiit points for ankles. There are no hit points for shoulders, elbow, foreheads, necks, wrists, shins, guts, and so forth. THere is just hiit points, for everything, in one fell swoop.

Understanding that is one of the most important things for people to learn -- there is no "i attack and sweep their feet out from under them" in D&D. 

If the game was just "hit points are only body stuff" then no one would have more than 10 hit points as a player. Most folks would only have 5 or 6. that's why weapons do damage the way they do. The system is set up so that having more hit points reflects having more skill, being better at avoiding blows, being harder to hurt, and so forth -- not actual physical stuff.

The same applies to attacks -- you are not "just swinging a sword" when you attack, the roll represents the success of getting through another's defense, skill, hardiness, and so forth -- but your character is swinging a sword a lot in six seconds, parrying and riposting and ducking, and moving around in a small space. Each attack represents the chance of breaking through the enemy defense in that moment, and the higher the level the more often one will see that chance.

So the answer is, you don't figure out how to do that. That isn't how D&D combat works. Monsters and characters cannot pick a particular place to use a weapon.

The one exception that I know of, going back to the early days, is the Vorpal weapon -- and that has always been a huge problem. But they are also magical.

I don't use called shot and attacks don't reduce speed unless specifically noted otherwise ie ray of frost 

Agree with the above.  Aiming for a very specific target (eye, sword hand, et) opens a can of worms that the game (and the DM) cannot handle

It's not a dumb question, but it does introduce complications to combat that are more fun in theory than they are in practice. Combat can be messy and slow and chaotic with normal rules, but if you have to keep track of hobbling and weakened strikes, etc., it makes it all that much slower and more confusing. 

Called shots usually make an attack more powerful than it should be by imposing secondary consequences on an enemy. As stated above, there are special actions and abilities that impose those secondary conditions, so getting them just because you say you want them is poor cricket. And boy, do players hate it when you turn it on them. This is also why I don't allow disarming in my games. It's all fun and games until the monsters smack your holy symbol or sword out of your hands with one of their 3 attacks.

Aside from the unnecessary mechanical crunch... to me anyway, called shots feel like players kind of dictating to the DM how the combat should go. I don't particularly care for it. 

If you want to try "called shots" then you might find this article interesting: https://www.d20pfsrd.com/gamemastering/other-rules/called-shots/

From reading through it it seems more geared to a 3e version of d&d but you could tweak it quite easily if you felt inclined to implement it.

Nothing wrong with calling a shot if it is for flavor only, attack the ankle and you have the same AC and the same damage as if they do not call the shot. A halfling in melee with a giant might only be able to attack the ankle!

A reason not mentioned why allowing a secondary effect is it can step on other players toes, if a player decides to play a battle master for tripping attack, it would be pretty disheartening if everyone can do the same think just by aiming for the ankle or a spellcaster learns blindness only ti find the fighter can do the same thing without a resource cost by aiming for the eye.

If you absolutely must have called shots they must have downsides to make them only situationally useful, an ankle attack might do no damage but on a hit the targets movement speed is reduced, or it might add to the targets AC and do both. The problem is getting something that is balanced (and yes enemies can do the same thing to the PCs)

RAW anything like this falls under optional rules. If they're looking to attack a specified area then the AC should be adjusted accordingly. If the target's AC is 17 and they want to hit the ankle then add at least 5, making the AC 22. (With the ankles being small and moving, I'd probably add a 10 to the AC if I were to allow this.) Then you could rule that after a certain amount of damage is taken the target's speed is reduced or they might fall prone and be unable to stand. There are optional combat rules in the Dungeon Master's Guide you may want to check them out to see if any of them are what you want to add to your game. 

However, anything the PCs can do to the NPCs the NPCs can also do to the PCs. Make sure all the players are on board with this change to the combat since their characters' ankles are going to be getting targeted too. 

If I were going to allow it it would look something like this 

Called shots are rolled at a disadvantage 

Head AC+6 If target is not wearing a helmet  they must make a Constitution saving throw at disadvantage if they fail they are stunned for 1d4  rounds
Arm/Leg  AC+3 if the target limb   damage equal or greater than 1/3 or your total hit points  A Constitution Save at disadvantage if they fail the limb is considered crippled until seen by a magical  healer or the  character has completed in a healers care for 1 week  

I do not support more specific called shots

You could, but I’d make them roll to hit with disadvantage

The others are probably right that this is best avoided, but it definitely sucks to shut down a player who's actually trying to be creative and tactical rather than treating combat as a set of buttons to push. I might not apply mechanical consequences one way or another, but sort of play the wound into the enemy's tactics in a "soft" way, like make them hesitant to move a lot versus standing and fighting in the ankle case.

The first thing I would do is ask why. If it's just a flavor thing then I'll just let them roll a normal attack and let them describe it hitting the ankle. If, on the other hand, they want to inflict a condition I would first look to see if anything they could normally do would inflict that condition and let them do that but flavor it as an attack.