Is there a way to set a subclass to use Ki points as spell slots. 

I am building a monk subclass that gains spells depending on level. I'd like to allow the monk subclass to use its Ki points as the spell slots, i.e. Spending 1 Ki point per level of the spell, so a 3rd level spell will cost 3 Ki points to cast. 

Also, How do I add Ki points to the monk class. I am trying to make an ability that adds Ki points every other level.

The balance for Ki to spell level isn’t a 1-1, look at the Way of the 4 Elements Monk for an idea of the scale. It uses the same scale as the Sorcerer’s uses to convert Sorcery Points to Spell Slots. 1st-level = 2 Ki, 2nd-level = 3 Ki, 3rd-level = 5 Ki, 4th = 6ki, 5th = 7 Ki. That being said, it’s your homebrew, do what you want. I believe setting the spells to consume “charges” would do what you are looking for if I remember correctly. As far as I know, there is no way to add additional Ki.

I would start with doubling the ki costs given that monks recharge on a short rest instead of only on a long rest like a Sorcerer does.

I don't think you can make spells consume Ki (the 4 elements monk doesn't).

You can make a feature that has a limited use action called extra Ki, but it will be separate from normal ki.

Quote from DxJxC >>

I don't think you can make spells consume Ki (the 4 elements monk doesn't).

You can make a feature that has a limited use action called extra Ki, but it will be separate from normal ki.

 Please tell me more

Quote from IamSposta >>

Quote from DxJxC >>

I don't think you can make spells consume Ki (the 4 elements monk doesn't).

You can make a feature that has a limited use action called extra Ki, but it will be separate from normal ki.

 Please tell me more

 More about what? That the official monk subclass features don't auto-consume Ki? Or that homebrew subclass features can make actions with limited numbers of uses?

Quote from Warranto >>

I would start with doubling the ki costs given that monks recharge on a short rest instead of only on a long rest like a Sorcerer does.

 This doesn't seem necessary. Sorcerers are also full casters, so they don't need to use as many points to make slots. And monks have a lot more features that use Ki quickly.

Also if you compare the potential spellcasting ability of a monk using Ki to any other caster and they most closely compare to a warlock with (more flexible spell levels and) half the the number of equal level spells per rest. If you limit their max slot level like 4 elements does, they almost exactly a half warlock that uses spell points instead of pact slots.

Ah, I misunderstood your previous post. My bad.

True enough. It was merely intended to be a starting point.

I was making an, perhaps unfounded, assumption that this was to be a more versatile spell caster than a 4 elements monk (which as you pointed out has its own limitations to balance it), otherwise the subclass would just a different flavour of elemental monk (and no need to make any alterations). The 4 elements monk can only cast spells up to 5th level, with no upcasting available. If you wanted a monk to cast more powerful spells for merely a ki cost, it would need to be re-balanced so that casting multiple high level spells come at a greater cost to using your other abilities (in your example, the Warlock has their once per day high level spells). 

Newest version on page 2 of this thread.

Your balance is off, they should cap at 5th level spells. Cantrip-6th makes them more magic than Rangers, Paladins, and even Artificers. Other Monks like the 4 Elements Monk doesn’t get 2nd-level spells until 6th level. Also, because Monks regain their Ki on a short rest the Ki/spell level should be higher to match the Sorcery Point/Spell Slot ratio. Especially because you intend to give them a whackton of free extra Ki. You are adding almost 50% more Ki! This is about as balanced as the US national budget. It seems to me that you are trying to balance this closer to a spellcaster, when it would be more appropriate to compare it to the Arcane Trickster or the Eldritch Knight.

Yeah, those spells are too cheap and level up too quickly. Especially since you are giving them 13 extra Ki (you should get rid of the 14th level extra Ki).

Spell level. monk level - Ki cost

1. 3 -- 2
2. 5 -- 3
3. 9 -- 5
4. 13 - 6
5. 17 - 7

Monk Subclasses only gain features at levels 3, 6, 11, and 17. And Dex for a Spellcasting Ability is too much, Monks already benefit from Wis, and that makes more sense anyway, Int makes the most sense based on your description of “seekers of knowledge.” There’s more, but I don’t have the energy for all that.

I'm going back to the drawing board completely on this. So I guess this forum is no longer necessary. I will be scrapping the whole 'spellcasting' gimmick in favor of just defining a few new skills that might look like certain spells that I wanted to add to the subclass.

There’s nothing wrong with the Spellcasting, just has to be toned down. If you base it a little closer off Wot4E Monk as a balancing point, it should be fine. Good luck.

I'm working on that part. It's hard to balance that with the thought I had it my head on how I was hoping to make this monk play. I wanted this monk to be an interrogation style monk based on seeking truth from those it questions. Utilizing spells that would allow for such interrogations made sense in my mind. I'll read over the Wot4E Monk information and see if that style of Monk spell play would fit this subclass, or if I should just customize some abilities that the Monk will gain at the different level gains. 

I originally had thought of the Arcane Trickster subclass for Rogue when I was building this. After your comment, I quickly realized that I had gone overboard and completely overpowered this subclass. 

I'm creating a new campaign in a new world with new deities, subclasses, domains, etc., and this is definitely not a class I would want to try and DM for.

That’s not a problem, just use the chart that DxJxC gave you. 

Spell level. monk level - Ki cost

1. 3 -- 2
2. 5 -- 3
3. 9 -- 5
4. 13 - 6
5. 17 - 7

And maybe drop all that extra Ki, they do get it all back on a short rest after all.

I had seen that chart, and I agree that seems fairer, but I did want to integrate up to a few 6th level spells. True seeing was the main one I wanted to integrate, but I might be able to get away with that as a higher-level ability rather than a spell, and give it limited uses, like once per long rest or something. True Seeing just makes sense to me, thematically, as a seeker of knowledge and truth. I have already decided to drop the extra Ki part, because of your mentioned reason (short rest reset). 

I will say, though, in all of my home games, I have yet to see anyone play a 4 elements monk because they seem a bit underpowered compared to some of the other monk subclasses. Most prefer to simply play an elemental wizard, sorcerer, or cleric over the elemental Monk version. Because of this, I wanted to create something for my home group that would actually entice them to take a chance on a subclass that is different. 

With that last part said, however, the group I play with tend to lean more toward min-maxing so they would figure out how to break the hell out of class I attempted to make originally. 

If they get 1 or 2 6th-level spells they could cast 1ce/Long Rest as a capstone at level 17 is no problemo. 

Look at it this way. A Sorcerer has the exact same number of spell slots a Wizard has right? Only they don’t because they can use Font of Magic, thereby creating more spell slots in a day than any other caster except maybe a Warlock who takes a lot of short rests. With Font of magic, they can even get more spells off in a day than a Wizard with Arcane Recovery. A Spellcasting monk is like a Sorcerer without spells slots, who recharged on a Short Rest like a Warlock. That could potentially mean with enough Short Rests, a Monk could outcast a Sorcerer.

The only real difference is that the Sorcerer gets Spell Slots and Metamagic, and a Monk gets better Hit Dice, better Combat Stats, better AC, etc. The reason most people pan the Wot4E Monk is because their abilities cost so much Ki compared to Flurry of Blows and Stunning Strike. The spells you have picked are generally more powerful, like those UA spells, and you’re not padding the list with other abilities like the Wot4E abilities that are not based on spells. 

I suggest that you base it as close as you can on Wot4E Monk, just don’t tell your players that. Put it in front of them, and see what they say. Remember, you’re going to end up rebalancing things over and over as they playtest the subclass for you. Whatever they do all the time is probably too powerful for the cost, and whatever they never do is probably too Ki expensive for the power. That’s when you adjust things, but you’re never gonna know without playtesting it.