I tried searching the site to see if this was brought up before, but I didn't find anything.

I'm not a fan of the spell slot system and would prefer a mana point system instead.

The concept is fairly straight forward and would be fairly easy to convert spell slots into mana points.  1 spell slot x spell slot level = mana points.  When you cast a spell, the spell level = the number of mana points used.  If you want to increase the power of the spell, dump in more mana.

I would love to see something like this as a homebrew "house rules" option, set at the campaign level so that it applies to all spell casters that use spell slots.

The math is fairly straight forward, but I would imagine the programming might be challenging.  But, being a programmer myself, it should be doable.

The campaign has a "use mana point system" box checked.  All characters attached to the campaign get flagged accordingly.  Any character that is flagged displays "Max Mana Points" instead of the spell slot table, with Max calculated as described above.  Then there is a "current mana points" box which will reduce either when a spell is cast or manually reduced.  Short and Long rests can restore mana points in a similar way to restoring spell slots.  Spell slot restored x spell slot level = mana points restored.

This system makes more sense to me intuitively when it comes to using magic.  If I have enough energy to cast a 9th level spell, I should have enough energy to cast nine 1st level spells.   If all you have left is a 9th level spell slot and you need to cast a lower level spell, it doesn't make sense that the whole slot is used and the extra energy is wasted.  There is no way that an Archmage with all their experience and training wouldn't be able to manage their spell casting energy in a more efficient manner.

Is something like that possible?

This is already in the Dungeon Master’s Guide as a variant. D&D Beyond don’t have any plans to add it to the site because it’d be an expensive-to-develop feature that very few people would use.

I read up on the variant in the DMG and I didn't like it either.  It's not an equivalent exchange compared to my suggested alternative, it's more complicated, and it's too limiting with how it handles higher level spells.  I can certainly understand why they wouldn't want to program it as well as why few people would use that particular variant.

But my suggested alternative is not the same and should be fairly easy to program compared to what's in the DMG.  It's just applying math to existing data that is already programmed into their database and then toggling a "switch" to display it compared to the existing system.

As for how many people would use it, who can say?  Perhaps it's worth creating a poll on the subject and float it out there to see how many people might be interested in the proposed alternative?

If they aren't interested in it, then that's that, and I'll either drop the idea or come up with some sort of workaround within the existing limitations.

They aren’t a 1-1 conversion because the spell levels don’t correlate 1-1. One 3rd level slot does not equal three 1st level slots when calculating an order of magnitude.

Sorcerers in 3e used that same system and the majority of people houseruled them to use spell slots instead because people found the points to be unbalanced and require too much bookkeeping.

@IamSposta, fair point regarding spell slot/spell point costs not being 1 to 1 based on magnitude.

Edit: went back and did some math based on damage causing spells and the average maximum damage for those spells at any given level.  Threw out those that modify weapons and some of the outliers that were unusually low or high (like the insanely overpowered Meteor Swarm).  Looking just at the proportion of max damage dealt out on average per spell level, the spell point progression that is in the DMG isn't too bad, but still could use some tweaks to suit my tastes.

Ya know, it occurs to me that when the alternative spell point system gets implemented, one way to allow for home brew alternatives would be to provide the following options (at the campaign level?):

1. Allow customization of how many spell points are required to cast a spell for any given spell level.

2. Allow customization of how many spell points a caster can have at any given level.

3. Allow customization for the rule of the maximum number of high level spells per spell level of 6th and above can be cast before requiring a long rest, with "0" allowing for no limit.  Could be for all of them or a separate rule for each (such as 3 6th, 2 7th, 2 8th, 1 9th),

I am using a spell point system in my game at the moment but only for the Soc.  This make the class feel diffrent then the wiz with out being got like.  Spell cost 1 point per spell level and share the point with your Socorey points.You have a max of 3xlevel but on short rest you regain your chr mod.   To make it work for my player (only level 3 so far).  I made a feat SOC SPELL POINTS that I add to the player.  It keep the number on this sheet as he play using his phone as a sheet.  Like right now it show 9 point that he can click off or add on the short and it reset to full on long rest.

When he should be getting level 6 or higher spell it will be treated like the warlock higher slot of 1 a day per spell level.

For those of you who might know of the World of Warcraft Mana system, it's a different variation on a theme that has been very successful in my own campaign.  While it does naturally limit the number of spells a caster might use in a given encounter, it does force some defined resource management questions instead of blasting everything every combat.  Our current style doesn't have significant multiple combats which tends to let caster types shine a little brighter when the PC knows they will get another full rest before the next combat.  That being said, a more limited mana base is a greater boon for our campaign style.

Basic Guidelines:
Spells cost Level+6 (minus the caster's proficiency modifier).  Thus a second level spell cast by a 3rd level caster costs 6 points, the same spell cast by a 9th level caster costs 4 points.  Mana is calculated thus, Casting Stat+Highest Access Spell Level+Prof Modifier.  A ninth level cleric with a 20 Wisdom would have 20+5+4 = 29 Mana; twenty for their wisdom, five for the highest Access Spell Level which is fifth, and four for their proficiency modifier, totaling 29.
There are a few other guidelines, and either your class is a full caster (ie. Cleric, Druid, Sorcerer, etc), or a 'half caster' (ie. Paladin, Ranger, etc).

While I understand this formula doesn't work for all campaign styles, it does work in mine which tends to have a more martial bend.  Some alternate methods to calculate Mana would be very useful in any upcoming updates.

I have developed a rough mana system which I use with my players in several campaigns and they all seem to enjoy it. half and full casters have their own chart for mana pool size as well as I have a chart for how much each spell level cost. The idea behind it is flexible magic at the cost of fewer or more spells. For example a level 20 full caster using my system could cast potentially 3, 9th level spells in a single long rest, however they could potentially lose a level, gain exhaustion, and could do nearly nothing else. Or they could use nearly 100, 1st level spells instead of using higher level magic. I am still working out how warlocks might function with this system however so far none of my players are playing a warlock so its not a primary focus. A casters mana pool is based off a chart which I am figuring out a revision for so I can give an exact formula but at the time being, they look at the chart, then they add their spell casting modifer * half their level (rounded down) to their mana pool to get their max level. Players who multiclass only have mana equal to their spell casing level so lets say a player has 3 levels of fighter and 2 levels of sorcerer. They would have a mana pool equal to that of a level 2 sorcerer + spell mod * 2 (half of 5 rounded down). If you or anyone else looking at this forum would like any of the specifics of mana pool size, spell cost, etc feel free to email my non personal email johnsmith_12340@hotmail.com 

You guys are overthinking this. A level one slot is 1 point, a level two slot is 2 points, and so on. Write your total down. Casting a level 1 spell costs 1 point, casting a level 2 spell costs 2 points, and so on. But now, you have a pool and can cast your highest level spell as many times as you wish until you run out of mana and need to rest. Simple.

Quote from HorrorMinion >>

You guys are overthinking this. A level one slot is 1 point, a level two slot is 2 points, and so on. Write your total down. Casting a level 1 spell costs 1 point, casting a level 2 spell costs 2 points, and so on. But now, you have a pool and can cast your highest level spell as many times as you wish until you run out of mana and need to rest. Simple.

 
I'm like a year late to this convo. but it's not quite that simple

Let's take 9th-level spells for example, they can't be cast until you have a 9th-level spell slot which is level 17 for full caster characters. 

However on a 1 to 1 conversion basis like you're saying they would be able to cast a level 9, a 9 cost mana spell at level 4. AND it wouldn't be their only spell that combat, as theyd have one mana left over. 

A level 20 full caster has a total of 22 spell slots - 2 of which can be level 8 and above. With the one-to-one mana conversion, that's 9 level 9 spells and a 10th level 8 spell before they run out of their 89 mana. (having done the spell slot conversion math - I have the table if anyone wants it) 

Now sure you could go through and assign each spell a mana value that better befits their magnitude, but that's a lot of work, and even with a formulaic approach, like say that mana cost is the spell level times half the spell level again ( 9 * 9/2 = 40.5 rounded to 41 ) giving you two level 9 spells in combat but still feeling more balanced, that's great, but you'd still be ham stringing the punching power of casters, as they would lose the volume of spells they can cast with spell slots. Not to mention the added extra maths; If you think tracking spell slots gets cumbersome, try calculating mana cost first, then deducting it from your total... sure it would fit some tables but not many that I've been at / run

Then there's the inevitable question from casters: "Can we have mana potions like health potions"  - which creates more work in homebrewing them because you look like you are nerfing casters if you say no. 

TL;DR balancing game mechanics as a DM is more complicated than it first appears. 

Given how spellslots tend to be easy on the bookkeeping I tend to like adding mana points/spell points as an addition potential reward instead of magical items. These spell points are tacked separately and can be used to upcast spells for 1 mana point per spell level and to empower certain items. You can also use them to restore 1 spellslot using an action at a cost of 3 mana points/spell level.

This allows these points to rly do something other then spell slots and allows me to give them to everyone in the party. If in different amounts.