I need some help with formulas for figuring damage in my RPG. Not only the formulas, but ideas on how stats change at level up. I'm using a real-time combat system, where a level 1 monster should take about 4 hits to kill. I just have simple stats like strength & vitality for both the player and the monsters.

Like kazzmir said, it all depends on what kind of system you are using. Will enemies have armor? Or simply just a health attribute?

I think a very simple way to do it if your ONLY two attributes are strength and vitality is this. The player has a set "core" strength for each level he is on. IE, level 1 starts at say, 10str. Then there is additional strength that's added on. For instance, a sword or something. So you could just create a random number from the core strength up to the core strentgh + the sword strength.

As for making the enemies the right difficutly. Find out what general equipment the player will have at said level in said area, and determine how much hp it should have if you multiply the total damage the player should do by 4. Simple.

I would think there will be more stuff like weapons, armor, and other things that will play a role. I guess it depends on how you want to do things. Maybe you could elaborate a little more?

I'm sorry if my question was vague...
I meant how should I balance the stats and stuff? How much should your stats raise each level? Should I have a choice like in Diablo? How much should the exp requirement change per level? Do any of you have documentation or anything on this? My basic stats are as follows:
HP (max 1000)
Strength (max 100)
Vitality (max 100)

The strength is added to by the equipped weapon's power, which is mostly between 1 & 15 (15 are the strongest weapons). The defense is added to by shields and armour.

I run into the problem that enemies the same level and above the player will usually take <=0 damage from my attacks! The same works for the player when he's getting attacked! It's all very confusing to me...

they are added? That's not a very good way of doing things if you ask me.
So if I have 'the all powerful sword of wacking' (15) and my str is 70, I do just as well as my brother with 'the useless sword of wanking' (1) who has str 84.
He's str isn't that much better than mine, I should be doing more damage with my cool sword.
I guess what I'm saying is that you should multiply some stuff.
I think what Diablo uses is
total_damage = weapon_damage*(1+str/100)
for simple cases.
ie. each point of strength adds 1% damage.
With your kind of numbers though, you might want to try something like 5%

Don't be afraid to experiment with numbers and formulas. Think to yourself, "if I increase the weapon power by X, what do I want the effect to be for all the different characters?".

My favoriting function for game balance is arctan
that sexy little curve is useful for so many things...

Here's how I do it in my MUD.
(This code is for regular attacks)

  obj = HasWeapon(agg);

  if(obj)
   {
  dmg += (int)(agg->att.att_str * 1.25);
   }
  else
    dmg += (int)(agg->att.att_str * 0.75);

  dmg += rand()%(agg->level+agg->att.att_lck);

  

  dmg = CalcDamage(agg,tgt,dmg,DMG_WEAPON);

Some irrelevant code removed.

Then CalcDamage looks like this:

int CalcDamage(CHARACTER *agg, CHARACTER *tgt, int dmg, int type)
 {
  int ret = dmg;
  int acc = tgt->att.att_agi - agg->att.att_agi;
  
  if(acc > 0)
   {
    if((rand()%acc)>>4 > 0)
    ret = 0;
   }

  if(ret < 0) ret = 0;
    
  return ret;
 }

Obviously incomplete.
I need to change 1.25 in the first part of the code to a value which is given by the weapon (obj), rather than a static value. I prefer using a float like that to calculate the strength of a weapon. That way you have a much wider variation.
Some swords might be 1.25, but raise your strength by 5.. and another sword might be 2.0, but lower your strength because it's so fricken heavy.

What also needs to be added is the damage type specific stuff. So if you attack ice with fire, it would hit for more [edit: not defend better you idiot, derezo!]. This gets extremely confusing with my setup, and is why I haven't done it yet

I also have a 'damage' function which actually handles taking away the hp, killing the target, and everything else involved in that. It also handles spells that may be affecting the target or caster, and adjusts the damage according to that.

I didn't read many of the posts, but there's a way I do it for now

For magic and skills, I do something completely different.. but it's a lot more code and I wont bother posting it because it probably wont make any sense
The code is available on my site if you really want it. heh..

float e_value(float x,float lim,float xx){
float y,z,b;
z=pow(4,1/xx);
b=log(lim)/log(z);
y=lim-pow(z,b-x);
return(y);
}

A little bit of work on paper yielded that this was the same as:

float e_value(float x,float lim,float xx){
return lim*(1-pow(4, -x/xx));

So if that's what you're using, you might want to check my maths with a short test program, and if I'm right, switch to what I said (would be faster).
Also, it would be a fair bit faster to calculate exp(-x/xx) than pow(4, -x/xx). I'm not sure about this from a computing point of view (haven't tested it), but from a mathimatics point of view it is*. I'd test it now, but I'm not on my own computer...
Is there some special reason you are using base 4 instead of the natural base (e)?

Aside from all that, your graph reminds me of arctan...

*
pow(4, x) is defined as exp(x*log(4))

Yeah!