Many situations can be simulated using the generic fields below. For example: if you use a masterwork weapon, give it a +1 enhancement and subtract 1 off its average damage. About the only things you cannot simulate right now are natural weapons and weapons that deal extra damage on crits (i.e. Icy Burst). Support for these might be added in at some future point in time.
The Combat Calculator attempts to error check your input, but due to the complexity of the game it is not always possible to tell if the input is actually in error. For example, Greater Weapon Focus is normally limited to 8th level fighters -- but Order of the Bow Initiates get it for free. Likewise, charging normally only allows you a single attack -- but Cavaliers can make Full Mounted Attacks. So, the combat calculator will helpfully select "Only 1 Attack" if you are charging, but you can still unselect it manually if you are playing a Cavalier.
It will work with both 3.0 and 3.5, but you'll have to manually enforce the appropriate rules (for example, it won't complain if you have keen and improved critical on a weapon, which is legal in 3.0 but doesn't work in 3.5). Questions or comments should be sent to ShakaUVM (on Reddit).
Sparticus Deathblow Half-Orc Brb1/exMnk1/Rog5 (3.5)
Sparticus, 12th level LG Devoted Defender (3.0)
Sparticus, 12th level LG Devoted Defender (3.5)
Frauline Nong, 14th level LC Half-Orc Sacred Fist Barbarian
Grishnak Elfslayer, 18th level LC wizard while self buffed
Grishnak Elfslayer, 18th level LC wizard while fully buffed including TT