So, calculating damage is a bitch. There's a page on it now, but it isn't the most enlightening.
One way to solve this is to list the stand alone stat effects of each item, that is the effects each item have on your stats if you were playing as Isaac and had no other item. Now, this isn't a completely accurate account of the effects of each item - for example, damage ups tend to get less effective the more of them you have - but hey, what the hell.
A list of stat upgrades (excluding health since that's obvious)
|Item||Stand alone Damage||Stand alone Tears (Fire Rate)||Stand alone Speed||Extra Notes|
+16% after 1st kill, +29% after 2nd kill, +42% after 3rd kill, +53% after 4th kill, +64% after 5th kill, maximum of +122% (after about 11 kills)
|Blood Of The Martyr||+48%|
|Book Of Belial||+84%|
|Bucket Of Lard||-20%|
|Cancer||+25%||Fire rate increases dramatically when tears stat is higher.|
|Chemical Peel||+84% to every other tear (as Cain, 50% chance for tear to have +84% damage)|
Damage increase starts after charging for 7 frames (about 0.23 seconds).
+39% damage after a third of a second's charge, +83% damage after half a second's charge, +175% damage after a second's charge, maximum is +222% damage after 4/3 seconds charge
|The Inner Eye||-58%|
|Lord Of The Pit||+30%|
|Lump of Coal||+120% for every second the tear stays in the air|
|Money Equals Power||+22% with 10 coins, +31% with 15 coins, +40% with 20 coins, +48% with 25, +56% with 30, +71% with 40, +84% with 50, +140% with 99.|
|My Little Unicorn/ VII The Chariot||+28%|
|Odd Mushroom (Large)||+16%||-10%|
|Odd Mushroom (Thin)||-28%||+90%||+30%|
|Holding A Pony / White Pony||+50%||Actual effect is to set your speed to 150% base speed if it would be lower than that otherwise.|
|Razor Blade||+56% after 1st use, +97% after 2nd use, +131% after 3rd use|
|Red Patch||When hit: 20% chance to increase by 78% (further hits stack but don't care about that)|
|Sacred Heart||+159%||-46%||Effect on the tears stat decreases significantly if you have Tears Ups.|
|SMB Super Fan||+16%||+8%||-20%|
|Tough Love||Teeth have +220% damage|
|Whore of Babylon||+67% if it triggered in a different room and hasn't left, +90% if it triggered by falling below half a red heart in the room you're in, +125% if triggered by III The Empress||+30%|
|Wire Coat Hanger!||+30%|
There are, of course, some items (e.g. Technology 2, Mom's Knife, Brimstone) that are too complicated to explain even when they're the only item you have.