User blog comment:Krufix/Last Whisper - When to get?/@comment-77.105.18.208-20121106161755

Ok here is the math that I took into account.

Youmuu's Ghostblade gives you 20 armor penetration. Cost: 2678

Last Whisper gives you 40% armor penetration. Cost: 2290

If enemy has less than 50 armor Youmuu will be a better choice since it will do a flat reduction of 20. If enemy has more than 50 armor Last Whisper will reduce more (it reduces 20 armor when enemy has 50). It can boost your damage dramatically.

Situation with The Black Cleaver is somewhat similar. If enemy has more than 112 armor, use Last Whisper, if not, use The Black Cleaver.

Now the actual advice on when to use it is a bit tricky. If you ask me it should always be a must have in every build for a carry since it allows you a greater damage output and it is very cheap for the stats it gives all together. I mean BF Sword gives you 45 AD for 1650 gold. For 640 more gold you get almost as much AD (40) and a huge boost of armor penetration.

Now I will try to compar it with Bloodthirster. Lets asume you have Infinity Edge and LW and on other IE and BT.

On one with LW you will have 120 attack damage while on other you have 180. Now the only factor to take into account is the ammount of armor that enemy has. If enemy has 100 armor it reduces physical damage by 50%. That means that BT and IE will do just 90 damage while LW and IE will do 74 damage since 100 armor gets reduced to 60 that reduces physical damage by 38%.

BT and IE wins but as add more and more armor LW will do bigger ammount of damage. So by my free guess, if enemy that you are attacking has less than 130 armor BT and IE will do bigger damage, if enemy has more, LW is your weapon of choice.