First, (ON WINDOWS 7 & Below) Do Ctrl+ALT+Delete, open Task Manager, Go To the PERFORMANCE Tab. View "Physical Memory (RAM)" and look at Total. The number that is for Total (example: 8151 = 8gb of RAM, 5675 = 5gm of RAM), now remember your number! (FOR WINDOWS 8.1/8) go to Performance, Find Resource Monitor near the bottom, click it, then go to the Memory tab, Look at the bottom of the window and find (Instead of Total) "Available". (For Example, mine is 6013, so 6gb RAM! (Tip: IF YOU HAVE A 32bit COMPUTER! MAKE SURE YOU HAVE 32bit JAVA!) Now, Open the Minecraft Launcher. Once it's loaded, go to 'New Profile', and click it. Then, name the profile whatever you want (ex: Your Username or something random like FPS+ Cause 'o' RAM!) and go to the very bottom area of the profile editor and find 'JVM Arguments' (Make Sure The Box Next To 'JVM Arguments' Is Checked/Ticked). Inside the Typing Area of JVM Arguments, PASTE the Following Text Inside it:  -Xmx2048M -Xms2048M. NOW, I Place of 2048, you may use the number you remembered from Task Manager, and look at the list below, and find what amount of RAM you want to allocate. Reccommended is 2048, unless you only have 1gb of RAM. (For Example, i have 6gb, so i might allocate 3gb of RAM, because note: DONT GIVE ALL YOUR RAM TO MINECRAFT, OR IMPORTANT BACKGROUND PROGRAMS WON'T HAVE THEIR DESERVED ALLOCATED RAM! Then, when you have copy/pasted the code into JVM Arguments (and optionally edited it) click 'Save Profile'. Play the game, and see your game not have Lag Spikes, and NEVER say 'Ran Out Of Memory!'  YOUR SET! Make sure to give a Diamond if it helped! Check out my Channel: (PieNinjaProductions) and Check Out My Custom Maps and Mini-Games on my Profile! THANKS AND HAVE A GOOD DAY! (If you have probs or need help, comment!)



LIST OF EXCEPTED RAM FOR JVM ARG.:

1GB = 1024
2GB = 2048
3GB = 3072
4GB = 4096
5GB = 5120
6GB = 6144
7GB = 7168
8GB =8192
9GB = 9216
10GB = 10240