Mass is what cells, pellets, viruses (except teams mode), spawners, and ejected pellets are made of. Cells need to eat mass to grow bigger. It is measured with a number, which is shown on the player's cell, if this has been activated in the user preferences.
The default starting mass without an account is 10 mass.
Earning Higher Starting Mass
Main page: Levels
Players may register an account in agar.io with either Facebook  or Google+[], which will allow them to start with a higher mass (except when playing Battle Royale). If a player levels up with that account, they will receive +1 starting mass. This rule is broken when the player reaches level 24 when logged into that account, and the maximum starting mass is 43 with the exception of a starting mass boost multiplier. See more information about greater starting mass at the Levels page.
Main page: Virus
In FFA mode, if a player runs into a virus that player will gain the amount of mass the virus has, although still splitting the player's cell as a virus normally does. The default virus mass is 100, unless a player throws mass into it. When a player has 16 cells, a player may gain that virus's mass without splitting any more if the virus is eaten.
Other Smaller Cells
Main page: Ejecting
You can eject your mass out of yourself to give it to others by pressing W
Other players instead of ejecting mass, will split and give at least one of their cells feeding their friends.
Getting big is one thing, but staying big is another. These are common mistakes that players make that cost them mass unnecessarily:
Minimising self feeding
When mass is ejected, the mass used is 18 mass per pellet, but the mass increase on consumption is only 14 mass per pellet. Thus, self feeding wastes some mass and should be used sparingly.
It should only be used when there is danger or potential danger coming after you and you know you won't merge anytime soon, or when it allows you to eat other cells easily. These are examples of scenarios where you should not self feed:
- When you are oversplit in many pieces but there are no enemy cells in sight.
- When you are oversplit and being chased, the chasers are not huge and not dangerously close, you split some time ago and your pieces are merging soon.
- If you are teaming properly, you should not have to use self feeding at all.
Avoiding Anti-Team Penalty
While anti-team was made to make teaming on FFA/Experimental and cross-teaming on Teams not worth it, it still accidentally affects solo players all the time. Fortunately, there are still ways to avoid it.
- Avoid hitting viruses accidentally.
- If you do hit one or get hit, try to stay in 16 pieces. If a few of your small pieces fly into other cells and the sniper tries to shoot you again, split either on the virus or away, depending on the situation. You'll be in 16 pieces again and won't pop on the virus or get anti-team.
- Avoid splitting into 16 pieces at once.