ことにする and ことになる

ことにする and ことになる are essentially the same as Nounにする and Nounになる, but with nominalized verbs instead. This is achieved through using こと to nominalize the verb. 

Reminder:

• する implies a decision or effort by an actor
• なる implies a natural change or result. (no actor)

～ことにする

Decide to do
Simple present Verb +ことにする (to decide to do)
Simple negative verb+ことにする (decide to not do)

ことにする means someone “decided for themselves to do X”. It is important that the decision was made by the person and not for them. 

See also Nounにする

Examples

• たべることにする
  (will) decide to eat. 
  
  
• うんどうすることにした
   Decided to exercise
  
  
• つかれたから、寝ることにした
  I was tired, so (I) decided to sleep. 
  
  
• 忙しくて、寝ないことにした
  (I) was busy, so (I) decided to not sleep. 
  
  

ことにしている

ことにする can also be used in present progressive tense, to express an ongoing decision: 

• うんどうすることにしている
  Currently I have decided to exercise. (exercise more, everyday, etc).
  
  
• おかしを食べないことにしている
  Currently I have decided to not eat candy. 
  
  

You might have noticed that ことにしている comes out as “have decided”.
This pattern literally translates as something like “I have deciding to”. It is for reasons like this that it is important to keep in mind that ている is not always the present progressive as we think of it, but a current state. 

～ことになる

Outcome
Simple present +ことになる (to become)
Simple negative verb +ことになる (to not become)

～ことになる indicates that an action was decided by someone else, OR, naturally came to be. This pattern is usually past tense or present progressive tense. 
Typically, the “deciders” are your boss, parents, workplace, or nature. 
It is usually translated as “become” or “became”, but it really indicates an unchangeable or unavoidable outcome. 

Examples

• さけをのむことになった
  It became that (I) drank. (naturally, or no choice, i.e. peer pressure) 
  
  
• うんどうすることになった
  Became that (I) exercise. (you decided naturally or no choice)
  
  
• つかれたから、ねることになった
  I was tired, so fell asleep. (You didn’t choose to sleep)
  
  
• 忙しくて、ねないことになった
  (I) was busy, so I didn’t sleep. (Didn’t choose to not sleep)
  
  

See also Nounになる

ことになっている

Like ことにする, ことになる can be used in the present progressive tense to express a current state:

• テレビをみないことになっている
  It’s become that I don’t’ watch tv. (decided for you, or no time)
  
  
• おかしを食べないことになっている
  It became that (I) don’t eat candy. 
  
  
• こうかいすることになるよ
  You’ll come to regret it. 
  
  
• こうかいすることになっている 
  It has become that I regret it. 
  
  
• 5時に会うことになっている 
  It has become that we are meeting at 5:00. (We are meeting at 5). 
  
  

Tense

The tense of the verb before ことになる can cause some confusion. So here are some comparison sentences:

• リンゴ5個をたべたことになる 
  It will be that I have eaten 5 apples.
  
  
• リンゴ5個をたべることになる
   It will be that I eat 5 apples.
  
  
• リンゴ5個を食べたことになった
   It became that I had eaten 5 apples
  
  
• リンゴ5個を食べることになった
   It became that I will have eaten 5 apples. (No choice)

See this stack exchange post if you want to see even more on the issue.

Comparison

ことにする and ことになる can both mean “decided”. However the difference is Who or What decided.

• にする – It is always the subject who chooses to do something. (choice)
• になる – Indicates some irresistible force decided it (i.e., your boss, parents, or nature). (no choice)

Please remember that ことになる is still “became” (i.e. nature). 

• あきになって、はっぱがおちることになった
  It became fall, and the leaves started (became) to fall from the trees. 
  
  

Japanese people also love to use ことになる for things like けっこん

• 彼女とけっこんすることになった
  It became that I married her. 
  
  i.e. “So naturally I married her.” / It just happened. (No thinking about it, natural occurrence)

Which sounds way better than:

• 彼女とけっこんすることにした
  I decided to marry her. (emphasis on the decision to marry)