What is the formula for min-raise and reraise?

I am a newbie.I played four person pokerTH game many times on computer but failed to understand following scenarios.I hope understanding these will help me become a better player. Kindly clear my confusions if possible(four players are SB , BB , Dealer and Player 1)
(The formula for Raise appears to be= Previous Bet +Previous Bet – BB but it doesn’t seem to apply to all scenarios below)
All Scenarios relate to preflop only.

Scenario 1:
I am playing Small Blind. I post small blind $10. Big Blind posts $20. Player 1 calls $20. Dealer calls $20. Next I can Fold , Call($10) , or Raise. My question is what should be my minimum raise and why ? (Computer says $30 )

Scenario 2:
I am playing Dealer. SB posts $10. BB posts $20 . Player 1 bets $40. Next I can Fold , Call ($40),Raise. My question is what should be my minimum raise and why ? (Computer says $60)

Scenario 3:
I am the Player One. SB posts $10.BB posts $20. Next I can Fold ,Call ($20) or Raise. My question is what should be my minimum raise and why ? (Computer says $40)

Scenario 4:
I am playing Big Blind. SB posts $10. I play $20. Player 1 folds. Dealer calls $20. SB bets $30. Next I get the option to Fold , Call ($20) or Raise. My question is what should be my minimum raise and why ?(Computer says $40)

In short what is the formula for minimum raise and re-raise?

Thanks!!

I hope I am able to communicate the essence of my problem. Any help will be appreciated.

Hi dkvlko.

I think you are making this a lot harder than what it really is.

Minraise = 1 BB

In poker, position plays a huge part on how you should play different hands.
In the scenarios you describe Player 1 is what we call "Under The Gun" or UTG. That means he is the first to act preflop. BB is last to act preflop.

So if Player 1 wants to be in the pot he has to pay 1 BB, or he could chose to raise. Then minraise is 1BB.

Let us say Player1 chooses to minraise, making the price to be in the pot 2 BB. The player who comes after, in your scenario the Dealer, now has to pay 2BB to be in the pot, or he could choose to raise. Still minraise is 1 BB.

The players who have SB and BB always have to pay that for this hand no matter what.

SB = 1/2 BB

Let us now say the Dealer chooses to minraise. That now means 3 BB to be in the pot.

SB, who comes after, now has to pay 2,5 BB since he has already paid 1/2 BB, making it 3 BB in total to be in the pot, or he could fold and lose 1/2 BB.

The same now goes for BB, who have to pay 2 BB to be in the pot, or fold and lose 1 BB.

Player 1 has to pay 1 BB to be in the pot, making it 3 BB in total.

So to be in the pot, you have to pay the price the other players have made it. If you have already paid a part of the price, as for SB and BB and also Player 1 since he got reraised, you have to pay what the other players have raised on top of that.

It is common that the BB goes up as the game progresses depending on what type of game you play.
On ranking games here the BB doubles after every 11th hand.
Another option is to use time, say it goes up every 5 minutes for example.

I hope this helped.

Thanks !!
That solved my problem for the preflop round. I got confused by the variety of answers on the internet.
Is the logic for min-raising remains the same for the other rounds like turn etc..?

Consider the following situation :
In the TURN round only two players are left. Me and Player One.
1. I bet $20
2. Player One bets $120
3. I get the next chance to either Call ($100) or Raise. Computer tells me the min raise should be $200.
According to you Raise amount should be Call + Big Blind but in this round there is no big blind. So what is happening here?

EDIT:I found an inconsistency in the logic. While playing pokerTH only 3 players remains in preflop round.
Consider the moves(I am the dealer) :
1. SB posts $10
2. BB posts $20
3. I bet $40
4. SB calls $30
5. BB bets $80
Now what should be my options for raising given that I can call for $60? Computer tells me I can raise by $120. How is that possible ? AS per my understanding , Min Raise = 1 BB therefore Total Raise amount = Call amount + 1 BB = 60 +20 = $80 but that is not happening. Please explain. Thanks a lot.