How much to bet: the right size for every bet

Deciding to bet is half the decision. The other half is how much. A bet that's too small lets your opponent see cards cheaply; one that's too big scares off exactly the player you wanted to call. The right size depends on what you want to happen.

Before the flop: the standard raise

First in: raise to 2.5 to 3 times the big blind. At a 10/20 table, something between 50 and 60.
For each player who already called: add one more big blind per player. With two players who already called 20, go to about 100.
Re-raise (3-bet): about 3 times the raise you are facing when you act after the raiser, and a bit more when you act before.

Use the same size with every hand you raise. If you raise 3 times with A-A and only 2 times with 7-7, anyone paying attention will read your cards from the amount.

After the flop: in fractions of the pot

After the flop, think of the bet as a fraction of the pot. The most used sizes:

1/3 of the pot: on boards that barely help the caller (for example, an Ace and two low cards after you raised). Cheap, and it still makes worse hands fold.
1/2 to 2/3 of the pot: the everyday size. It charges draws a high price and still gets called by worse hands.
The whole pot or more: on dangerous boards with many draws, or on the river, when you want the most from a very strong hand (or a believable bluff).

What the bet is for

Before choosing the size, know the reason for the bet. There are basically three:

Value: you think you have the best hand and want worse hands to pay. Bet the most a worse hand would still call.
Protection: you are probably ahead, but many cards can turn the hand around. Bet to charge draws a high price.
Bluff: you want better hands to fold. Bet enough to be believable, no more and no less. More in How to bluff.

If you can't say why you are betting, checking may be better. Betting a medium hand that only gets called by better hands and only makes worse hands fold is the bet that wins the least.

On the river, the bet is different

On the river there are no more cards to come, so there is no protection: nobody has a draw to pay for. Two reasons to bet remain, value and bluff. That changes the question:

With a very strong hand, ask what the biggest bet is that a worse hand still calls. Against players who call a lot, the whole pot or more.
With a good hand that isn't the best, a smaller bet (1/3 to 1/2 of the pot) still gets called by worse pairs that would fold to a big bet.
With a hand that sometimes wins and loses to everything that calls, check. If your opponent bets, decide by the pot odds.

Common sizing mistakes

The minimum bet out of habit. Betting 1 big blind into a pot of 20 protects nothing and says nothing. Almost always, either bet for real or check.
Shoving a strong hand into a small pot. Only hands that beat you will call. A normal bet would win chips from worse hands.
A size that gives your hand away. Betting big only when you are strong, or only when you bluff, is a pattern opponents learn fast.
Forgetting what's left. If the bet leaves you with so few chips that folding later makes no sense, it may be better to go all-in right away, or to bet less.

Think about the stack

Bet sizes grow every street: a half-pot bet on the flop, once called, becomes a much bigger pot on the turn. If you have a very strong hand and want to reach the river with all the chips in the middle, start building the pot early. If your hand is medium, smaller bets keep the pot under control.

A useful reference: with three bets of 2/3 of the pot (flop, turn and river), the chips in the middle grow about 12 times. If the pot on the flop had 10 big blinds and all three bets were called, it ends with more than 120: each player put about 60 big blinds into the hand.