Well we aren't adding up every single chip, that'd make the math more difficult.
It's more of an estimation, and it's actually easier than you might think.
Preflop is pretty easy so I won't dive in too deep.
Blinds are 50/100
2 people limp for 100
just add it up ~ 3 big blinds are in the pot, plus half of 1.
3.5:1 on a call.
People generally size their bets relative to the pot size.
So when villain bets roughly
1/2 the pot, assuming no one calls before you, you're getting 3:1 pot odds.
To get the pot odds I took the bet size and just added it.
Villain bet 1/2 pot:
1+2 = 3.
3:1 pot odds.
Villain bets the full pot 1/1:
2:1 pot odds.
Villain bets a mere 1/4 pot:
5:1 pot odds.
This only works when the fraction starts with a 1, so if villain bets 3/4 or 2/3 pot then this won't work, but I think it's a good starting point.
It's also important to note if anyone calls before you, then you will have to make an adjustment.
Villain bets 1/2 pot, well we already know that gives us 3:1.
But someone calls before you.
That would then change your pot odds to 4:1, thus lowering the amount of equity needed to continue.