Adding One More Leg to Your Multi? Here’s How It Increases Risk and House Advantage

In this article, I'll calculate the vig on each leg and show how it compounds. I’ll also track how your true win probability collapses and then flip it around because if you actually have a positive edge on each leg, adding legs is one of the smartest things you can do.

What Is Vig, and Why Does It Compound?

Vig (vigorish), also called the overround or margin, is the betting site's built-in profit cut on every market. To calculate it, you add the implied probability for all possible bets in a market and subtract 100%. So, for a BTTS market, you’d add implied probability for Yes and No and then minus 100%. 

Formula 

Vig per leg = (1/Yes odds + 1/No odds - 1) x 100  

Example: Chelsea vs Manchester United

(1/1.48) + (1/2.60) = 0.6757 + 0.3846 = 1.0603 (Vig = 6.03%)

On a single bet, a 6% vig is uncomfortable but manageable. The danger is during a multi - each leg's vig multiplies.

The 3-Leg Multi

The 3-Leg Multi

I recently placed a R100 stake on three EPL BTTS Yes selections at Betway, with a 3% win boost applied to the potential return:

The compounded vig across the three legs was 19.01% (1.063 x 1.0590 x 1.0599), while after removing the vig from the odds, my true win compound probability was only 18.1%. My expected value was -R16.38 per R100 staked, calculated as: (0.181 × 4.62) − 1 = −0.1638, or −16.38% EV.

Adding the Fourth Leg

Adding the Fourth Leg

I was considering adding another leg to my multi: Brighton vs Chelsea BTTS Yes at 1.57 (No at 2.37). This extra selection caused the odds to jump to 7.26, the win boost to surge to 4%, and my potential return to rise to R755.89.

However, adding just one more leg pushed the bookmaker’s compounded margin from 19.1% to 26.1%, a sharp 7 percentage point increase in their edge. At the same time, my true win probability dropped from 18.1% to 10.9%.

In practical terms, I’ve gone from roughly a 1-in-5.5 chance to a 1-in-9 shot. The potential payout looks more attractive. But unless you're betting with free bets it doesn’t come close to offsetting the accumulator variance and how much harder the bet becomes to actually win, which is why I ultimately decided to stick with the 3-leg multi.

How Edge Compounds

The same compounding mechanism that works against you when you have zero edge is your friend when you have positive expected value. Let's say your edge on each leg is either -2% or +2%, you just use a geometric progression to determine your expected return.

If you're backing legs with no edge, adding more legs is compounding a guaranteed drain. But if you've identified real value, then adding more positive-edge legs compounds that advantage just as aggressively as the vig compounds against you.