This is an alternative lines calculator. You enter the market spread and the vig-free price for that side, and it outputs the fair probabilities and American odds for a full range of alternate spreads. It shows you what the “true” line prices should be.

It's different for every sport. In basketball spreads, we use historical data to get parameters, and treat the point spread as a threshold in a Normal distribution of scoring margins. The user’s vig-free probability “anchors” the mean of that distribution. From there, we shift the line up and down by 0.5-point increments and recalculate the probability of covering using the same distribution.

The calculator requires a vig-free price as input. That means you should first remove the bookmaker’s margin from the odds being offered (for example, from a -110/-110 market). The model then uses that fair price to anchor the distribution. If you use raw (vig-included) prices, the results will be biased.

Different leagues have different scoring environments and volatility. A football game and a basketball game don’t have the same distribution of margins. The calculator uses a Normal distribution with a standard deviation tuned to WNBA games. For other leagues, we’ll use different values to reflect their scoring profiles.

Totals influence variance (higher-scoring sports/events generally produce wider margins). Right now, the calculator assumes a fixed standard deviation for each league. More advanced versions can adjust based on the game’s total, but this version keeps it simple.

They’re mathematically consistent with the assumptions of the model. That means they’re a strong approximation, but not perfect reality. Real-world pricing can differ. The calculator is best used as a reference point for whether an alternative line is priced fairly.