NFL key numbers explained. Why 3 and 7 are the only halves worth buying.

Plain-English answer

Key numbers in NFL betting are the most common final-margin gaps between two teams. The two dominant key numbers are 3 (most common, ~15% of all NFL games end on a 3-point margin) and 7 (second most common, ~9%), driven by field-goal frequency and touchdown-plus-XP. Tier-2 keys are 10 (~5%), 6 (~4%), and 14 (~3%). Half-points across these numbers carry roughly 5x the average half-point value because the line literally moves the cover-or-not coin flip on the modal final margin. The rule: buying a half-point that crosses 3 is worth roughly 25 cents of price (e.g. -3 -110 to -2.5 -135 is fair); buying off a non-key (e.g. 4.5 to 4) is roughly 5 cents fair. Retail buys uniformly across the half-point board because the book sells them at uniform prices; sharps buy only off key numbers because that is where the math breaks the book's flat pricing.

Compare what the book charges to what the half-point is worth. Books typically charge a flat 10 to 18 cents for any half-point buy. The math:

• -3 to -2.5: book charges 12 to 18c, fair is 25c. Buy.
• +3 to +3.5: book charges 12 to 18c, fair is 25c. Buy.
• -7 to -6.5: book charges 10 to 14c, fair is 18c. Buy.
• -4 to -3.5: book charges 10c, fair is 5c. Skip.
• -7.5 to -8: book charges 10c, fair is 5c. Skip.

The simplest test: buy only when the half-point step changes which side of 3 or 7 the spread sits on. Anything else is the book charging key-number prices for non-key value. Confirm with no-vig pricing on the post-buy line.

Use it live

Pair the key-number read with teaser math to extend the leverage, middling for the same key-number-driven gap math, and EV to confirm the post-buy price still clears.