What “Under 21.5” means (short)

“Under 21.5” in tennis betting is a total-games market for the match. It means you are betting that the match will have fewer than 21.5 games — in practice that means 21 games or fewer. Bookmakers use the “.5” so there’s never a push.

How you count games

• Count the number of games played in each set (e.g., a 6–3 set = 6 + 3 = 9 games).
• Add the games from all sets.
• A 7–6 set counts as 7 + 6 = 13 games (the tiebreak is reflected by the 7–6 score — you don’t add tiebreak pointsseparately).
• Be careful: tournament or bookmaker rules vary for special deciding tiebreaks or walkovers/retirements — always check the market rules.

Examples (step-by-step arithmetic)

1. Match ends 6–3, 6–4
   Set1 games = 6 + 3 = 9.
   Set2 games = 6 + 4 = 10.
   Total = 9 + 10 = 19 → 19 ≤ 21 → Under 21.5 wins.
2. Match ends 6–4, 7–6
   Set1 games = 6 + 4 = 10.
   Set2 games = 7 + 6 = 13.
   Total = 10 + 13 = 23 → 23 > 21.5 → Under 21.5 loses.
3. Match ends 6–1, 6–2
   Set1 games = 6 + 1 = 7.
   Set2 games = 6 + 2 = 8.
   Total = 7 + 8 = 15 → Under 21.5 wins.
4. Three-set match 6–2, 3–6, 6–4
   Set1 = 6 + 2 = 8.
   Set2 = 3 + 6 = 9.
   Set3 = 6 + 4 = 10.
   Total = 8 + 9 + 10 = 27 → Under 21.5 loses.
5. Best-of-5 example 6–1, 6–1, 6–1
   Set1 = 6 + 1 = 7.
   Set2 = 6 + 1 = 7.
   Set3 = 6 + 1 = 7.
   Total = 7 + 7 + 7 = 21 → 21 ≤ 21 → Under 21.5 wins (so note: format matters).

Settlement edge cases — what to watch for

• Walkover before match start: usually bets are void.
• Retirement mid-match: rules vary — some books settle using the score at retirement, others void total-games markets unless a minimum number of games were played.
• Deciding match tiebreaks / special rules (e.g., a 10-point match tiebreak instead of a full final set): some books count that as a single game, some count it differently — check the market rules for that sportsbook.
• Always read the bookmaker’s market rules if you want certainty.

Quick betting tips

• Check match format (Bo3 vs Bo5). Under 21.5 is easier to hit in Bo3 than Bo5, but as example #5 shows it’s still possible in Bo5.
• Favor heavy favorites who often win in straight sets with big margins (e.g., many 6–2 / 6–3 scores).
• Avoid matches where both players often push tiebreaks (lots of 7–6 sets raise the total).
• Surface and playing styles matter: aggressive servers and short points can go either way (fewer games if one player dominates, or more close holds and tiebreaks).
• Check recent form, head-to-head, and fatigue (a tired underdog can make an early, short match more likely).
• Compare rules between bookmakers — a retirement policy can swing whether a bet is paid or void.