luckadjusted
NFL Week 9 · 2026

Week 15 2021 · December 19, 2021

No FTN charting for this game (drops and interception-worthy throws are not modelled).

Falcons at 49ers, Week 15 2021: who deserved to win?

The 49ers beat the Falcons 31–13, and the replays back it up: with the way both teams actually played, the 49ers win 9 of 10.

Before kickoff the 49ers were 76% favorites. The biggest swing (Q2, 2:00 left): Defensive Pass Interference on the Falcons (Fabian Moreau), 14 yards, worth 12 points of win probability to the 49ers.

Final

Atlanta Falcons 13
San Francisco 49ers 31

December 19, 2021 · Levi's Stadium · Closing line: 49ers −9.5

Luck index +9 49ers about even · −9 Falcons

Each team’s own plays from this game, resampled into new drives 4,000 times: with the way both teams actually played, how often does this performance win? How the replay works

Ten replays. Falcons win 1, 49ers win 9.

  1. 1 ATL 24 SF 17
  2. 2 ATL 30 SF 33
  3. 3 ATL 13 SF 17
  4. 4 ATL 24 SF 31
  5. 5 ATL 16 SF 26
  6. 6 ATL 17 SF 30
  7. 7 ATL 29 SF 44
  8. 8 ATL 27 SF 45
  9. 9 ATL 7 SF 29
  10. 10 ATL 10 SF 38

Three other ways to ask

Each answers a different question, so none of them replaces the replay above.

The closing betting line, turned into a probability. What everyone knew before the game.

The line plus what this game showed, weighted by how much one game can tell. It stays close to the line because one game says little.

Every kick, fumble bounce, drop, conversion and judgment call redrawn; everything else fixed. The narrowest view of luck.

How 4,000 replays ended

Falcons win 9% of them, 49ers 91%, counting the 2% still level after regulation as half each. The line is what actually happened: 49ers by 18.

Show as a table
Final margin in the replaysShare of replays
Falcons by 17+0.6%
Falcons by 9–161.8%
Falcons by 1–86.4%
Level after regulation1.6%
49ers by 1–817.8%
49ers by 9–1622.6%
49ers by 17+49.2%

Where the luck was

What the luck was worth

Every lucky and unlucky play, weighted by the moment it happened: its swing in win probability, added up by kind. A missed kick in a decided game counts for nothing; the same kick at the end of a tie game can be most of a win.

Swings add up across a game, so in one that went back and forth they can pass 100%. On the scoreboard the same luck was worth +4.0 points to the 49ers: the final margin was 49ers by 18, without the luck 49ers by 14. Luck shares are the fitted ones (how much of each kind is luck).

The kicks

How likely each kicker was to make that kick (his own record, the distance and the weather), and what happened.

  • Y. Koo, 48 yds made · 81% expected
  • R. Gould, 30 yds made · 96% expected
  • Y. Koo, 22 yds made · 99% expected
Bar: expected chance of a make. Filled dot: made. Open dot: missed or blocked.

Penalties: Falcons 4 for 29 yards, 49ers 5 for 35 yards

Every accepted flag, in order, and what it did to the helped team’s chance of winning at that moment, including any play it wiped out (points on the scoreboard in grey). A flag in a decided game is worth nothing; one at the turn of a close game can be worth most of a win. Judgment calls count half toward luck; pre-snap fouls, which repeat, don’t count. How calls are valued

WhenFlagWin probability for
Q1 9:37 False Start on the Falcons, 5 yards · pre-snap +6% SF +1.2 pts
Q1 0:42 Defensive Too Many Men on Field on the 49ers, 5 yards · pre-snap +1% ATL +1.0 pts
Q2 13:30 False Start on the 49ers (Brandon Aiyuk), 5 yards · pre-snap +2% ATL +0.6 pts
Q2 7:50 Neutral Zone Infraction on the 49ers (Arden Key), 5 yards · pre-snap +4% ATL +1.0 pts
Q2 4:50 Roughing the Passer on the 49ers (Arden Key), 15 yards +6% ATL +1.7 pts
Q2 4:29 False Start on the Falcons, 5 yards · pre-snap +2% SF +0.8 pts
Q2 2:00 Defensive Pass Interference on the Falcons (Fabian Moreau), 14 yards +12% SF +1.7 pts
Q3 8:31 False Start on the Falcons (Kyle Pitts), 5 yards · pre-snap 0% SF +0.6 pts
Q4 9:36 Neutral Zone Infraction on the 49ers (Kentavius Street), 5 yards · pre-snap 0% ATL +0.5 pts

Falcons penalties, every game · 49ers penalties, every game · All teams, 2021

When it happened

49ers’ chance of winning

As low as 34% and as high as 100%. Numbers mark the biggest luck moments, listed below.

nflfastR’s in-game win probability. It ignores the betting line, so it starts near even. Shaded above the middle while the 49ers were ahead in it, below while the Falcons were.
#WhenWhat happenedWin probability forNext drive
1 Q1 15:00 The 49ers fumbled and the Falcons recoveredFumble bounces · loose balls are close to 50/50 +5% ATL SF: Punt
2 Q1 12:32 The Falcons were stopped on 4th & 13rd & 4th downs · 69% expected +11% SF ATL: Field goal
3 Q1 9:37 False Start on the Falcons, 5 yardsPre-snap flags +6% SF SF: Field goal
4 Q1 9:15 The Falcons converted 3rd & 63rd & 4th downs · 39% expected +9% ATL ATL: Field goal
5 Q1 2:07 The 49ers were stopped on 3rd & 83rd & 4th downs · 35% expected +5% ATL SF: Field goal
6 Q1 0:42 The Falcons converted 3rd & 23rd & 4th downs · 63% expectedDefensive Too Many Men on Field on the 49ers, 5 yardsPre-snap flags +5% ATL ATL: Punt
7 Q2 4:50 Roughing the Passer on the 49ers (Arden Key), 15 yardsJudgment calls · 8 flags in 1039 snaps before, position avg 1 per 266 +6% ATL ATL: Touchdown
8 Q2 2:00 Defensive Pass Interference on the Falcons (Fabian Moreau), 14 yardsJudgment calls · 10 flags in 1908 snaps before, position avg 1 per 213 +12% SF SF: Touchdown

Missing players

Key players (usually on the field for half the snaps or more) who missed this game or part of it, valued by position and quality. The same players are missing in every replay, so this counts as luck in points only.

  • Mike McGlinchey (SF T), out (IR) · 0.9 pts
  • Emmanuel Moseley (SF CB), out (IR) · 0.6 pts
  • Mohamed Sanu (SF WR), out (IR) · 0.4 pts
  • Azeez Al-Shaair (SF LB), out · 0.3 pts
  • Elijah Mitchell (SF RB), out · 0.1 pts
  • Erik Harris (ATL FS), out (IR) · 0.3 pts

Adjust the assumptions

Disagree with how much of a fumble bounce or a pass interference call is luck? Change it and the coin-flip replay reruns in your browser.

Only coin-flips re-rolled

Falcons 0% · 49ers 100%

Final margin
49ers by 18
Luck in points
4.0 for the 49ers
Luck-adjusted margin
49ers by 14.0
Replay with same day's form
Falcons 9% · 49ers 91% unchanged: why
How much of each is luck?

0% treats it as skill, 100% as pure luck. The tick is our fit.

What counts

None by default: our tests measured no carryover after a lucky break. Slide it up to count a break for more than its own swing. Why

Flip a play

Force the other outcome, in this game and in every replay: a made kick becomes a miss, a stop becomes a conversion, a flag goes the other way.

About these numbers

Play-by-play and win probability from nflfastR via nflverse; kicks valued against each kicker’s own record; drops and interception-worthy throws from FTN charting (not available for this game); injuries from snap counts and injury reports. No FTN charting for this game (drops and interception-worthy throws are not modelled).

Methodology and validation · All Week 15 2021 games · Falcons · 49ers

More from Week 15 2021