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William Kempner

2026
Marlins· RHP · throws R· 12.1 IP· 2.92 ERA· 0.97 WHIP· 50 batters faced

Grade

His grade shows how good he has been, and at what.
Dbetter than 38% of qualified pitchers at limiting bases
RelieverCommand Artist

Rarely walks anyone and lets the defense work.

Kempner has been below average at limiting bases

The simulator expects him to allow 0.460 bases per batter faced, better than 38% of qualified pitchers. His best skill is limiting hard contact (89th percentile). Short relief outings push strikeout rates up a little, so read his K% spoke with that in mind.

His grade rests on 50 batters faced, with data through 2026-09-27.

How to read this chart
Each spoke is a skill estimate adjusted for sample size, not a raw season stat, and farther out is better. A ↓ marks a stat where a lower number is better, and its spoke is flipped so farther out still means better. The middle ring marks the league median and the outer ring the league best. Percentiles compare him with 517 qualified pitchers, those with 100+ batters faced, starters and relievers together. He has not reached that bar himself, so he is measured against them rather than counted among them.

Plays like…

Each of these pitchers shares his archetype and sits closest to him in xEB/PA.

Martín PérezBravesNick FrassoRockiesYerry De los SantosYankeesMichael KingPadres

Pitcher type

This map shows what kind of pitcher he is, next to the league.

Arsenal

These cards show what he throws and what each pitch does.

William Kempner leans on his four-seam fastball (48% of pitches). No pitch has beaten league-average results yet, and the four-seam fastball has come closest, with a 39% whiff rate against a league median of 21%.

Pitch mix & swing-and-miss

+8 bases better than a replacement arm this season

He allowed about 18 estimated bases, against a replacement arm's 26 over the same batters faced.

Replacement level is per-batter and can't be split by pitch type, so the bars below use league average.

All pitches

He cost 1.5 bases against league average across 96 pitches, or 1.6 per 100 pitches.

Four-Seam Fastball
48%
95.5 mph · RHP median 95.1
39% whiffs · Lg median 21%
cost 1.6 per 1001.5 total
Sweeper
not charted
22%
81.6 mph · RHP median 83.5
—
—
Sinker
not charted
15%
94.0 mph · RHP median 94.7
—
—
Cutter
not charted
13%
87.3 mph · RHP median 90.1
—
—

Only his four-seam fastball is in the charts below, because the rest were tracked too rarely to plot.

He rarely throws his changeup (3% of his pitches).

How to read this chart
Each row is a pitch he throws at least 3% of the time, from pitch tracking in the regular season. The bar is how often he throws it, then its average velocity and its whiff rate, the share of swings that miss. Beside the velocity and the whiff rate is the median for that pitch, among pitchers of his hand for velocity and across the league for whiffs. More whiffs are better for a pitcher, while usage and velocity describe how he pitches. The Bases vs average bars score each pitch against a league-average pitch in the same count, and a bar to the left (saved) is better.

Mix by batter side

Four-Seam Fastball
47% vs LHH · RHP median 40%
52% vs RHH · RHP median 33%
How to read this chart
Each row shows how often he throws a pitch to left-handed hitters (LHH) and right-handed hitters (RHH), next to the median for pitchers of his hand (RHP median). Medians come from the 384 right-handed pitchers who throw that pitch at least 3% of the time. A mix is a plan, so a higher or lower share is neither better nor worse. Overall results by batter side are under Platoon splits.

William Kempner's pitch movement, 2026

His Four-Seam Fastball has moved 2+ inches from its early-season shape.

The verdict above compares each pitch with his own early-season outings in 2026, not with other pitchers, after taking out the league-wide drift in break.

William Kempner's pitch movement, 202612181218in← glove sidearm side →more risemore drop
Four-Seam FastballFF
Rise 12.3 in
−3.4 vs Lg median
Break 16.8 in
+9.1 vs Lg median
Vs. early season changed
Early season → nowLg medianNormal wobble around the dot
How to read this chart
Each dot is a pitch's current break in inches, averaged over his latest outings. Higher is more rise and further right is more arm-side break, and neither is better or worse. The faint trail runs from his early-season outings to the dot, and the soft halo shows how far normal noise can move it. League median is the small hollow ring, and a pitch marked * has none because too few pitchers throw it. The “changed” verdict removes a league-wide drift that the trail still shows, so a long trail can read as steady. The by-appearance chart below has the exact dates. More on the About page

By appearance

These charts follow his velocity, movement and mix from outing to outing.

These charts follow his velocity, movement and pitch mix one outing at a time, and each pitch keeps the same color in every chart. The velocity and movement charts also shade the middle of the league for scale. Only his four-seam fastball is in these charts, because the rest were tracked too rarely to plot.

Platoon splits

This section splits his results against lefties and righties.

Platoon splits (EB/PA allowed)

He handles righty bats better than lefty bats, by 0.041 EB/PA allowed.

50% probable range: +0.033 to +0.049

vs LHB33 PA0.434

vs LHB: shrunk estimate 0.434 EB/PA, 50% probable range 0.412 to 0.458, based on 33 plate appearances. Raw (unshrunk) rate: 0.343 EB/PA. League median vs LHB: 0.451 EB/PA.

His raw rate vs LHB (0.343) is off this scale. Small samples can push a raw rate this far, which is why the dot, the shrunk estimate, is the number to trust.

vs RHB17 PA0.393

vs RHB: shrunk estimate 0.393 EB/PA, 50% probable range 0.370 to 0.416, based on 17 plate appearances. Raw (unshrunk) rate: 0.471 EB/PA. League median vs RHB: 0.435 EB/PA.

His raw rate vs RHB (0.471) is off this scale. Small samples can push a raw rate this far, which is why the dot, the shrunk estimate, is the number to trust.

0.3590.3860.4140.4410.468
← betterworse →
Below league medianAbove50% probable rangeHis raw rateMiddle half of the league

His split is about typical for a righty arm (median 0.042). He allows less than the league median against both sides, so even his weaker side is a strength.

How to read this chart
  • Each dot is his estimate against that side, with its 50% probable range as the bar. These are bases allowed, so lower is better.
  • The diamond is his raw rate, which small samples make noisy, so the simulator pulls the dot toward a more trustworthy value.
  • The dashed tick is the league median, and the shaded strip is the middle half of pitchers with 100 or more batters faced.
  • The two splits average back to his modeled overall EB/PA allowed, not the xEB/PA headline.
  • Tap or hover a mark for its number. LHB and RHB are left- and right-handed batters. More on the About page

Contact allowed

These cards show what hitters do when they make contact.

Batters go to the pull side off him most often, but they do the most damage up the middle.

Pull side and opposite field depend on which side each batter hits from, so this pools every hitter he faced.

Batted balls allowed

Every ball hit off William Kempner this season is drawn where it landed.
hit · out
EBlowhigh
vs Dominic CanzoneHome run
106 mph · 32° · worth 2.4 bases
07/09 vs Mariners
vs Kahlil WatsonDouble
95 mph · 13° · worth 1.8 bases
07/11 vs Guardians
vs Junior CamineroSingle
90 mph · 11° · worth 1.0 bases
06/05 vs Rays
vs Luke RaleySingle
77 mph · 20° · worth 1.0 bases
07/09 vs Mariners
vs Brayan RocchioSingle
98 mph · 12° · worth 1.0 bases
07/11 vs Guardians

Luck check

How to read this chart
Dot color is estimated bases (EB), what the contact was worth no matter what the defense did with it. Darker means more bases, so a dark hollow ring in the outfield is a crushed ball his defense ran down. Tap or hover a dot or a row to link the two, and use the ▶ links to watch a play on Baseball Savant.

Best games

These are his biggest outings this season.

Best games

These are his best outings this season, ranked by bases saved against a replacement arm.