diff --git a/doc/source/whatsnew/v0.21.1.txt b/doc/source/whatsnew/v0.21.1.txt
index 793e9bf17bac9..b0a8dd724bd9c 100644
--- a/doc/source/whatsnew/v0.21.1.txt
+++ b/doc/source/whatsnew/v0.21.1.txt
@@ -117,7 +117,7 @@ Reshaping
 Numeric
 ^^^^^^^
 
--
+- Fixed incorrect maximum :func:`Series.rank` percentile for large arrays (:issue:`18271`)
 -
 -
 
diff --git a/pandas/_libs/algos_rank_helper.pxi.in b/pandas/_libs/algos_rank_helper.pxi.in
index 0945aec638b1d..df659ef56f82d 100644
--- a/pandas/_libs/algos_rank_helper.pxi.in
+++ b/pandas/_libs/algos_rank_helper.pxi.in
@@ -60,7 +60,7 @@ def rank_1d_{{dtype}}(object in_arr, ties_method='average', ascending=True,
         float64_t sum_ranks = 0
         int tiebreak = 0
         bint keep_na = 0
-        float count = 0.0
+        float64_t count = 0.0
     tiebreak = tiebreakers[ties_method]
 
     {{if dtype == 'float64'}}
@@ -234,7 +234,7 @@ def rank_2d_{{dtype}}(object in_arr, axis=0, ties_method='average',
         float64_t sum_ranks = 0
         int tiebreak = 0
         bint keep_na = 0
-        float count = 0.0
+        float64_t count = 0.0
 
     tiebreak = tiebreakers[ties_method]