diff --git a/src/Prometheus/Math.php b/src/Prometheus/Math.php
index 3b3029d6..f9ae146f 100644
--- a/src/Prometheus/Math.php
+++ b/src/Prometheus/Math.php
@@ -21,14 +21,11 @@ public function quantile(array $arr, float $q): float
             return 0;
         }
 
-        $allindex = ($count - 1) * $q;
-        $intvalindex = (int) $allindex;
-        $floatval = $allindex - $intvalindex;
-        if ($count > $intvalindex + 1) {
-            $result = $floatval * ($arr[$intvalindex + 1] - $arr[$intvalindex]) + $arr[$intvalindex];
-        } else {
-            $result = $arr[$intvalindex];
+        $j = floor($count * $q);
+        $r = $count * $q - $j;
+        if (0.0 === $r) {
+            return $arr[$j - 1];
         }
-        return $result;
+        return $arr[$j];
     }
 }
diff --git a/tests/Test/BlackBoxTest.php b/tests/Test/BlackBoxTest.php
index ab6c82e3..fd451792 100644
--- a/tests/Test/BlackBoxTest.php
+++ b/tests/Test/BlackBoxTest.php
@@ -176,11 +176,11 @@ public function summariesShouldIncrementAtomically(): void
         $body = (string)$metricsResult->getBody();
 
         self::assertThat($body, self::stringContains(<<<EOF
-test_some_summary{type="blue",quantile="0.01"} 1.09
-test_some_summary{type="blue",quantile="0.05"} 1.45
-test_some_summary{type="blue",quantile="0.5"} 5.5
-test_some_summary{type="blue",quantile="0.95"} 9.55
-test_some_summary{type="blue",quantile="0.99"} 9.91
+test_some_summary{type="blue",quantile="0.01"} 1
+test_some_summary{type="blue",quantile="0.05"} 1
+test_some_summary{type="blue",quantile="0.5"} 5
+test_some_summary{type="blue",quantile="0.95"} 10
+test_some_summary{type="blue",quantile="0.99"} 10
 test_some_summary_count{type="blue"} 10
 test_some_summary_sum{type="blue"} 55
 EOF
diff --git a/tests/Test/Prometheus/AbstractSummaryTest.php b/tests/Test/Prometheus/AbstractSummaryTest.php
index 0a74d1c6..955c7ed0 100644
--- a/tests/Test/Prometheus/AbstractSummaryTest.php
+++ b/tests/Test/Prometheus/AbstractSummaryTest.php
@@ -277,19 +277,19 @@ public function itShouldObserveComputeMedianCorrectlyWithEvenSerieLength(): void
                                     'name' => 'test_some_metric',
                                     'labelNames' => ['quantile'],
                                     'labelValues' => [0.1],
-                                    'value' => 1.9,
+                                    'value' => 1,
                                 ],
                                 [
                                     'name' => 'test_some_metric',
                                     'labelNames' => ['quantile'],
                                     'labelValues' => [0.5],
-                                    'value' => 5.5,
+                                    'value' => 5,
                                 ],
                                 [
                                     'name' => 'test_some_metric',
                                     'labelNames' => ['quantile'],
                                     'labelValues' => [0.9],
-                                    'value' => 9.1,
+                                    'value' => 9,
                                 ],
                                 [
                                     'name' => 'test_some_metric_count',
@@ -446,19 +446,19 @@ public function itShouldIgnoreSerieItemsOlderThanMaxAgeSeconds(): void
                                     'name' => 'test_some_metric',
                                     'labelNames' => ['quantile'],
                                     'labelValues' => [0.1],
-                                    'value' => 1.9,
+                                    'value' => 1,
                                 ],
                                 [
                                     'name' => 'test_some_metric',
                                     'labelNames' => ['quantile'],
                                     'labelValues' => [0.5],
-                                    'value' => 5.5,
+                                    'value' => 5,
                                 ],
                                 [
                                     'name' => 'test_some_metric',
                                     'labelNames' => ['quantile'],
                                     'labelValues' => [0.9],
-                                    'value' => 9.1,
+                                    'value' => 9,
                                 ],
                                 [
                                     'name' => 'test_some_metric_count',
diff --git a/tests/Test/Prometheus/MathTest.php b/tests/Test/Prometheus/MathTest.php
index 1f35ae22..0980845e 100644
--- a/tests/Test/Prometheus/MathTest.php
+++ b/tests/Test/Prometheus/MathTest.php
@@ -30,9 +30,9 @@ public function providerQuantileSuccess(): array
     {
         return [
             'Even serie' => [[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 0.5, 5],
-            'Odd serie' => [[1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 0.5, 5.5],
+            'Odd serie' => [[1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 0.5, 5],
             'Even float serie' => [[0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.10], 0.5, 0.5],
-            'Odd float serie' => [[0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.10], 0.5, 0.55],
+            'Odd float serie' => [[0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.10], 0.5, 0.5],
             'Empty serie' => [[], 0.5, 0],
         ];
     }