diff --git a/quantile/stream.go b/quantile/stream.go
index 56d722e..f4cabd6 100644
--- a/quantile/stream.go
+++ b/quantile/stream.go
@@ -133,7 +133,7 @@
 		if l == 0 {
 			return 0
 		}
-		i := int(float64(l) * q + 0.5) // Add 0.5 because casting float to int always rounds down.
+		i := int(math.Ceil(float64(l) * q))
 		if i > 0 {
 			i -= 1
 		}