diff --git a/promql/engine.go b/promql/engine.go
index 11a7ad22ec..57a1f41bb8 100644
--- a/promql/engine.go
+++ b/promql/engine.go
@@ -4418,9 +4418,9 @@ func extendFloats(floats []FPoint, mint, maxt int64, smoothed bool) []FPoint {
 		lastSampleIndex--
 	}
 
-	// TODO: Preallocate the length of the new list.
-	out := make([]FPoint, 0)
-	// Create the new floats list with the boundary samples and the inner samples.
+	count := max(lastSampleIndex-firstSampleIndex+1, 0)
+	out := make([]FPoint, 0, count+2)
+
 	out = append(out, FPoint{T: mint, F: left})
 	out = append(out, floats[firstSampleIndex:lastSampleIndex+1]...)
 	out = append(out, FPoint{T: maxt, F: right})