diff --git a/promql/functions.go b/promql/functions.go
index 20a76b9e7..251838607 100644
--- a/promql/functions.go
+++ b/promql/functions.go
@@ -145,8 +145,17 @@ func funcIncrease(ev *evaluator, args Expressions) model.Value {
 
 // === irate(node model.ValMatrix) Vector ===
 func funcIrate(ev *evaluator, args Expressions) model.Value {
+	return instantValue(ev, args[0], true)
+}
+
+// === idelta(node model.ValMatric) Vector ===
+func funcIdelta(ev *evaluator, args Expressions) model.Value {
+	return instantValue(ev, args[0], false)
+}
+
+func instantValue(ev *evaluator, arg Expr, isRate bool) model.Value {
 	resultVector := vector{}
-	for _, samples := range ev.evalMatrix(args[0]) {
+	for _, samples := range ev.evalMatrix(arg) {
 		// No sense in trying to compute a rate without at least two points. Drop
 		// this vector element.
 		if len(samples.Values) < 2 {
@@ -157,7 +166,7 @@ func funcIrate(ev *evaluator, args Expressions) model.Value {
 		previousSample := samples.Values[len(samples.Values)-2]
 
 		var resultValue model.SampleValue
-		if lastSample.Value < previousSample.Value {
+		if isRate && lastSample.Value < previousSample.Value {
 			// Counter reset.
 			resultValue = lastSample.Value
 		} else {
@@ -169,38 +178,10 @@ func funcIrate(ev *evaluator, args Expressions) model.Value {
 			// Avoid dividing by 0.
 			continue
 		}
-		// Convert to per-second.
-		resultValue /= model.SampleValue(sampledInterval.Seconds())
 
-		resultSample := &sample{
-			Metric:    samples.Metric,
-			Value:     resultValue,
-			Timestamp: ev.Timestamp,
-		}
-		resultSample.Metric.Del(model.MetricNameLabel)
-		resultVector = append(resultVector, resultSample)
-	}
-	return resultVector
-}
-
-// === idelta(node model.ValMatric) Vector ===
-func funcIdelta(ev *evaluator, args Expressions) model.Value {
-	resultVector := vector{}
-	for _, samples := range ev.evalMatrix(args[0]) {
-		// No sense in trying to compute a rate without at least two points. Drop
-		// this vector element.
-		if len(samples.Values) < 2 {
-			continue
-		}
-
-		lastSample := samples.Values[len(samples.Values)-1]
-		previousSample := samples.Values[len(samples.Values)-2]
-		resultValue := lastSample.Value - previousSample.Value
-
-		sampledInterval := lastSample.Timestamp.Sub(previousSample.Timestamp)
-		if sampledInterval == 0 {
-			// Avoid dividing by 0.
-			continue
+		if isRate {
+			// Convert to per-second.
+			resultValue /= model.SampleValue(sampledInterval.Seconds())
 		}
 
 		resultSample := &sample{