diff --git a/promql/functions.go b/promql/functions.go
index ff54e24d7a..251838607e 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 {
@@ -169,8 +178,11 @@ func funcIrate(ev *evaluator, args Expressions) model.Value {
 			// Avoid dividing by 0.
 			continue
 		}
-		// Convert to per-second.
-		resultValue /= model.SampleValue(sampledInterval.Seconds())
+
+		if isRate {
+			// Convert to per-second.
+			resultValue /= model.SampleValue(sampledInterval.Seconds())
+		}
 
 		resultSample := &sample{
 			Metric:    samples.Metric,
@@ -956,6 +968,12 @@ var functions = map[string]*Function{
 		ReturnType: model.ValVector,
 		Call:       funcIrate,
 	},
+	"idelta": {
+		Name:       "idelta",
+		ArgTypes:   []model.ValueType{model.ValMatrix},
+		ReturnType: model.ValVector,
+		Call:       funcIdelta,
+	},
 	"label_replace": {
 		Name:       "label_replace",
 		ArgTypes:   []model.ValueType{model.ValVector, model.ValString, model.ValString, model.ValString, model.ValString},
diff --git a/promql/testdata/functions.test b/promql/testdata/functions.test
index 58b1ee5720..cbc789fd55 100644
--- a/promql/testdata/functions.test
+++ b/promql/testdata/functions.test
@@ -85,6 +85,28 @@ eval instant at 30m irate(http_requests[50m])
 
 clear
 
+# Tests for delta().
+load 5m
+	http_requests{path="/foo"}	0 50 100 150 200
+	http_requests{path="/bar"}	200 150 100 50 0
+
+eval instant at 20m delta(http_requests[20m])
+	{path="/foo"} 200
+	{path="/bar"} -200
+
+clear
+
+# Tests for idelta().
+load 5m
+	http_requests{path="/foo"}	0 50 100 150
+	http_requests{path="/bar"}	0 50 100 50
+
+eval instant at 20m idelta(http_requests[20m])
+	{path="/foo"} 50
+	{path="/bar"} -50
+
+clear
+
 # Tests for deriv() and predict_linear().
 load 5m
 	testcounter_reset_middle	0+10x4 0+10x5
diff --git a/promql/testdata/legacy.test b/promql/testdata/legacy.test
index 6da0281a7b..1556485f8c 100644
--- a/promql/testdata/legacy.test
+++ b/promql/testdata/legacy.test
@@ -91,11 +91,6 @@ eval instant at 50m x{y="testvalue"}
 	x{y="testvalue"} 100
 
 
-# Deltas should be adjusted for target interval vs. samples under target interval.
-eval instant at 50m delta(http_requests{group="canary", instance="1", job="app-server"}[18m])
-	{group="canary", instance="1", job="app-server"} 288
-
-
 # Rates should calculate per-second rates.
 eval instant at 50m rate(http_requests{group="canary", instance="1", job="app-server"}[50m])
 	{group="canary", instance="1", job="app-server"} 0.26666666666666666