diff --git a/promql/functions.go b/promql/functions.go
index 8d2a2830775..4e47c412f4f 100644
--- a/promql/functions.go
+++ b/promql/functions.go
@@ -46,55 +46,15 @@ func funcTime(ev *evaluator, args Expressions) model.Value {
 
 // === delta(matrix model.ValMatrix) Vector ===
 func funcDelta(ev *evaluator, args Expressions) model.Value {
-	// This function still takes a 2nd argument for use by rate() and increase().
-	isCounter := len(args) >= 2 && ev.evalInt(args[1]) > 0
 	resultVector := vector{}
-
-	// If we treat these metrics as counters, we need to fetch all values
-	// in the interval to find breaks in the timeseries' monotonicity.
-	// I.e. if a counter resets, we want to ignore that reset.
-	var matrixValue matrix
-	if isCounter {
-		matrixValue = ev.evalMatrix(args[0])
-	} else {
-		matrixValue = ev.evalMatrixBounds(args[0])
-	}
-	for _, samples := range matrixValue {
+	for _, samples := range ev.evalMatrixBounds(args[0]) {
 		// No sense in trying to compute a delta without at least two points. Drop
 		// this vector element.
 		if len(samples.Values) < 2 {
 			continue
 		}
 
-		var (
-			counterCorrection model.SampleValue
-			lastValue         model.SampleValue
-		)
-		for _, sample := range samples.Values {
-			currentValue := sample.Value
-			if isCounter && currentValue < lastValue {
-				counterCorrection += lastValue - currentValue
-			}
-			lastValue = currentValue
-		}
-		resultValue := lastValue - samples.Values[0].Value + counterCorrection
-
-		targetInterval := args[0].(*MatrixSelector).Range
-		sampledInterval := samples.Values[len(samples.Values)-1].Timestamp.Sub(samples.Values[0].Timestamp)
-		if sampledInterval == 0 {
-			// Only found one sample. Cannot compute a rate from this.
-			continue
-		}
-		// Correct for differences in target vs. actual delta interval.
-		//
-		// Above, we didn't actually calculate the delta for the specified target
-		// interval, but for an interval between the first and last found samples
-		// under the target interval, which will usually have less time between
-		// them. Depending on how many samples are found under a target interval,
-		// the delta results are distorted and temporal aliasing occurs (ugly
-		// bumps). This effect is corrected for below.
-		intervalCorrection := model.SampleValue(targetInterval) / model.SampleValue(sampledInterval)
-		resultValue *= intervalCorrection
+		resultValue := samples.Values[len(samples.Values)-1].Value - samples.Values[0].Value
 
 		resultSample := &sample{
 			Metric:    samples.Metric,
@@ -109,8 +69,7 @@ func funcDelta(ev *evaluator, args Expressions) model.Value {
 
 // === rate(node model.ValMatrix) Vector ===
 func funcRate(ev *evaluator, args Expressions) model.Value {
-	args = append(args, &NumberLiteral{1})
-	vector := funcDelta(ev, args).(vector)
+	vector := funcIncrease(ev, args).(vector)
 
 	// TODO: could be other type of model.ValMatrix in the future (right now, only
 	// MatrixSelector exists). Find a better way of getting the duration of a
@@ -124,8 +83,35 @@ func funcRate(ev *evaluator, args Expressions) model.Value {
 
 // === increase(node model.ValMatrix) Vector ===
 func funcIncrease(ev *evaluator, args Expressions) model.Value {
-	args = append(args, &NumberLiteral{1})
-	return funcDelta(ev, args).(vector)
+	resultVector := vector{}
+	for _, samples := range ev.evalMatrix(args[0]) {
+		// No sense in trying to compute an increase without at least two points. Drop
+		// this vector element.
+		if len(samples.Values) < 2 {
+			continue
+		}
+
+		var (
+			counterCorrection model.SampleValue
+			lastValue         model.SampleValue
+		)
+		for _, sample := range samples.Values {
+			currentValue := sample.Value
+			if currentValue < lastValue {
+				counterCorrection += lastValue - currentValue
+			}
+			lastValue = currentValue
+		}
+		resultValue := lastValue - samples.Values[0].Value + counterCorrection
+		resultSample := &sample{
+			Metric:    samples.Metric,
+			Value:     resultValue,
+			Timestamp: ev.Timestamp,
+		}
+		resultSample.Metric.Del(model.MetricNameLabel)
+		resultVector = append(resultVector, resultSample)
+	}
+	return resultVector
 }
 
 // === irate(node model.ValMatrix) Vector ===
diff --git a/promql/testdata/functions.test b/promql/testdata/functions.test
index 3972053dfe8..d313ca3a00f 100644
--- a/promql/testdata/functions.test
+++ b/promql/testdata/functions.test
@@ -63,6 +63,11 @@ eval instant at 50m increase(http_requests[50m])
 	{path="/foo"} 100
 	{path="/bar"}  90
 
+# Tests for increase().
+eval instant at 51m increase(http_requests[50m])
+	{path="/foo"}  90
+	{path="/bar"}  80
+
 clear
 
 # Tests for irate().
@@ -87,10 +92,10 @@ load 5m
 	http_requests{job="app-server", instance="1", group="canary"}		0+80x10
 
 # deriv should return the same as rate in simple cases.
-eval instant at 50m rate(http_requests{group="canary", instance="1", job="app-server"}[60m])
+eval instant at 50m rate(http_requests{group="canary", instance="1", job="app-server"}[50m])
 	{group="canary", instance="1", job="app-server"} 0.26666666666666666
 
-eval instant at 50m deriv(http_requests{group="canary", instance="1", job="app-server"}[60m])
+eval instant at 50m deriv(http_requests{group="canary", instance="1", job="app-server"}[50m])
 	{group="canary", instance="1", job="app-server"} 0.26666666666666666
 
 # deriv should return correct result.
diff --git a/promql/testdata/legacy.test b/promql/testdata/legacy.test
index 01ca435c683..b7a428d7e0c 100644
--- a/promql/testdata/legacy.test
+++ b/promql/testdata/legacy.test
@@ -194,18 +194,18 @@ eval instant at 50m sum(http_requests) by (job) + min(http_requests) by (job) +
 	{job="api-server"} 1750 
 
 
-# Deltas should be adjusted for target interval vs. samples under target interval.
+# Deltas should not 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
+	{group="canary", instance="1", job="app-server"} 240
 
 
 # Rates should calculate per-second rates.
 eval instant at 50m rate(http_requests{group="canary", instance="1", job="app-server"}[60m])
-	{group="canary", instance="1", job="app-server"} 0.26666666666666666
+	{group="canary", instance="1", job="app-server"} 0.2222222222222222
 
 # Counter resets at in the middle of range are handled correctly by rate().
 eval instant at 50m rate(testcounter_reset_middle[60m])
-	{} 0.03
+	{} 0.025
 
 
 # Counter resets at end of range are ignored by rate().