diff --git a/go.mod b/go.mod
index e296bd3..998cbf5 100644
--- a/go.mod
+++ b/go.mod
@@ -3,3 +3,5 @@ module github.com/go-chi/httprate
 go 1.17
 
 require github.com/cespare/xxhash/v2 v2.3.0
+
+require golang.org/x/sync v0.7.0 // indirect
diff --git a/go.sum b/go.sum
index eb7f94a..09aebbf 100644
--- a/go.sum
+++ b/go.sum
@@ -2,3 +2,5 @@ github.com/cespare/xxhash/v2 v2.1.2 h1:YRXhKfTDauu4ajMg1TPgFO5jnlC2HCbmLXMcTG5cb
 github.com/cespare/xxhash/v2 v2.1.2/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
 github.com/cespare/xxhash/v2 v2.3.0 h1:UL815xU9SqsFlibzuggzjXhog7bL6oX9BbNZnL2UFvs=
 github.com/cespare/xxhash/v2 v2.3.0/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
+golang.org/x/sync v0.7.0 h1:YsImfSBoP9QPYL0xyKJPq0gcaJdG3rInoqxTWbfQu9M=
+golang.org/x/sync v0.7.0/go.mod h1:Czt+wKu1gCyEFDUtn0jG5QVvpJ6rzVqr5aXyt9drQfk=
diff --git a/local_counter_test.go b/local_counter_test.go
index e19ce2f..c6fa7a1 100644
--- a/local_counter_test.go
+++ b/local_counter_test.go
@@ -6,6 +6,8 @@ import (
 	"sync"
 	"testing"
 	"time"
+
+	"golang.org/x/sync/errgroup"
 )
 
 func TestLocalCounter(t *testing.T) {
@@ -16,74 +18,128 @@ func TestLocalCounter(t *testing.T) {
 		windowLength:     time.Second,
 	}
 
-	// Time = NOW()
 	currentWindow := time.Now().UTC().Truncate(time.Second)
 	previousWindow := currentWindow.Add(-time.Second)
 
-	for i := 0; i < 5; i++ {
-		curr, prev, _ := limitCounter.Get(fmt.Sprintf("key-%v", i), currentWindow, previousWindow)
-		if curr != 0 {
-			t.Errorf("unexpected curr = %v, expected %v", curr, 0)
-		}
-		if prev != 0 {
-			t.Errorf("unexpected prev = %v, expected %v", prev, 0)
-		}
-
-		_ = limitCounter.IncrementBy(fmt.Sprintf("key-%v", i), currentWindow, 1)
-		_ = limitCounter.IncrementBy(fmt.Sprintf("key-%v", i), currentWindow, 99)
-
-		curr, prev, _ = limitCounter.Get(fmt.Sprintf("key-%v", i), currentWindow, previousWindow)
-		if curr != 100 {
-			t.Errorf("unexpected curr = %v, expected %v", curr, 100)
-		}
-		if prev != 0 {
-			t.Errorf("unexpected prev = %v, expected %v", prev, 0)
-		}
+	type test struct {
+		name        string        // In each test do the following:
+		advanceTime time.Duration // 1. advance time
+		incrBy      int           // 2. increase counter
+		prev        int           // 3. check previous window counter
+		curr        int           //    and current window counter
 	}
 
-	// Time++
-	currentWindow = currentWindow.Add(time.Second)
-	previousWindow = previousWindow.Add(time.Second)
-
-	for i := 0; i < 5; i++ {
-		curr, prev, _ := limitCounter.Get(fmt.Sprintf("key-%v", i), currentWindow, previousWindow)
-		if curr != 0 {
-			t.Errorf("unexpected curr = %v, expected %v", curr, 0)
-		}
-		if prev != 100 {
-			t.Errorf("unexpected prev = %v, expected %v", prev, 100)
-		}
-		_ = limitCounter.IncrementBy(fmt.Sprintf("key-%v", i), currentWindow, 50)
+	tests := []test{
+		{
+			name: "t=0s: init",
+			prev: 0,
+			curr: 0,
+		},
+		{
+			name:   "t=0s: increment 1",
+			incrBy: 1,
+			prev:   0,
+			curr:   1,
+		},
+		{
+			name:   "t=0s: increment by 99",
+			incrBy: 99,
+			prev:   0,
+			curr:   100,
+		},
+		{
+			name:        "t=1s: move clock by 1s",
+			advanceTime: time.Second,
+			prev:        100,
+			curr:        0,
+		},
+		{
+			name:   "t=1s: increment by 20",
+			incrBy: 20,
+			prev:   100,
+			curr:   20,
+		},
+		{
+			name:   "t=1s: increment by 20",
+			incrBy: 20,
+			prev:   100,
+			curr:   40,
+		},
+		{
+			name:        "t=2s: move clock by 1s",
+			advanceTime: time.Second,
+			prev:        40,
+			curr:        0,
+		},
+		{
+			name:   "t=2s: incr++",
+			incrBy: 1,
+			prev:   40,
+			curr:   1,
+		},
+		{
+			name:   "t=2s: incr+=9",
+			incrBy: 9,
+			prev:   40,
+			curr:   10,
+		},
+		{
+			name:   "t=2s: incr+=20",
+			incrBy: 20,
+			prev:   40,
+			curr:   30,
+		},
+		{
+			name:        "t=4s: move clock by 2s",
+			advanceTime: 2 * time.Second,
+			prev:        0,
+			curr:        0,
+		},
 	}
 
-	// Time++
-	currentWindow = currentWindow.Add(time.Second)
-	previousWindow = previousWindow.Add(time.Second)
+	concurrentRequests := 1000
 
-	for i := 0; i < 5; i++ {
-		curr, prev, _ := limitCounter.Get(fmt.Sprintf("key-%v", i), currentWindow, previousWindow)
-		if curr != 0 {
-			t.Errorf("unexpected curr = %v, expected %v", curr, 0)
+	for _, tt := range tests {
+		if tt.advanceTime > 0 {
+			currentWindow = currentWindow.Add(tt.advanceTime)
+			previousWindow = previousWindow.Add(tt.advanceTime)
 		}
-		if prev != 50 {
-			t.Errorf("unexpected prev = %v, expected %v", prev, 50)
-		}
-		_ = limitCounter.IncrementBy(fmt.Sprintf("key-%v", i), currentWindow, 99)
-	}
 
-	// Time += 10
-	currentWindow = currentWindow.Add(10 * time.Second)
-	previousWindow = previousWindow.Add(10 * time.Second)
+		if tt.incrBy > 0 {
+			var g errgroup.Group
+			for i := 0; i < concurrentRequests; i++ {
+				i := i
+				g.Go(func() error {
+					key := fmt.Sprintf("key:%v", i)
+					return limitCounter.IncrementBy(key, currentWindow, tt.incrBy)
+				})
+			}
+			if err := g.Wait(); err != nil {
+				t.Errorf("%s: %v", tt.name, err)
+			}
+		}
 
-	for i := 0; i < 5; i++ {
-		curr, prev, _ := limitCounter.Get(fmt.Sprintf("key-%v", i), currentWindow, previousWindow)
-		if curr != 0 {
-			t.Errorf("unexpected curr = %v, expected %v", curr, 0)
+		var g errgroup.Group
+		for i := 0; i < concurrentRequests; i++ {
+			i := i
+			g.Go(func() error {
+				key := fmt.Sprintf("key:%v", i)
+				curr, prev, err := limitCounter.Get(key, currentWindow, previousWindow)
+				if err != nil {
+					return fmt.Errorf("%q: %w", key, err)
+				}
+				if curr != tt.curr {
+					return fmt.Errorf("%q: unexpected curr = %v, expected %v", key, curr, tt.curr)
+				}
+				if prev != tt.prev {
+					return fmt.Errorf("%q: unexpected prev = %v, expected %v", key, prev, tt.prev)
+				}
+				return nil
+			})
 		}
-		if prev != 0 {
-			t.Errorf("unexpected prev = %v, expected %v", prev, 0)
+		if err := g.Wait(); err != nil {
+			t.Errorf("%s: %v", tt.name, err)
 		}
-		_ = limitCounter.IncrementBy(fmt.Sprintf("key-%v", i), currentWindow, 99)
 	}
 }