diff --git a/2021/day06/main.go b/2021/day06/main.go
new file mode 100644
index 0000000..22ef7ee
--- /dev/null
+++ b/2021/day06/main.go
@@ -0,0 +1,66 @@
+package main
+
+import (
+	_ "embed"
+	"flag"
+	"fmt"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/cast"
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+//go:embed input.txt
+var input string
+
+func init() {
+	// do this in init (not main) so test file has same input
+	input = strings.TrimRight(input, "\n")
+	if len(input) == 0 {
+		panic("empty input.txt file")
+	}
+}
+
+// 180/1133
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	ans := step(input, part)
+	util.CopyToClipboard(fmt.Sprintf("%v", ans))
+	fmt.Println("Output:", ans)
+}
+
+func step(input string, part int) int {
+	state := make([]int, 9)
+	for _, num := range strings.Split(input, ",") {
+		daysLeft := cast.ToInt(num)
+		state[daysLeft]++
+	}
+
+	// different number of days for part 1 vs part 2
+	days := 80
+	if part == 2 {
+		days = 256
+	}
+
+	// count down to zero
+	// then it makes a new lanternfish with a count of 8
+	for d := 0; d < days; d++ {
+		save := state[0]
+		for i := 0; i < len(state)-1; i++ {
+			state[i] = state[i+1]
+		}
+		state[8] = save
+		state[6] += save
+	}
+
+	// count up final sum
+	var sum int
+	for _, n := range state {
+		sum += n
+	}
+	return sum
+}
diff --git a/2021/day06/main_test.go b/2021/day06/main_test.go
new file mode 100644
index 0000000..d5a7a94
--- /dev/null
+++ b/2021/day06/main_test.go
@@ -0,0 +1,51 @@
+package main
+
+import (
+	"testing"
+)
+
+var example = `3,4,3,1,2`
+
+func Test_part1(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		part  int
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			part:  1,
+			want:  5934, // after 80 days
+		},
+		{
+			name:  "actual",
+			input: input,
+			part:  1,
+			want:  362666,
+		},
+		{
+			name:  "example",
+			input: example,
+			part:  2,
+			want:  26984457539,
+		},
+		{
+			name:  "actual",
+			input: input,
+			part:  2,
+			want:  1640526601595,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if tt.part == 0 {
+				t.Error("part value cannot be zero")
+			}
+			if got := step(tt.input, tt.part); got != tt.want {
+				t.Errorf("step() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}