diff --git a/2021/day03/main.go b/2021/day03/main.go
new file mode 100644
index 0000000..82aad8e
--- /dev/null
+++ b/2021/day03/main.go
@@ -0,0 +1,150 @@
+package main
+
+import (
+	_ "embed"
+	"flag"
+	"fmt"
+	"strconv"
+	"strings"
+
+	"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")
+	}
+}
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	if part == 1 {
+		ans := part1(input)
+		util.CopyToClipboard(fmt.Sprintf("%v", ans))
+		fmt.Println("Output:", ans)
+	} else {
+		ans := part2(input)
+		util.CopyToClipboard(fmt.Sprintf("%v", ans))
+		fmt.Println("Output:", ans)
+	}
+}
+
+func part1(input string) int {
+	var gamma, epsilon string
+	binaries := strings.Split(input, "\n")
+	for i := 0; i < len(binaries[0]); i++ {
+		var zeroes, ones int
+		for _, b := range binaries {
+			if b[i] == '0' {
+				zeroes++
+			} else {
+				ones++
+			}
+		}
+
+		// gamma is the most common bit at each index (accross all bits)
+		// epsilon is the opposite
+		if zeroes > ones {
+			gamma += "0"
+			epsilon += "1"
+		} else {
+			gamma += "1"
+			epsilon += "0"
+
+		}
+	}
+
+	// multiply together for final ans
+	// actual answer in decimal, not binary
+
+	e, err := strconv.ParseInt(epsilon, 2, 64)
+	if err != nil {
+		panic(err)
+	}
+	g, err := strconv.ParseInt(gamma, 2, 64)
+	if err != nil {
+		panic(err)
+	}
+	return int(e * g)
+}
+
+func part2(input string) int {
+	// filtering values until one remains
+
+	// consider just first bit of each number
+	//   only keep numbers for the correct bit criteria
+	// stop when only one number is left, otherwise continue onto the next bit
+	nums := strings.Split(input, "\n")
+	// assume this will work and len(nums) will eventually hit 1
+	for i := 0; len(nums) > 1; i++ {
+		var zeroes, ones int
+		for _, n := range nums {
+			if n[i] == '0' {
+				zeroes++
+			} else {
+				ones++
+			}
+		}
+		// bit criteria:
+		//   oxygen: most common value of the bit, 1s win ties
+		//   CO2:    opposite, 0s win ties
+		keepChar := "1" // 1s win ties for oxygen
+		if zeroes > ones {
+			keepChar = "0"
+		}
+
+		var newNums []string
+		for _, n := range nums {
+			if string(n[i]) == keepChar {
+				newNums = append(newNums, n)
+			}
+		}
+		nums = newNums
+	}
+	oxygen := nums[0]
+
+	// copy pasta for co2
+	nums = strings.Split(input, "\n")
+	// assume this will work and len(nums) will eventually hit 1
+	for i := 0; len(nums) > 1; i++ {
+		var zeroes, ones int
+		for _, n := range nums {
+			if n[i] == '0' {
+				zeroes++
+			} else {
+				ones++
+			}
+		}
+		// bit criteria:
+		//   oxygen: most common value of the bit, 1s win ties
+		//   CO2:    opposite, 0s win ties
+		keepChar := "0" // 0s win ties for co2
+		if ones < zeroes {
+			keepChar = "1"
+		}
+
+		var newNums []string
+		for _, n := range nums {
+			if string(n[i]) == keepChar {
+				newNums = append(newNums, n)
+			}
+		}
+		nums = newNums
+	}
+	co2 := nums[0]
+
+	// multiplying the oxygen generator rating by the CO2 scrubber rating.
+	o, _ := strconv.ParseInt(oxygen, 2, 64)
+	c, _ := strconv.ParseInt(co2, 2, 64)
+
+	return int(c * o)
+}
diff --git a/2021/day03/main_test.go b/2021/day03/main_test.go
new file mode 100644
index 0000000..3e958fb
--- /dev/null
+++ b/2021/day03/main_test.go
@@ -0,0 +1,70 @@
+package main
+
+import (
+	"testing"
+)
+
+var example = `00100
+11110
+10110
+10111
+10101
+01111
+00111
+11100
+10000
+11001
+00010
+01010`
+
+func Test_part1(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			want:  198,
+		},
+		{
+			name:  "actual",
+			input: input,
+			want:  3320834,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := part1(tt.input); got != tt.want {
+				t.Errorf("part1() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_part2(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			want:  230,
+		},
+		{
+			name:  "actual",
+			input: input,
+			want:  4481199,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := part2(tt.input); got != tt.want {
+				t.Errorf("part2() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}