diff --git a/2021/day03/main.go b/2021/day03/main.go
new file mode 100644
index 0000000..c8138d9
--- /dev/null
+++ b/2021/day03/main.go
@@ -0,0 +1,122 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"strings"
+)
+
+func parseInput(file string) []string {
+	content, err := os.ReadFile(file)
+	if err != nil {
+		log.Fatalf("Failed to read input file: %v", err)
+	}
+	return strings.Split(strings.TrimSpace(string(content)), "\n")
+}
+
+func PartOne(data []string) int {
+	bitlen := len(data[0])
+	ones := make([]int, bitlen)
+
+	for _, line := range data {
+		for i := range bitlen {
+			if line[i] == '1' {
+				ones[i]++
+			}
+		}
+	}
+
+	var gamma int
+	total := len(data)
+	for idx := range bitlen {
+		gamma <<= 1
+		if ones[idx] > total-ones[idx] {
+			gamma |= 1
+		}
+	}
+
+	mask := (1 << bitlen) - 1
+	epsilon := mask ^ gamma
+	return gamma * epsilon
+}
+
+func PartTwo(data []string) int {
+	bitlen := len(data[0])
+
+	oxygenIndices := make([]int, len(data))
+	for idx := range oxygenIndices {
+		oxygenIndices[idx] = idx
+	}
+
+	co2Indices := make([]int, len(data))
+	for idx := range co2Indices {
+		co2Indices[idx] = idx
+	}
+
+	for indice := 0; indice < bitlen && len(oxygenIndices) > 1; indice++ {
+		ones := 0
+		for _, idx := range oxygenIndices {
+			if data[idx][indice] == '1' {
+				ones++
+			}
+		}
+		target := byte('1')
+		if ones*2 < len(oxygenIndices) {
+			target = '0'
+		}
+		writeIdx := 0
+		for readIdx := 0; readIdx < len(oxygenIndices); readIdx++ {
+			if data[oxygenIndices[readIdx]][indice] == target {
+				oxygenIndices[writeIdx] = oxygenIndices[readIdx]
+				writeIdx++
+			}
+		}
+		oxygenIndices = oxygenIndices[:writeIdx]
+	}
+
+	for idx := 0; idx < bitlen && len(co2Indices) > 1; idx++ {
+		ones := 0
+		for _, indice := range co2Indices {
+			if data[indice][idx] == '1' {
+				ones++
+			}
+		}
+		target := byte('0')
+		if ones*2 < len(co2Indices) {
+			target = '1'
+		}
+		writeIdx := 0
+		for readIdx := 0; readIdx < len(co2Indices); readIdx++ {
+			if data[co2Indices[readIdx]][idx] == target {
+				co2Indices[writeIdx] = co2Indices[readIdx]
+				writeIdx++
+			}
+		}
+		co2Indices = co2Indices[:writeIdx]
+	}
+
+	oxygenRating := 0
+	for _, char := range data[oxygenIndices[0]] {
+		oxygenRating <<= 1
+		if char == '1' {
+			oxygenRating |= 1
+		}
+	}
+
+	co2Rating := 0
+	for _, char := range data[co2Indices[0]] {
+		co2Rating <<= 1
+		if char == '1' {
+			co2Rating |= 1
+		}
+	}
+
+	return oxygenRating * co2Rating
+}
+
+func main() {
+	data := parseInput("input.txt")
+	fmt.Println("Part 1:", PartOne(data))
+	fmt.Println("Part 2:", PartTwo(data))
+}