diff --git a/2021/03/part1.mjs b/2021/03/part1.mjs
new file mode 100644
index 0000000..e34d64d
--- /dev/null
+++ b/2021/03/part1.mjs
@@ -0,0 +1,25 @@
+import fs from "fs-extra";
+import fp from "lodash/fp.js";
+import _ from "lodash";
+
+export const processFile = async (f) =>
+    fs
+        .readFile(f, "utf8")
+        .then((x) => x.split("\n").filter((x) => x))
+        .then((lines) =>
+            lines.map((l) => l.split("").map((x) => parseInt(x)))
+        );
+
+const mostCommon = (numbers) => (_.sum(numbers) * 2 > numbers.length ? 1 : 0);
+
+export const toDecimal = (numbers) => numbers.reduce((a, x) => a * 2 + x, 0);
+
+export const solution = (entries) => {
+    const groups = _.zip(...entries).map(mostCommon);
+
+    const gamma = toDecimal(groups);
+
+    const epsilon = toDecimal(groups.map((x) => 1 - x));
+
+    return gamma * epsilon;
+};
diff --git a/2021/03/part2.mjs b/2021/03/part2.mjs
new file mode 100644
index 0000000..6419a66
--- /dev/null
+++ b/2021/03/part2.mjs
@@ -0,0 +1,27 @@
+import fp from "lodash/fp.js";
+import _ from "lodash";
+import u from "updeep";
+
+import { toDecimal } from "./part1.mjs";
+
+function findNumber(entries, index, most) {
+    // which number to filter on
+    const mostSeen =
+        _.sum(entries.map((e) => e[index])) * 2 >= entries.length ? 1 : 0;
+
+    const filter = most ? mostSeen : 1 - mostSeen;
+
+    const filtered = entries.filter((e) => e[index] === filter);
+
+    if (filtered.length === 1) return filtered[0];
+
+    return findNumber(filtered, index + 1, most);
+}
+
+export const solution = (entries) => {
+    const oxyGenerator = toDecimal(findNumber(entries, 0, true));
+
+    const scrubber = toDecimal(findNumber(entries, 0, false));
+
+    return oxyGenerator * scrubber;
+};
diff --git a/2021/03/sample b/2021/03/sample
new file mode 100644
index 0000000..a6366a8
--- /dev/null
+++ b/2021/03/sample
@@ -0,0 +1,12 @@
+00100
+11110
+10110
+10111
+10101
+01111
+00111
+11100
+10000
+11001
+00010
+01010
diff --git a/2021/03/test.mjs b/2021/03/test.mjs
new file mode 100644
index 0000000..e9099ee
--- /dev/null
+++ b/2021/03/test.mjs
@@ -0,0 +1,18 @@
+import tap from "tap";
+
+import * as p1 from "./part1.mjs";
+import * as p2 from "./part2.mjs";
+
+
+const sample = p1.processFile("sample");
+const input = p1.processFile("input");
+
+tap.test("part1", async (t) => {
+    t.equal(p1.solution(await sample), 198);
+    t.equal(p1.solution(await input), 3813416 );
+});
+
+tap.test("part2", async (t) => {
+    t.equal(p2.solution(await sample), 230);
+    t.equal(p2.solution(await input), 2990784);
+});