diff --git a/2021/README.md b/2021/README.md
index fb040e3..135e9eb 100644
--- a/2021/README.md
+++ b/2021/README.md
@@ -16,7 +16,7 @@
 | :-: | :-: | :-: | :-: | :-: | :-:  | :-: |
 |     |     |     | [1] | [2] | [3]  | [4] |
 | [5] | [6] | [7] | [8] | [9] | [10] | [11]|
-| [12]| 13  | 14  | 15  | 16  | 17   | 18  |
+| [12]| [13]| 14  | 15  | 16  | 17   | 18  |
 | 19  | 20  | 21  | 22  | 23  | 24   | 25  |
 
 <!-- links -->
@@ -33,3 +33,4 @@
 [10]: ./lib/2021/10.ex
 [11]: ./lib/2021/11.ex
 [12]: ./lib/2021/12.ex
+[13]: ./lib/2021/13.ex
diff --git a/2021/lib/2021/13.ex b/2021/lib/2021/13.ex
new file mode 100644
index 0000000..44dd411
--- /dev/null
+++ b/2021/lib/2021/13.ex
@@ -0,0 +1,73 @@
+import AOC
+
+aoc 2021, 13 do
+  def input(input_string \\ input_string()) do
+    [dots_string, folds_string] =
+      input_string
+      |> String.split("\n\n", trim: true)
+
+    dots =
+      dots_string
+      |> String.split("\n", trim: true)
+      |> Enum.map(fn line ->
+        [x, y] =
+          String.split(line, ",")
+          |> Enum.map(&String.to_integer/1)
+
+        {x, y}
+      end)
+      |> MapSet.new()
+
+    folds =
+      folds_string
+      |> String.split("\n", trim: true)
+      |> Enum.map(fn
+        "fold along y=" <> y -> {:y, String.to_integer(y)}
+        "fold along x=" <> x -> {:x, String.to_integer(x)}
+      end)
+
+    {dots, folds}
+  end
+
+  def do_fold({:y, line}, dots) do
+    for {x, y} <- dots, into: MapSet.new() do
+      if y > line do
+        {x, line - (y - line)}
+      else
+        {x, y}
+      end
+    end
+  end
+
+  def do_fold({:x, line}, dots) do
+    for {x, y} <- dots, into: MapSet.new() do
+      if x > line do
+        {line - (x - line), y}
+      else
+        {x, y}
+      end
+    end
+  end
+
+  def p1 do
+    {dots, [fold | _folds]} = input()
+
+    do_fold(fold, dots) |> MapSet.size()
+  end
+
+  def p2 do
+    {dots, folds} = input()
+
+    final_dots = Enum.reduce(folds, dots, &do_fold/2)
+
+    max_x = final_dots |> Enum.map(&elem(&1, 0)) |> Enum.max()
+    max_y = final_dots |> Enum.map(&elem(&1, 1)) |> Enum.max()
+
+    for x <- 0..max_x, into: "" do
+      for y <- 0..max_y, into: "" do
+        if MapSet.member?(final_dots, {x, y}), do: "#", else: "."
+      end <> "\n"
+    end
+    |> IO.puts()
+  end
+end