diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
index faa1752..0388be6 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/AdventOfCode2023.FSharp.Lib.fsproj
@@ -23,6 +23,7 @@
     <Compile Include="Day10.fs" />
     <Compile Include="Day11.fs" />
     <Compile Include="Day12.fs" />
+    <Compile Include="Day13.fs" />
   </ItemGroup>
 
 </Project>
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day13.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day13.fs
new file mode 100644
index 0000000..9728f54
--- /dev/null
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day13.fs
@@ -0,0 +1,123 @@
+namespace AdventOfCode2023
+
+open System
+
+[<RequireQualifiedAccess>]
+module Day13 =
+
+    let rowToInt (row : ReadOnlySpan<char>) : uint32 =
+        let mutable mult = 1ul
+        let mutable answer = 0ul
+
+        for c = row.Length - 1 downto 0 do
+            if row.[c] = '#' then
+                answer <- answer + mult
+
+            mult <- mult * 2ul
+
+        answer
+
+    let colToInt (grid : ReadOnlySpan<char>) (rowLength : int) (colNum : int) =
+        let mutable mult = 1ul
+        let mutable answer = 0ul
+
+        for i = grid.Count '\n' - 1 downto 0 do
+            if grid.[i * (rowLength + 1) + colNum] = '#' then
+                answer <- answer + mult
+
+            mult <- mult * 2ul
+
+        answer
+
+    let verifyHorizontalReflection (group : ResizeArray<'a>) (smaller : int) (bigger : int) : bool =
+        let midPoint = (smaller + bigger) / 2
+
+        let rec isOkWithin (curr : int) =
+            if smaller + curr > midPoint then
+                true
+            else if group.[smaller + curr] = group.[bigger - curr] then
+                isOkWithin (curr + 1)
+            else
+                false
+
+        if not (isOkWithin 0) then
+            false
+        else
+
+        smaller = 0 || bigger = group.Count - 1
+
+    /// Find reflection among rows
+    [<TailCall>]
+    let rec findRow (rows : ResizeArray<'a>) (currentLine : int) =
+        if currentLine = rows.Count - 1 then
+            None
+        else
+            let mutable answer = UInt32.MaxValue
+            let mutable i = currentLine
+
+            while i < rows.Count - 1 do
+                i <- i + 1
+
+                if currentLine % 2 <> i % 2 then
+                    if rows.[i] = rows.[currentLine] then
+                        if verifyHorizontalReflection rows currentLine i then
+                            answer <- uint32 (((currentLine + i) / 2) + 1)
+                            i <- Int32.MaxValue
+
+            if answer < UInt32.MaxValue then
+                Some answer
+            else
+                findRow rows (currentLine + 1)
+
+    let part1 (s : string) =
+        let mutable s = s.AsSpan ()
+        use lines = StringSplitEnumerator.make' '\n' s
+        let rows = ResizeArray ()
+        let cols = ResizeArray ()
+        let mutable answer = 0ul
+
+        while not s.IsEmpty do
+            rows.Clear ()
+            cols.Clear ()
+            let index = s.IndexOf "\n\n"
+
+            let group =
+                if index < 0 then
+                    // last group
+                    s
+                else
+                    s.Slice (0, index + 1)
+
+            let lineLength = s.IndexOf '\n'
+            cols.EnsureCapacity lineLength |> ignore
+
+            for col = 0 to lineLength - 1 do
+                cols.Add (colToInt group lineLength col)
+
+            for row in StringSplitEnumerator.make' '\n' group do
+                if not row.IsEmpty then
+                    rows.Add (rowToInt row)
+
+            match findRow rows 0 with
+            | Some rowIndex -> answer <- answer + 100ul * rowIndex
+            | None ->
+                let colIndex = Option.get (findRow cols 0)
+                answer <- answer + colIndex
+
+            if index < 0 then
+                s <- ReadOnlySpan<char>.Empty
+            else
+                s <- s.Slice (index + 2)
+
+        answer
+
+    let part2 (s : string) =
+        use mutable lines = StringSplitEnumerator.make '\n' s
+
+        let mutable answer = 0uL
+
+        for line in lines do
+            if not line.IsEmpty then
+                ()
+
+        answer
diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs
index 25640b4..a41eec9 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp/Program.fs
@@ -252,6 +252,22 @@ module Program =
             Console.WriteLine (part2.ToString ())
             Console.Error.WriteLine ((1_000.0 * float sw.ElapsedTicks / float Stopwatch.Frequency).ToString () + "ms")
 
+        Console.WriteLine "=====Day 13====="
+
+        do
+            let input = Path.Combine (dir.FullName, "day13.txt") |> File.ReadAllText
+
+            sw.Restart ()
+            let part1 = Day13.part1 input
+            sw.Stop ()
+            Console.WriteLine (part1.ToString ())
+            Console.Error.WriteLine ((1_000.0 * float sw.ElapsedTicks / float Stopwatch.Frequency).ToString () + "ms")
+            sw.Restart ()
+            let part2 = Day13.part2 input
+            sw.Stop ()
+            Console.WriteLine (part2.ToString ())
+            Console.Error.WriteLine ((1_000.0 * float sw.ElapsedTicks / float Stopwatch.Frequency).ToString () + "ms")
+
         endToEnd.Stop ()
 
         Console.Error.WriteLine (
diff --git a/AdventOfCode2023.FSharp/Test/Test.fsproj b/AdventOfCode2023.FSharp/Test/Test.fsproj
index 5af2d07..18f4264 100644
--- a/AdventOfCode2023.FSharp/Test/Test.fsproj
+++ b/AdventOfCode2023.FSharp/Test/Test.fsproj
@@ -21,6 +21,7 @@
     <Compile Include="TestDay10.fs" />
     <Compile Include="TestDay11.fs" />
     <Compile Include="TestDay12.fs" />
+    <Compile Include="TestDay13.fs" />
     <EmbeddedResource Include="samples\day1.txt"/>
     <EmbeddedResource Include="samples\day1part1.txt"/>
     <EmbeddedResource Include="samples\day2.txt"/>
@@ -36,6 +37,7 @@
     <EmbeddedResource Include="samples\day10.txt" />
     <EmbeddedResource Include="samples\day11.txt" />
     <EmbeddedResource Include="samples\day12.txt" />
+    <EmbeddedResource Include="samples\day13.txt" />
   </ItemGroup>
 
   <ItemGroup>
diff --git a/AdventOfCode2023.FSharp/Test/TestDay13.fs b/AdventOfCode2023.FSharp/Test/TestDay13.fs
new file mode 100644
index 0000000..1a6fb47
--- /dev/null
+++ b/AdventOfCode2023.FSharp/Test/TestDay13.fs
@@ -0,0 +1,68 @@
+namespace AdventOfCode2023.Test
+
+open System
+
+open AdventOfCode2023
+open NUnit.Framework
+open FsUnitTyped
+open System.IO
+
+[<TestFixture>]
+module TestDay13 =
+
+    [<Test>]
+    let ``rowToInt test`` () =
+        Day13.rowToInt ("#.##..##.".AsSpan ()) |> shouldEqual 358ul
+
+    [<Test>]
+    let ``colToInt test`` () =
+        let s =
+            """#.##..##.
+..#.##.#.
+##......#
+##......#
+..#.##.#.
+..##..##.
+#.#.##.#.
+"""
+
+        Day13.colToInt (s.AsSpan ()) 9 0
+        |> shouldEqual (List.sum [ 1 ; 8 ; 16 ; 64 ] |> uint32)
+
+    [<Test>]
+
+    let sample = Assembly.getEmbeddedResource typeof<Dummy>.Assembly "day13.txt"
+
+    [<Test>]
+    let part1Sample () =
+        sample |> Day13.part1 |> shouldEqual 405ul
+
+    [<Test>]
+    let part2Sample () =
+        sample |> Day13.part2 |> shouldEqual 525152uL
+
+    [<Test>]
+    let part1Actual () =
+        let s =
+            try
+                File.ReadAllText (Path.Combine (__SOURCE_DIRECTORY__, "../../inputs/day13.txt"))
+            with
+            | :? DirectoryNotFoundException
+            | :? FileNotFoundException ->
+                Assert.Inconclusive ()
+                failwith "unreachable"
+
+        Day13.part1 s |> shouldEqual 30158ul
+
+    [<Test>]
+    let part2Actual () =
+        let s =
+            try
+                File.ReadAllText (Path.Combine (__SOURCE_DIRECTORY__, "../../inputs/day13.txt"))
+            with
+            | :? DirectoryNotFoundException
+            | :? FileNotFoundException ->
+                Assert.Inconclusive ()
+                failwith "unreachable"
+
+        Day13.part2 s |> shouldEqual 3384337640277uL
diff --git a/AdventOfCode2023.FSharp/Test/samples/day13.txt b/AdventOfCode2023.FSharp/Test/samples/day13.txt
new file mode 100644
index 0000000..3b6b5cc
--- /dev/null
+++ b/AdventOfCode2023.FSharp/Test/samples/day13.txt
@@ -0,0 +1,15 @@
+#.##..##.
+..#.##.#.
+##......#
+##......#
+..#.##.#.
+..##..##.
+#.#.##.#.
+
+#...##..#
+#....#..#
+..##..###
+#####.##.
+#####.##.
+..##..###
+#....#..#