diff --git a/lib/image_plug/param_parser/twicpics.ex b/lib/image_plug/param_parser/twicpics.ex
index 1116ef8..b50a29a 100644
--- a/lib/image_plug/param_parser/twicpics.ex
+++ b/lib/image_plug/param_parser/twicpics.ex
@@ -14,6 +14,7 @@ defmodule ImagePlug.ParamParser.Twicpics do
     "contain" => {ImagePlug.Transform.Contain, Twicpics.Transform.ContainParser},
     "contain-min" => {ImagePlug.Transform.Contain, Twicpics.Transform.ContainMinParser},
     "contain-max" => {ImagePlug.Transform.Contain, Twicpics.Transform.ContainMaxParser},
+    "inside" => {ImagePlug.Transform.Contain, Twicpics.Transform.InsideParser},
     "cover" => {ImagePlug.Transform.Cover, Twicpics.Transform.CoverParser},
     "cover-min" => {ImagePlug.Transform.Cover, Twicpics.Transform.CoverMinParser},
     "cover-max" => {ImagePlug.Transform.Cover, Twicpics.Transform.CoverMaxParser},
diff --git a/lib/image_plug/param_parser/twicpics/transform/contain_max_parser.ex b/lib/image_plug/param_parser/twicpics/transform/contain_max_parser.ex
index 964bc8a..9588999 100644
--- a/lib/image_plug/param_parser/twicpics/transform/contain_max_parser.ex
+++ b/lib/image_plug/param_parser/twicpics/transform/contain_max_parser.ex
@@ -13,13 +13,20 @@ defmodule ImagePlug.ParamParser.Twicpics.Transform.ContainMaxParser do
   ## Examples
 
       iex> ImagePlug.ParamParser.Twicpics.Transform.ContainMaxParser.parse("250x25.5")
-      {:ok, %ImagePlug.Transform.Contain.ContainParams{width: {:pixels, 250}, height: {:pixels, 25.5}, constraint: :max}}
+      {:ok, %ImagePlug.Transform.Contain.ContainParams{type: :dimensions, width: {:pixels, 250}, height: {:pixels, 25.5}, constraint: :max, letterbox: false}}
   """
 
   def parse(input, pos_offset \\ 0) do
     case SizeParser.parse(input, pos_offset) do
       {:ok, %{width: width, height: height}} ->
-        {:ok, %ContainParams{width: width, height: height, constraint: :max}}
+        {:ok,
+         %ContainParams{
+           type: :dimensions,
+           width: width,
+           height: height,
+           constraint: :max,
+           letterbox: false
+         }}
 
       {:error, _reason} = error ->
         Utils.update_error_input(error, input)
diff --git a/lib/image_plug/param_parser/twicpics/transform/contain_min_parser.ex b/lib/image_plug/param_parser/twicpics/transform/contain_min_parser.ex
index 7ace043..7bb40b0 100644
--- a/lib/image_plug/param_parser/twicpics/transform/contain_min_parser.ex
+++ b/lib/image_plug/param_parser/twicpics/transform/contain_min_parser.ex
@@ -13,13 +13,20 @@ defmodule ImagePlug.ParamParser.Twicpics.Transform.ContainMinParser do
   ## Examples
 
       iex> ImagePlug.ParamParser.Twicpics.Transform.ContainMinParser.parse("250x25.5")
-      {:ok, %ImagePlug.Transform.Contain.ContainParams{width: {:pixels, 250}, height: {:pixels, 25.5}, constraint: :min}}
+      {:ok, %ImagePlug.Transform.Contain.ContainParams{type: :dimensions, width: {:pixels, 250}, height: {:pixels, 25.5}, constraint: :min, letterbox: false}}
   """
 
   def parse(input, pos_offset \\ 0) do
     case SizeParser.parse(input, pos_offset) do
       {:ok, %{width: width, height: height}} ->
-        {:ok, %ContainParams{width: width, height: height, constraint: :min}}
+        {:ok,
+         %ContainParams{
+           type: :dimensions,
+           width: width,
+           height: height,
+           constraint: :min,
+           letterbox: false
+         }}
 
       {:error, _reason} = error ->
         Utils.update_error_input(error, input)
diff --git a/lib/image_plug/param_parser/twicpics/transform/contain_parser.ex b/lib/image_plug/param_parser/twicpics/transform/contain_parser.ex
index 0c23935..92d6576 100644
--- a/lib/image_plug/param_parser/twicpics/transform/contain_parser.ex
+++ b/lib/image_plug/param_parser/twicpics/transform/contain_parser.ex
@@ -13,13 +13,20 @@ defmodule ImagePlug.ParamParser.Twicpics.Transform.ContainParser do
   ## Examples
 
       iex> ImagePlug.ParamParser.Twicpics.Transform.ContainParser.parse("250x25.5p")
-      {:ok, %ImagePlug.Transform.Contain.ContainParams{width: {:pixels, 250}, height: {:percent, 25.5}, constraint: :none}}
+      {:ok, %ImagePlug.Transform.Contain.ContainParams{type: :dimensions, width: {:pixels, 250}, height: {:percent, 25.5}, constraint: :none, letterbox: false}}
   """
 
   def parse(input, pos_offset \\ 0) do
     case SizeParser.parse(input, pos_offset) do
       {:ok, %{width: width, height: height}} ->
-        {:ok, %ContainParams{width: width, height: height, constraint: :none}}
+        {:ok,
+         %ContainParams{
+           type: :dimensions,
+           width: width,
+           height: height,
+           constraint: :none,
+           letterbox: false
+         }}
 
       {:error, _reason} = error ->
         Utils.update_error_input(error, input)
diff --git a/lib/image_plug/param_parser/twicpics/transform/cover_max_parser.ex b/lib/image_plug/param_parser/twicpics/transform/cover_max_parser.ex
index c22d612..3f22470 100644
--- a/lib/image_plug/param_parser/twicpics/transform/cover_max_parser.ex
+++ b/lib/image_plug/param_parser/twicpics/transform/cover_max_parser.ex
@@ -13,7 +13,7 @@ defmodule ImagePlug.ParamParser.Twicpics.Transform.CoverMaxParser do
   ## Examples
 
       iex> ImagePlug.ParamParser.Twicpics.Transform.CoverMaxParser.parse("250x25.5")
-      {:ok, %ImagePlug.Transform.Cover.CoverParams{width: {:pixels, 250}, height: {:pixels, 25.5}, constraint: :max}}
+      {:ok, %ImagePlug.Transform.Cover.CoverParams{type: :dimensions, width: {:pixels, 250}, height: {:pixels, 25.5}, constraint: :max}}
   """
 
   def parse(input, pos_offset \\ 0) do
diff --git a/lib/image_plug/param_parser/twicpics/transform/cover_min_parser.ex b/lib/image_plug/param_parser/twicpics/transform/cover_min_parser.ex
index 4621d5a..2b26ade 100644
--- a/lib/image_plug/param_parser/twicpics/transform/cover_min_parser.ex
+++ b/lib/image_plug/param_parser/twicpics/transform/cover_min_parser.ex
@@ -13,7 +13,7 @@ defmodule ImagePlug.ParamParser.Twicpics.Transform.CoverMinParser do
   ## Examples
 
       iex> ImagePlug.ParamParser.Twicpics.Transform.CoverMinParser.parse("250x25.5")
-      {:ok, %ImagePlug.Transform.Cover.CoverParams{width: {:pixels, 250}, height: {:pixels, 25.5}, constraint: :min}}
+      {:ok, %ImagePlug.Transform.Cover.CoverParams{type: :dimensions, width: {:pixels, 250}, height: {:pixels, 25.5}, constraint: :min}}
   """
 
   def parse(input, pos_offset \\ 0) do
diff --git a/lib/image_plug/param_parser/twicpics/transform/inside_parser.ex b/lib/image_plug/param_parser/twicpics/transform/inside_parser.ex
new file mode 100644
index 0000000..b4e2b52
--- /dev/null
+++ b/lib/image_plug/param_parser/twicpics/transform/inside_parser.ex
@@ -0,0 +1,55 @@
+defmodule ImagePlug.ParamParser.Twicpics.Transform.InsideParser do
+  alias ImagePlug.ParamParser.Twicpics.SizeParser
+  alias ImagePlug.ParamParser.Twicpics.RatioParser
+  alias ImagePlug.ParamParser.Twicpics.Utils
+  alias ImagePlug.Transform.Contain.ContainParams
+
+  @doc """
+  Parses a string into a `ImagePlug.Transform.Contain.ContainParams` struct.
+
+  Syntax:
+  * `inside=<size>`
+  * `inside=<ratio>`
+
+  ## Examples
+
+  iex> ImagePlug.ParamParser.Twicpics.Transform.InsideParser.parse("250x25.5p")
+  {:ok, %ImagePlug.Transform.Contain.ContainParams{type: :dimensions, width: {:pixels, 250}, height: {:percent, 25.5}, constraint: :none, letterbox: true}}
+
+  iex> ImagePlug.ParamParser.Twicpics.Transform.InsideParser.parse("1.5:2")
+  {:ok, %ImagePlug.Transform.Contain.ContainParams{type: :ratio, ratio: {1.5, 2}, letterbox: true}}
+  """
+
+  def parse(input, pos_offset \\ 0) do
+    if String.contains?(input, ":"),
+      do: parse_ratio(input, pos_offset),
+      else: parse_size(input, pos_offset)
+  end
+
+  defp parse_ratio(input, pos_offset) do
+    case RatioParser.parse(input, pos_offset) do
+      {:ok, %{width: width, height: height}} ->
+        {:ok, %ContainParams{type: :ratio, ratio: {width, height}, letterbox: true}}
+
+      {:error, _reason} = error ->
+        Utils.update_error_input(error, input)
+    end
+  end
+
+  defp parse_size(input, pos_offset) do
+    case SizeParser.parse(input, pos_offset) do
+      {:ok, %{width: width, height: height}} ->
+        {:ok,
+         %ContainParams{
+           type: :dimensions,
+           width: width,
+           height: height,
+           constraint: :none,
+           letterbox: true
+         }}
+
+      {:error, _reason} = error ->
+        Utils.update_error_input(error, input)
+    end
+  end
+end
diff --git a/lib/image_plug/transform/contain.ex b/lib/image_plug/transform/contain.ex
index 10a8792..4ea2089 100644
--- a/lib/image_plug/transform/contain.ex
+++ b/lib/image_plug/transform/contain.ex
@@ -8,32 +8,78 @@ defmodule ImagePlug.Transform.Contain do
   alias ImagePlug.TransformState
 
   defmodule ContainParams do
-    defstruct [:width, :height, :constraint]
+    defstruct [:type, :ratio, :width, :height, :constraint, :letterbox]
 
     @type t ::
             %__MODULE__{
-              width: ImagePlug.imgp_length(),
-              height: ImagePlug.imgp_length() | :auto,
-              constraint: :regular | :min | :max
+              type: :ratio,
+              ratio: ImagePlug.imgp_ratio(),
+              letterbox: boolean()
             }
             | %__MODULE__{
+                type: :dimensions,
+                width: ImagePlug.imgp_length(),
+                height: ImagePlug.imgp_length() | :auto,
+                constraint: :regular | :min | :max,
+                letterbox: boolean()
+              }
+            | %__MODULE__{
+                type: :dimensions,
                 width: ImagePlug.imgp_length() | :auto,
                 height: ImagePlug.imgp_length(),
-                constraint: :regular | :min | :max
+                constraint: :regular | :min | :max,
+                letterbox: boolean()
               }
   end
 
   @impl ImagePlug.Transform
   def execute(%TransformState{} = state, %ContainParams{
+        type: :ratio,
+        ratio: {ratio_width, ratio_height},
+        # Note: Not letterboxing doesn't make sense with this implementation,
+        #       as the transformation would just return the same image
+        letterbox: letterbox
+      }) do
+    # compute target width and height based on the ratio
+    image_width = image_width(state)
+    image_height = image_height(state)
+
+    target_ratio = ratio_width / ratio_height
+    original_ratio = image_width / image_height
+
+    {target_width, target_height} =
+      if original_ratio > target_ratio do
+        # wider image: scale height to match ratio
+        {image_width, round(image_width / target_ratio)}
+      else
+        # taller image: scale width to match ratio
+        {round(image_height * target_ratio), image_height}
+      end
+
+    execute(state, %ContainParams{
+      type: :dimensions,
+      width: target_width,
+      height: target_height,
+      constraint: :none,
+      letterbox: letterbox
+    })
+  end
+
+  @impl ImagePlug.Transform
+  def execute(%TransformState{} = state, %ContainParams{
+        type: :dimensions,
         width: width,
         height: height,
-        constraint: constraint
+        constraint: constraint,
+        letterbox: letterbox
       }) do
     {target_width, target_height} = resolve_auto_size(state, width, height)
     {resize_width, resize_height} = fit_inside(state, target_width, target_height)
 
-    case maybe_scale(state, resize_width, resize_height, constraint) do
-      {:ok, scaled_image} -> state |> set_image(scaled_image) |> reset_focus()
+    with {:ok, state} <- maybe_scale(state, resize_width, resize_height, constraint),
+         {:ok, state} <- maybe_add_letterbox(state, letterbox, target_width, target_height) do
+      state |> reset_focus()
+    else
       {:error, error} -> add_error(state, {__MODULE__, error})
     end
   end
@@ -52,13 +98,13 @@ defmodule ImagePlug.Transform.Contain do
   def maybe_scale(%TransformState{} = state, width, height, :min) do
     if width > image_width(state) or height > image_height(state),
       do: do_scale(state, width, height),
-      else: {:ok, state.image}
+      else: {:ok, state}
   end
 
   def maybe_scale(%TransformState{} = state, width, height, :max) do
     if width < image_width(state) or height < image_height(state),
       do: do_scale(state, width, height),
-      else: {:ok, state.image}
+      else: {:ok, state}
   end
 
   def maybe_scale(%TransformState{} = state, width, height, _constraint),
@@ -67,6 +113,20 @@ defmodule ImagePlug.Transform.Contain do
   def do_scale(%TransformState{} = state, width, height) do
     width_scale = width / image_width(state)
     height_scale = height / image_height(state)
-    Image.resize(state.image, width_scale, vertical_scale: height_scale)
+
+    case Image.resize(state.image, width_scale, vertical_scale: height_scale) do
+      {:ok, resized_image} -> {:ok, set_image(state, resized_image)}
+      {:error, _reason} = error -> error
+    end
+  end
+
+  defp maybe_add_letterbox(state, letterbox?, width, height)
+  defp maybe_add_letterbox(%TransformState{} = state, false, width, height), do: {:ok, state}
+
+  defp maybe_add_letterbox(%TransformState{} = state, true, width, height) do
+    case Image.embed(state.image, width, height, background_color: :white) do
+      {:ok, letterboxed_image} -> {:ok, set_image(state, letterboxed_image)}
+      {:error, _reason} = error -> error
+    end
   end
 end
diff --git a/test/param_parser/twicpics_parser_test.exs b/test/param_parser/twicpics_parser_test.exs
index 8367d7a..0795035 100644
--- a/test/param_parser/twicpics_parser_test.exs
+++ b/test/param_parser/twicpics_parser_test.exs
@@ -155,9 +155,11 @@ defmodule ImagePlug.ParamParser.TwicpicsParserTest do
 
       assert {:ok,
               %Contain.ContainParams{
+                type: :dimensions,
                 width: to_result(width),
                 height: to_result(height),
-                constraint: :none
+                constraint: :none,
+                letterbox: false
               }} ==
                parsed
     end