diff --git a/src/ImageSharp/Formats/Heif/Av1/Av1BlockSize.cs b/src/ImageSharp/Formats/Heif/Av1/Av1BlockSize.cs
index 5a524c261d..92a651d60f 100644
--- a/src/ImageSharp/Formats/Heif/Av1/Av1BlockSize.cs
+++ b/src/ImageSharp/Formats/Heif/Av1/Av1BlockSize.cs
@@ -3,7 +3,7 @@
 
 namespace SixLabors.ImageSharp.Formats.Heif.Av1;
 
-internal enum Av1BlockSize
+internal enum Av1BlockSize : byte
 {
     // See sction 6.10.4 of the Av1 Specification.
 
diff --git a/src/ImageSharp/Formats/Heif/Av1/Av1LookupTables.cs b/src/ImageSharp/Formats/Heif/Av1/Av1LookupTables.cs
new file mode 100644
index 0000000000..a7e9ab7998
--- /dev/null
+++ b/src/ImageSharp/Formats/Heif/Av1/Av1LookupTables.cs
@@ -0,0 +1,34 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Heif.Av1;
+
+internal static class Av1LookupTables
+{
+    // The Subsampled_Size table in the spec (Section 5.11.38. Get plane residual size function).
+    public static Av1BlockSize[,,] SubSampledSize =
+    {
+        { { Av1BlockSize.Block4x4,      Av1BlockSize.Block4x4 },    { Av1BlockSize.Block4x4,    Av1BlockSize.Block4x4 } },
+        { { Av1BlockSize.Block4x8,      Av1BlockSize.Block4x4 },    { Av1BlockSize.Invalid,     Av1BlockSize.Block4x4 } },
+        { { Av1BlockSize.Block8x4,      Av1BlockSize.Invalid },     { Av1BlockSize.Block4x4,    Av1BlockSize.Block4x4 } },
+        { { Av1BlockSize.Block8x8,      Av1BlockSize.Block8x4 },    { Av1BlockSize.Block4x8,    Av1BlockSize.Block4x4 } },
+        { { Av1BlockSize.Block8x16,     Av1BlockSize.Block8x8 },    { Av1BlockSize.Invalid,     Av1BlockSize.Block4x8 } },
+        { { Av1BlockSize.Block16x8,     Av1BlockSize.Invalid },     { Av1BlockSize.Block8x8,    Av1BlockSize.Block8x4 } },
+        { { Av1BlockSize.Block16x16,    Av1BlockSize.Block16x8 },   { Av1BlockSize.Block8x16,   Av1BlockSize.Block8x8 } },
+        { { Av1BlockSize.Block16x32,    Av1BlockSize.Block16x16 },  { Av1BlockSize.Invalid,     Av1BlockSize.Block8x16 } },
+        { { Av1BlockSize.Block32x16,    Av1BlockSize.Invalid },     { Av1BlockSize.Block16x16,  Av1BlockSize.Block16x8 } },
+        { { Av1BlockSize.Block32x32,    Av1BlockSize.Block32x16 },  { Av1BlockSize.Block16x32,  Av1BlockSize.Block16x16 } },
+        { { Av1BlockSize.Block32x64,    Av1BlockSize.Block32x32 },  { Av1BlockSize.Invalid,     Av1BlockSize.Block16x32 } },
+        { { Av1BlockSize.Block64x32,    Av1BlockSize.Invalid },     { Av1BlockSize.Block32x32,  Av1BlockSize.Block32x16 } },
+        { { Av1BlockSize.Block64x64,    Av1BlockSize.Block64x32 },  { Av1BlockSize.Block32x64,  Av1BlockSize.Block32x32 } },
+        { { Av1BlockSize.Block64x128,   Av1BlockSize.Block64x64 },  { Av1BlockSize.Invalid,     Av1BlockSize.Block32x64 } },
+        { { Av1BlockSize.Block128x64,   Av1BlockSize.Invalid },     { Av1BlockSize.Block64x64,  Av1BlockSize.Block64x32 } },
+        { { Av1BlockSize.Block128x128,  Av1BlockSize.Block128x64 }, { Av1BlockSize.Block64x128, Av1BlockSize.Block64x64 } },
+        { { Av1BlockSize.Block4x16,     Av1BlockSize.Block4x8 },    { Av1BlockSize.Invalid,     Av1BlockSize.Block4x8 } },
+        { { Av1BlockSize.Block16x4,     Av1BlockSize.Invalid },     { Av1BlockSize.Block8x4,    Av1BlockSize.Block8x4 } },
+        { { Av1BlockSize.Block8x32,     Av1BlockSize.Block8x16 },   { Av1BlockSize.Invalid,     Av1BlockSize.Block4x16 } },
+        { { Av1BlockSize.Block32x8,     Av1BlockSize.Invalid },     { Av1BlockSize.Block16x8,   Av1BlockSize.Block16x4 } },
+        { { Av1BlockSize.Block16x64,    Av1BlockSize.Block16x32 },  { Av1BlockSize.Invalid,     Av1BlockSize.Block8x32 } },
+        { { Av1BlockSize.Block64x16,    Av1BlockSize.Invalid },     { Av1BlockSize.Block32x16,  Av1BlockSize.Block32x8 } },
+    };
+}
diff --git a/src/ImageSharp/Formats/Heif/Av1/Tiling/Av1TileDecoder.cs b/src/ImageSharp/Formats/Heif/Av1/Tiling/Av1TileDecoder.cs
index c286b34b6a..4308ff3326 100644
--- a/src/ImageSharp/Formats/Heif/Av1/Tiling/Av1TileDecoder.cs
+++ b/src/ImageSharp/Formats/Heif/Av1/Tiling/Av1TileDecoder.cs
@@ -451,7 +451,19 @@ internal class Av1TileDecoder : IAv1TileDecoder
 
     private Av1TransformSize GetSize(int plane, object transformSize) => throw new NotImplementedException();
 
-    private Av1BlockSize GetPlaneResidualSize(Av1BlockSize sizeChunk, int plane) => throw new NotImplementedException();
+    /// <summary>
+    /// 5.11.38. Get plane residual size function.
+    /// The GetPlaneResidualSize returns the size of a residual block for the specified plane. (The residual block will always
+    /// have width and height at least equal to 4.)
+    /// </summary>
+    private Av1BlockSize GetPlaneResidualSize(Av1BlockSize sizeChunk, int plane)
+    {
+        int subsamplingX = this.SequenceHeader.ColorConfig.SubSamplingX ? 1 : 0;
+        int subsamplingY = this.SequenceHeader.ColorConfig.SubSamplingY ? 1 : 0;
+        int subX = plane > 0 ? subsamplingX : 0;
+        int subY = plane > 0 ? subsamplingY : 0;
+        return Av1LookupTables.SubSampledSize[(byte)sizeChunk, subX, subY];
+    }
 
     /// <summary>
     /// 5.11.35. Transform block syntax.