diff --git a/src/ImGui.NET/ImGui.Manual.cs b/src/ImGui.NET/ImGui.Manual.cs
index ac6dd8b..84926a6 100644
--- a/src/ImGui.NET/ImGui.Manual.cs
+++ b/src/ImGui.NET/ImGui.Manual.cs
@@ -355,42 +355,74 @@ namespace ImGuiNET
             return result != 0;
         }
 
-        public static Vector2 CalcTextSize(
+        public static Vector2 CalcTextSize(string text)
+            => CalcTextSizeImpl(text);
+
+        public static Vector2 CalcTextSize(string text, int start)
+            => CalcTextSizeImpl(text, start);
+
+        public static Vector2 CalcTextSize(string text, float wrapWidth)
+            => CalcTextSizeImpl(text, wrapWidth: wrapWidth);
+
+        public static Vector2 CalcTextSize(string text, bool hideTextAfterDoubleHash)
+            => CalcTextSizeImpl(text, hideTextAfterDoubleHash: hideTextAfterDoubleHash);
+
+        public static Vector2 CalcTextSize(string text, int start, int length)
+            => CalcTextSizeImpl(text, start, length);
+
+        public static Vector2 CalcTextSize(string text, int start, bool hideTextAfterDoubleHash)
+            => CalcTextSizeImpl(text, start, hideTextAfterDoubleHash: hideTextAfterDoubleHash);
+
+        public static Vector2 CalcTextSize(string text, int start, float wrapWidth)
+            => CalcTextSizeImpl(text, start, wrapWidth: wrapWidth);
+
+        public static Vector2 CalcTextSize(string text, bool hideTextAfterDoubleHash, float wrapWidth)
+            => CalcTextSizeImpl(text, hideTextAfterDoubleHash: hideTextAfterDoubleHash, wrapWidth: wrapWidth);
+
+        public static Vector2 CalcTextSize(string text, int start, int length, bool hideTextAfterDoubleHash)
+            => CalcTextSizeImpl(text, start, length, hideTextAfterDoubleHash);
+
+        public static Vector2 CalcTextSize(string text, int start, int length, float wrapWidth)
+            => CalcTextSizeImpl(text, start, length, wrapWidth: wrapWidth);
+
+        public static Vector2 CalcTextSize(string text, int start, int length, bool hideTextAfterDoubleHash, float wrapWidth)
+            => CalcTextSizeImpl(text, start, length, hideTextAfterDoubleHash, wrapWidth);
+
+        private static Vector2 CalcTextSizeImpl(
             string text,
-            int? start = null,
+            int start = 0,
             int? length = null,
             bool hideTextAfterDoubleHash = false,
             float wrapWidth = -1.0f)
         {
             Vector2 ret;
-            byte* nativeText = null;
             byte* nativeTextStart = null;
             byte* nativeTextEnd = null;
             int textByteCount = 0;
-            int textByteSize = 0;
             if (text != null)
             {
-                textByteCount = Encoding.UTF8.GetByteCount(text);
-                textByteSize = Encoding.UTF8.GetByteCount("X");
+
+                int textToCopyLen = length.HasValue ? length.Value : text.Length;
+                textByteCount = Util.CalcUtf8(text, start, textToCopyLen);
                 if (textByteCount > Util.StackAllocationSizeLimit)
                 {
-                    nativeText = Util.Allocate(textByteCount + 1);
+                    nativeTextStart = Util.Allocate(textByteCount + 1);
                 }
                 else
                 {
                     byte* nativeTextStackBytes = stackalloc byte[textByteCount + 1];
-                    nativeText = nativeTextStackBytes;
+                    nativeTextStart = nativeTextStackBytes;
                 }
-                int nativeTextOffset = Util.GetUtf8(text, nativeText, textByteCount);
-                nativeText[nativeTextOffset] = 0;
-                nativeTextStart = nativeText + (start.HasValue ? (start.Value * textByteSize) : 0);
-                nativeTextEnd = length.HasValue ? nativeTextStart + (length.Value * textByteSize) : null;
+
+                int nativeTextOffset = Util.GetUtf8(text, nativeTextStart, textByteCount, start, textToCopyLen);
+                nativeTextStart[nativeTextOffset] = 0;
+                nativeTextEnd = nativeTextStart + nativeTextOffset;
             }
 
             ImGuiNative.igCalcTextSize(&ret, nativeTextStart, nativeTextEnd, *((byte*)(&hideTextAfterDoubleHash)), wrapWidth);
             if (textByteCount > Util.StackAllocationSizeLimit)
             {
-                Util.Free(nativeText);
+                Util.Free(nativeTextStart);
             }
 
             return ret;
diff --git a/src/ImGui.NET/Util.cs b/src/ImGui.NET/Util.cs
index 34280a0..0df92f0 100644
--- a/src/ImGui.NET/Util.cs
+++ b/src/ImGui.NET/Util.cs
@@ -33,6 +33,19 @@ namespace ImGuiNET
 
         internal static byte* Allocate(int byteCount) => (byte*)Marshal.AllocHGlobal(byteCount);
         internal static void Free(byte* ptr) => Marshal.FreeHGlobal((IntPtr)ptr);
+        internal static int CalcUtf8(string s, int start, int length)
+        {
+            if (start > s.Length - 1 || length > s.Length || start + length > s.Length)
+            {
+                throw new ArgumentOutOfRangeException();
+            }
+
+            fixed (char* utf16Ptr = s)
+            {
+                return Encoding.UTF8.GetByteCount(utf16Ptr + start, length);
+            }
+        }
+
         internal static int GetUtf8(string s, byte* utf8Bytes, int utf8ByteCount)
         {
             fixed (char* utf16Ptr = s)
@@ -40,5 +53,13 @@ namespace ImGuiNET
                 return Encoding.UTF8.GetBytes(utf16Ptr, s.Length, utf8Bytes, utf8ByteCount);
             }
         }
+
+        internal static int GetUtf8(string s, byte* utf8Bytes, int utf8ByteCount, int start = 0, int length = 0)
+        {
+            fixed (char* utf16Ptr = s)
+            {
+                return Encoding.UTF8.GetBytes(utf16Ptr + start, length, utf8Bytes, utf8ByteCount);
+            }
+        }
     }
 }