diff --git a/ShareX.ScreenCaptureLib/Forms/ImageSizeForm.Designer.cs b/ShareX.ScreenCaptureLib/Forms/ImageSizeForm.Designer.cs
index 8b5f0054a..04c9e8477 100644
--- a/ShareX.ScreenCaptureLib/Forms/ImageSizeForm.Designer.cs
+++ b/ShareX.ScreenCaptureLib/Forms/ImageSizeForm.Designer.cs
@@ -51,7 +51,6 @@ private void InitializeComponent()
             0,
             0});
             this.nudWidth.Name = "nudWidth";
-            this.nudWidth.ValueChanged += new System.EventHandler(this.nudWidth_ValueChanged);
             // 
             // nudHeight
             // 
@@ -62,7 +61,6 @@ private void InitializeComponent()
             0,
             0});
             this.nudHeight.Name = "nudHeight";
-            this.nudHeight.ValueChanged += new System.EventHandler(this.nudHeight_ValueChanged);
             // 
             // lblWidth
             // 
@@ -77,8 +75,11 @@ private void InitializeComponent()
             // cbAspectRatio
             // 
             resources.ApplyResources(this.cbAspectRatio, "cbAspectRatio");
+            this.cbAspectRatio.Checked = true;
+            this.cbAspectRatio.CheckState = System.Windows.Forms.CheckState.Checked;
             this.cbAspectRatio.Name = "cbAspectRatio";
             this.cbAspectRatio.UseVisualStyleBackColor = true;
+            this.cbAspectRatio.CheckedChanged += new System.EventHandler(this.cbAspectRatio_CheckedChanged);
             // 
             // btnOK
             // 
diff --git a/ShareX.ScreenCaptureLib/Forms/ImageSizeForm.cs b/ShareX.ScreenCaptureLib/Forms/ImageSizeForm.cs
index 7d5e23aae..c6e08e49f 100644
--- a/ShareX.ScreenCaptureLib/Forms/ImageSizeForm.cs
+++ b/ShareX.ScreenCaptureLib/Forms/ImageSizeForm.cs
@@ -39,17 +39,26 @@ public partial class ImageSizeForm : Form
     {
         public Size Result { get; private set; }
 
-        public ImageSizeForm()
+        private double widthRatio, heightRatio;
+        private bool ignoreValueChanged = true;
+
+        public ImageSizeForm(Size size)
         {
             InitializeComponent();
             Icon = ShareXResources.Icon;
-        }
 
-        public ImageSizeForm(Size size) : this()
-        {
             Result = size;
+            widthRatio = (double)size.Width / size.Height;
+            heightRatio = (double)size.Height / size.Width;
+
             nudWidth.SetValue(size.Width);
             nudHeight.SetValue(size.Height);
+            VerifySize();
+
+            nudWidth.TextChanged += NudWidth_TextChanged;
+            nudHeight.TextChanged += NudHeight_TextChanged;
+
+            ignoreValueChanged = false;
         }
 
         private void VerifySize()
@@ -57,19 +66,54 @@ private void VerifySize()
             btnOK.Enabled = nudWidth.Value > 0 && nudHeight.Value > 0;
         }
 
+        private void ApplyWidthAspectRatio()
+        {
+            if (!ignoreValueChanged)
+            {
+                if (cbAspectRatio.Checked)
+                {
+                    ignoreValueChanged = true;
+                    nudHeight.Value = (int)Math.Round((double)nudWidth.Value * heightRatio);
+                    ignoreValueChanged = false;
+                }
+
+                VerifySize();
+            }
+        }
+
+        private void ApplyHeightAspectRatio()
+        {
+            if (!ignoreValueChanged)
+            {
+                if (cbAspectRatio.Checked)
+                {
+                    ignoreValueChanged = true;
+                    nudWidth.Value = (int)Math.Round((double)nudHeight.Value * widthRatio);
+                    ignoreValueChanged = false;
+                }
+
+                VerifySize();
+            }
+        }
+
         private void ResizeSizeForm_Shown(object sender, EventArgs e)
         {
             this.ForceActivate();
         }
 
-        private void nudWidth_ValueChanged(object sender, EventArgs e)
+        private void NudWidth_TextChanged(object sender, EventArgs e)
         {
-            VerifySize();
+            ApplyWidthAspectRatio();
         }
 
-        private void nudHeight_ValueChanged(object sender, EventArgs e)
+        private void NudHeight_TextChanged(object sender, EventArgs e)
         {
-            VerifySize();
+            ApplyHeightAspectRatio();
+        }
+
+        private void cbAspectRatio_CheckedChanged(object sender, EventArgs e)
+        {
+            ApplyWidthAspectRatio();
         }
 
         private void btnOK_Click(object sender, EventArgs e)