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Auto Scale to Comp (Smart Fit & Fill Scale Expression)

About This Expression

This expression automatically scales a layer so it perfectly adapts to the composition size. Whether you’re working with images, videos, or precomps, it removes the need to manually resize or guess scale values.

You can choose between:

  • Fit → the entire layer is visible with no cropping
  • Fill → the comp is completely filled, even if some cropping occurs

There’s also a Maintain Ratio switch that lets you decide whether the original aspect ratio should be preserved or intentionally stretched.

This makes the expression ideal for responsive templates, multi-resolution projects, and fast layout work.

Expression

maintainRatio = effect("Maintain Ratio")("Checkbox");
fillMode = effect("Fit / Fill")("Checkbox");
lw = thisLayer.width;
lh = thisLayer.height;
cw = thisComp.width;
ch = thisComp.height;
aspectLayer = lw / lh;
aspectComp = cw / ch;
if (maintainRatio == 1) {
 if (fillMode == 0){
  // FIT (contain without cropping)
  scaleFactor = Math.min(cw / lw, ch / lh);
  [scaleFactor * 100, scaleFactor * 100];
 } else {
 // FILL (cover full comp, may crop)
 scaleFactor = Math.max(cw / lw, ch / lh);
 [scaleFactor * 100, scaleFactor * 100];
 }
} else {
 // No ratio constraint, stretch to fit/fill comp
 if (fillMode == 0) {
  [cw / lw * 100, ch / lh * 100];
  // Fit (stretch)
 } else {
  [cw / lw * 100, ch / lh * 100];
  // Fill (also stretches in this case)
 }
}

How We Use This Expression

Step 1: Select the Layer to Auto-Scale

This expression works on:

  • Images
  • Video layers
  • Precomps
  • Solids or backgrounds

Select the layer you want to automatically scale.

Step 2: Add Expression Controls

On the selected layer, add:

  • Checkbox Control → rename to Maintain Ratio
  • Checkbox Control → rename to Fit / Fill

These controls act as your scaling switches.

Step 3: Apply the Expression to Scale

  1. Press S to reveal Scale
  2. Hold Alt (Windows) / Option (Mac) and click the Scale stopwatch
  3. Paste the full expression
  4. Click outside the editor to apply

The layer will instantly resize based on the comp.

Step 4: Choose Fit or Fill Mode

Using the Fit / Fill checkbox:

  • Unchecked (0) → Fit inside comp (no cropping)
  • Checked (1) → Fill entire comp (may crop edges)

This behaves just like “Fit” and “Fill” options in design tools.

Step 5: Control Aspect Ratio

Using the Maintain Ratio checkbox:

  • Checked → keeps original proportions
  • Unchecked → stretches layer to exactly match comp size

This gives you full creative control over distortion vs accuracy.

Step 6: Resize the Comp Safely

One of the biggest advantages:

  • Change comp resolution
  • Swap footage sizes
  • Replace images

The layer always resizes correctly—no manual fixes required.

Step 7: Use in Templates & Reusable Projects

This expression is perfect for:

  • Social media templates
  • MOGRTs
  • Slideshow projects
  • Client-editable layouts

Clients can replace footage without breaking the design.

Best Use Cases in After Effects

  • Background images & videos
  • Responsive design templates
  • Vertical ↔ horizontal format switching
  • Multi-platform exports
  • Automated layout systems

Why This Expression Is a Must-Have

Manual scaling breaks the moment footage changes. This expression turns scaling into logic instead of guesswork, making projects faster and future-proof.

You get:

  • Automatic resizing
  • Zero keyframes
  • Consistent results
  • Clean, professional layouts

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