diff --git a/README.md b/README.md
index 1bf2948..812d272 100644
--- a/README.md
+++ b/README.md
@@ -251,10 +251,12 @@ Virtual disk images (VMDK, QCOW2) have two sizes:
 | **Actual size** | Real data in the image (sparse file) | ~7 GB |
 
 During analysis, the UI shows the virtual size (e.g. *"500.0 GB detected. Omit to
-auto-shrink to 30 GB"*). This is because the source image was created with a
+auto-shrink"*). This is because the source image was created with a
 large virtual disk, but only a fraction is used.
 
-**Target Disk Size field — what happens:**
+**Auto-shrink**: if you leave Target Disk Size blank, the disk shrinks to
+**10% of its virtual size, minimum 20 GB**. A 500 GB disk becomes 50 GB,
+an 80 GB disk becomes 20 GB. You can override this by entering a value.
 
 | You enter | Result |
 |-----------|--------|
diff --git a/backend/provisioner.py b/backend/provisioner.py
index 6504616..80cb50f 100644
--- a/backend/provisioner.py
+++ b/backend/provisioner.py
@@ -283,11 +283,15 @@ def _process_job(job_id: str, req: JobSubmissionRequest):
             current_gb = current_bytes / (1024 ** 3)
 
             target_gb = req.target_disk_size_gb
-            if target_gb is None and current_gb > 30:
-                target_gb = 30
-                logger.info("Auto-shrink: %.1f GiB → %d GiB", current_gb, target_gb)
-            elif target_gb is None:
-                target_gb = int(round(current_gb)) + 1
+            if target_gb is None:
+                # Auto-shrink: 10% of virtual size, minimum 20 GB
+                auto_target = max(int(current_gb * 0.1), 20)
+                if current_gb > auto_target:
+                    target_gb = auto_target
+                    logger.info("Auto-shrink: %.1f GiB → %d GiB (10%% rule)", current_gb, target_gb)
+                else:
+                    target_gb = int(round(current_gb)) + 1
+                    logger.info("Disk already <= target, no shrink needed")
 
             if target_gb < current_gb:
                 _update_job(job_id, progress=60,
diff --git a/frontend/templates/_analysis.html b/frontend/templates/_analysis.html
index 10012e1..7b15b41 100644
--- a/frontend/templates/_analysis.html
+++ b/frontend/templates/_analysis.html
@@ -43,7 +43,7 @@
         <input type="number" name="target_disk_size_gb" class="pve-input"
                value="{{ analysis.disk_size_gb }}"
                placeholder="Leave blank for auto-shrink">
-        <span class="pve-hint">{{ analysis.disk_size_gb }} GB detected. Omit to auto-shrink to 30 GB.</span>
+        <span class="pve-hint">{{ analysis.disk_size_gb }} GB detected. Omit to auto-shrink (10% of size, min 20 GB).</span>
       </div>
     </div>
     <div class="pve-row">