diff --git a/Sources/Stag/Capture/ThumbnailGeometry.swift b/Sources/Stag/Capture/ThumbnailGeometry.swift
new file mode 100644
index 0000000..673f18b
--- /dev/null
+++ b/Sources/Stag/Capture/ThumbnailGeometry.swift
@@ -0,0 +1,14 @@
+import CoreGraphics
+
+/// Pure sizing math for capture thumbnails, lifted out of CaptureManager so the
+/// aspect-fit rule can be tested without any AppKit drawing.
+enum ThumbnailGeometry {
+    /// Scales `size` down to fit within a `maxDimension` box (applied to each
+    /// side), preserving aspect ratio. Never upscales. Returns `.zero` for
+    /// non-positive input so callers can skip drawing.
+    static func fittedSize(for size: CGSize, maxDimension: CGFloat) -> CGSize {
+        guard size.width > 0, size.height > 0 else { return .zero }
+        let scale = min(maxDimension / size.width, maxDimension / size.height, 1.0)
+        return CGSize(width: size.width * scale, height: size.height * scale)
+    }
+}
diff --git a/Sources/Stag/CaptureManager.swift b/Sources/Stag/CaptureManager.swift
index b9d1aad..6092014 100644
--- a/Sources/Stag/CaptureManager.swift
+++ b/Sources/Stag/CaptureManager.swift
@@ -401,11 +401,8 @@ final class CaptureManager {
     }
 
     private func saveJPEGThumbnail(_ image: NSImage, to url: URL) {
-        let maxDim: CGFloat = 320
-        let w = image.size.width, h = image.size.height
-        guard w > 0, h > 0 else { return }
-        let scale = min(maxDim / w, maxDim / h, 1.0)
-        let thumbSize = CGSize(width: w * scale, height: h * scale)
+        let thumbSize = ThumbnailGeometry.fittedSize(for: image.size, maxDimension: 320)
+        guard thumbSize.width > 0 else { return }
         let thumb = NSImage(size: thumbSize)
         thumb.lockFocus()
         image.draw(in: NSRect(origin: .zero, size: thumbSize),
diff --git a/Tests/StagTests/ThumbnailGeometryTests.swift b/Tests/StagTests/ThumbnailGeometryTests.swift
new file mode 100644
index 0000000..1cd32c8
--- /dev/null
+++ b/Tests/StagTests/ThumbnailGeometryTests.swift
@@ -0,0 +1,43 @@
+import XCTest
+import CoreGraphics
+@testable import Stag
+
+/// Aspect-fit sizing extracted from CaptureManager.saveJPEGThumbnail.
+final class ThumbnailGeometryTests: XCTestCase {
+
+    func testLandscapeFitsToWidth() {
+        let s = ThumbnailGeometry.fittedSize(for: CGSize(width: 640, height: 320), maxDimension: 320)
+        XCTAssertEqual(s.width, 320, accuracy: 0.001)
+        XCTAssertEqual(s.height, 160, accuracy: 0.001)
+    }
+
+    func testPortraitFitsToHeight() {
+        let s = ThumbnailGeometry.fittedSize(for: CGSize(width: 320, height: 640), maxDimension: 320)
+        XCTAssertEqual(s.width, 160, accuracy: 0.001)
+        XCTAssertEqual(s.height, 320, accuracy: 0.001)
+    }
+
+    func testNeverUpscalesSmallImage() {
+        let input = CGSize(width: 100, height: 80)
+        let s = ThumbnailGeometry.fittedSize(for: input, maxDimension: 320)
+        XCTAssertEqual(s, input)
+    }
+
+    func testAspectRatioPreserved() {
+        let s = ThumbnailGeometry.fittedSize(for: CGSize(width: 1000, height: 250), maxDimension: 320)
+        XCTAssertEqual(s.width / s.height, 4.0, accuracy: 0.001)
+        XCTAssertLessThanOrEqual(s.width, 320)
+        XCTAssertLessThanOrEqual(s.height, 320)
+    }
+
+    func testZeroAndNegativeReturnZero() {
+        XCTAssertEqual(ThumbnailGeometry.fittedSize(for: .zero, maxDimension: 320), .zero)
+        XCTAssertEqual(ThumbnailGeometry.fittedSize(for: CGSize(width: -10, height: 50), maxDimension: 320), .zero)
+    }
+
+    func testSquareScalesUniformly() {
+        let s = ThumbnailGeometry.fittedSize(for: CGSize(width: 800, height: 800), maxDimension: 320)
+        XCTAssertEqual(s.width, 320, accuracy: 0.001)
+        XCTAssertEqual(s.height, 320, accuracy: 0.001)
+    }
+}