diff --git a/src/components/video-editor/exportDimensions.test.ts b/src/components/video-editor/exportDimensions.test.ts
index 5797daaed..05da6b631 100644
--- a/src/components/video-editor/exportDimensions.test.ts
+++ b/src/components/video-editor/exportDimensions.test.ts
@@ -2,6 +2,7 @@ import { describe, expect, it } from "vitest";
 import {
 	calculateMp4ExportDimensions,
 	calculateMp4SourceDimensions,
+	capExportCanvasDimensions,
 	shouldDebounceMp4SupportProbe,
 } from "./exportDimensions";
 
@@ -57,6 +58,98 @@ describe("calculateMp4SourceDimensions", () => {
 			height: 1080,
 		});
 	});
+
+	it("caps a 5K full-screen capture at 4K UHD for native exports", () => {
+		expect(calculateMp4SourceDimensions(5120, 2880, "native")).toEqual({
+			width: 3840,
+			height: 2160,
+		});
+	});
+
+	it("caps the portrait canvas from a 5K capture at 4K UHD", () => {
+		expect(calculateMp4SourceDimensions(5120, 2880, "9:16")).toEqual({
+			width: 2160,
+			height: 3840,
+		});
+	});
+
+	it("leaves a native crop of a 5K capture alone once it fits the budget", () => {
+		expect(
+			calculateMp4SourceDimensions(5120, 2880, "native", {
+				width: 0.5,
+				height: 1,
+			}),
+		).toEqual({
+			width: 2560,
+			height: 2880,
+		});
+	});
+
+	it("scales a native crop of a 5K capture that still exceeds the budget", () => {
+		expect(
+			calculateMp4SourceDimensions(5120, 2880, "native", {
+				width: 0.8,
+				height: 1,
+			}),
+		).toEqual({
+			width: 3434,
+			height: 2414,
+		});
+	});
+
+	it("leaves a 4K UHD source untouched", () => {
+		expect(calculateMp4SourceDimensions(3840, 2160, "native")).toEqual({
+			width: 3840,
+			height: 2160,
+		});
+		expect(calculateMp4SourceDimensions(3840, 2160, "9:16")).toEqual({
+			width: 2160,
+			height: 3840,
+		});
+	});
+});
+
+describe("capExportCanvasDimensions", () => {
+	it("returns dimensions inside the ceiling unchanged", () => {
+		expect(capExportCanvasDimensions(1920, 1080)).toEqual({ width: 1920, height: 1080 });
+		expect(capExportCanvasDimensions(2160, 3840)).toEqual({ width: 2160, height: 3840 });
+	});
+
+	it("leaves an ultrawide source inside the budget at its own size", () => {
+		expect(capExportCanvasDimensions(5120, 1440)).toEqual({ width: 5120, height: 1440 });
+	});
+
+	it("scales an ultrawide source above the budget by area, keeping its shape", () => {
+		expect(capExportCanvasDimensions(5120, 2160)).toEqual({ width: 4434, height: 1870 });
+	});
+
+	it("scales a 5K source down to the 4K UHD pixel budget", () => {
+		expect(capExportCanvasDimensions(5120, 2880)).toEqual({ width: 3840, height: 2160 });
+		expect(capExportCanvasDimensions(2880, 5120)).toEqual({ width: 2160, height: 3840 });
+	});
+
+	it("never exceeds the budget after rounding to even dimensions", () => {
+		for (const [width, height] of [
+			[5120, 2880],
+			[6016, 3384],
+			[5120, 2160],
+			[7680, 4320],
+			[2880, 5120],
+			[3441, 1441],
+		]) {
+			const capped = capExportCanvasDimensions(width, height);
+			expect(capped.width * capped.height).toBeLessThanOrEqual(3840 * 2160);
+			expect(capped.width % 2).toBe(0);
+			expect(capped.height % 2).toBe(0);
+		}
+	});
+
+	it("honours a custom budget", () => {
+		expect(capExportCanvasDimensions(2880, 5120, 2560 * 1440)).toEqual({
+			width: 1440,
+			height: 2560,
+		});
+	});
 });
 
 describe("calculateMp4ExportDimensions", () => {
@@ -81,6 +174,29 @@ describe("calculateMp4ExportDimensions", () => {
 		});
 	});
 
+	it("scales every tier from the capped canvas on a 5K portrait export", () => {
+		const sourceDimensions = calculateMp4SourceDimensions(5120, 2880, "9:16");
+
+		expect(
+			calculateMp4ExportDimensions(sourceDimensions.width, sourceDimensions.height, "source"),
+		).toEqual({
+			width: 2160,
+			height: 3840,
+		});
+		expect(
+			calculateMp4ExportDimensions(sourceDimensions.width, sourceDimensions.height, "good"),
+		).toEqual({
+			width: 1620,
+			height: 2880,
+		});
+		expect(
+			calculateMp4ExportDimensions(sourceDimensions.width, sourceDimensions.height, "medium"),
+		).toEqual({
+			width: 1296,
+			height: 2304,
+		});
+	});
+
 	it("scales portrait output dimensions from the aspect target", () => {
 		const sourceDimensions = calculateMp4SourceDimensions(1920, 1080, "9:16");
 
diff --git a/src/components/video-editor/exportDimensions.ts b/src/components/video-editor/exportDimensions.ts
index 2f9dcdc23..95dc9a08c 100644
--- a/src/components/video-editor/exportDimensions.ts
+++ b/src/components/video-editor/exportDimensions.ts
@@ -35,6 +35,37 @@ function normalizeEvenDimension(value: number): number {
 	return Math.max(2, Math.floor(value / 2) * 2);
 }
 
+/**
+ * Pixel budget for the export canvas that the quality tiers scale from.
+ *
+ * "Original" follows the source, and a full-screen capture of a 5K panel at backing
+ * scale 2 is 5120 x 2880. For a 9:16 export that produced a 2880 x 5120 canvas:
+ * 14.7 megapixels read back per frame and taller than H.264 hardware encoders accept,
+ * so the export stalled or failed. Capping the canvas at the area of 4K UHD keeps the
+ * largest tier encodable and every lower tier scales from the capped canvas, while a
+ * wide or tall source under that area (an ultrawide, for example) keeps its size.
+ * Sources at or below 4K UHD are unaffected.
+ */
+export const MAX_EXPORT_CANVAS_PIXELS = 3840 * 2160;
+
+export function capExportCanvasDimensions(
+	width: number,
+	height: number,
+	maxPixels: number = MAX_EXPORT_CANVAS_PIXELS,
+): { width: number; height: number } {
+	const pixels = width * height;
+
+	if (!Number.isFinite(pixels) || pixels <= 0 || pixels <= maxPixels) {
+		return { width: normalizeEvenDimension(width), height: normalizeEvenDimension(height) };
+	}
+
+	const scale = Math.sqrt(maxPixels / pixels);
+	return {
+		width: normalizeEvenDimension(width * scale),
+		height: normalizeEvenDimension(height * scale),
+	};
+}
+
 function fitAspectRatioWithinBounds(
 	maxWidth: number,
 	maxHeight: number,
@@ -73,7 +104,7 @@ export function calculateMp4SourceDimensions(
 	const aspectRatioValue = getAspectRatioValue(aspectRatio, sourceAspectRatio);
 
 	if (aspectRatio === "native") {
-		return { width: safeSourceWidth, height: safeSourceHeight };
+		return capExportCanvasDimensions(safeSourceWidth, safeSourceHeight);
 	}
 
 	const longSide = Math.max(safeSourceWidth, safeSourceHeight);
@@ -81,7 +112,8 @@ export function calculateMp4SourceDimensions(
 	const maxWidth = aspectRatioValue >= 1 ? longSide : shortSide;
 	const maxHeight = aspectRatioValue >= 1 ? shortSide : longSide;
 
-	return fitAspectRatioWithinBounds(maxWidth, maxHeight, aspectRatioValue);
+	const fitted = fitAspectRatioWithinBounds(maxWidth, maxHeight, aspectRatioValue);
+	return capExportCanvasDimensions(fitted.width, fitted.height);
 }
 
 export function calculateMp4ExportDimensions(