diff --git a/src/converters/ffmpeg.ts b/src/converters/ffmpeg.ts
index 8f263851..47451d42 100644
--- a/src/converters/ffmpeg.ts
+++ b/src/converters/ffmpeg.ts
@@ -1,5 +1,22 @@
-import { execFile as execFileOriginal } from "node:child_process";
-import { ExecFileFn } from "./types";
+import {
+  execFile as execFileOriginal,
+  type ChildProcess,
+  type ExecFileOptions,
+} from "node:child_process";
+
+// ffmpeg streams continuous progress to stderr, so a long conversion overflows
+// execFile's 1 MB default maxBuffer and fails with "stderr maxBuffer length
+// exceeded" (issue #565). Raise it well above that. The options object must be
+// passed before the callback: execFile ignores an options argument placed after
+// the callback, which is why the shared ExecFileFn type cannot carry it.
+const FFMPEG_MAX_BUFFER = 1024 * 1024 * 64; // 64 MB
+
+type FfmpegExecFile = (
+  cmd: string,
+  args: string[],
+  options: ExecFileOptions,
+  callback: (err: Error | null, stdout: string, stderr: string) => void,
+) => ChildProcess | void;
 
 // This could be done dynamically by running `ffmpeg -formats` and parsing the output
 export const properties = {
@@ -694,7 +711,7 @@ export async function convert(
   convertTo: string,
   targetPath: string,
   options?: unknown,
-  execFile: ExecFileFn = execFileOriginal, // to make it mockable
+  execFile: FfmpegExecFile = execFileOriginal as FfmpegExecFile, // to make it mockable
 ): Promise<string> {
   let extraArgs: string[] = [];
   let message = "Done";
@@ -739,6 +756,7 @@ export async function convert(
     execFile(
       "ffmpeg",
       [...ffmpegArgs, "-i", filePath, ...ffmpegOutputArgs, ...extraArgs, targetPath],
+      { maxBuffer: FFMPEG_MAX_BUFFER },
       (error, stdout, stderr) => {
         if (error) {
           reject(`error: ${error}`);
diff --git a/tests/converters/ffmpeg.test.ts b/tests/converters/ffmpeg.test.ts
index 6f0afc60..bd845c31 100644
--- a/tests/converters/ffmpeg.test.ts
+++ b/tests/converters/ffmpeg.test.ts
@@ -1,14 +1,18 @@
 import { beforeEach, expect, test } from "bun:test";
 import { convert } from "../../src/converters/ffmpeg";
+import type { ExecFileOptions } from "node:child_process";
 
 let calls: string[][] = [];
+let lastOptions: ExecFileOptions | undefined;
 
 function mockExecFile(
   _cmd: string,
   args: string[],
+  options: ExecFileOptions,
   callback: (err: Error | null, stdout: string, stderr: string) => void,
 ) {
   calls.push(args);
+  lastOptions = options;
   if (args.includes("fail.mov")) {
     callback(new Error("mock failure"), "", "Fake stderr: fail");
   } else {
@@ -18,6 +22,7 @@ function mockExecFile(
 
 beforeEach(() => {
   calls = [];
+  lastOptions = undefined;
   delete process.env.FFMPEG_ARGS;
 });
 
@@ -168,6 +173,7 @@ test("logs stderr when execFile returns only stderr and no error", async () => {
   const mockExecFileStderrOnly = (
     _cmd: string,
     _args: string[],
+    _options: ExecFileOptions,
     callback: (err: Error | null, stdout: string, stderr: string) => void,
   ) => {
     callback(null, "", "Only stderr output");
@@ -179,3 +185,13 @@ test("logs stderr when execFile returns only stderr and no error", async () => {
 
   expect(loggedMessage).toBe("stderr: Only stderr output");
 });
+
+test("passes a maxBuffer above the 1 MB default so long conversions don't overflow stderr (#565)", async () => {
+  await convert("in.mkv", "mkv", "h264.mp4", "out.mp4", undefined, mockExecFile);
+
+  // execFile's default maxBuffer is 1 MB; ffmpeg's progress output on a long
+  // encode exceeds it and the conversion fails with "stderr maxBuffer length
+  // exceeded". Lock in the raised buffer (must match FFMPEG_MAX_BUFFER in
+  // ffmpeg.ts) so a regression to a smaller-but-still-over-1-MB value is caught.
+  expect(lastOptions?.maxBuffer).toBe(1024 * 1024 * 64);
+});