diff --git a/crates/engine/src/media.rs b/crates/engine/src/media.rs
index 95a81a4b2..d5c8046bd 100644
--- a/crates/engine/src/media.rs
+++ b/crates/engine/src/media.rs
@@ -1256,6 +1256,7 @@ impl crate::Session {
         space: dac_pipeline::OutputSpace,
         depth: dac_pipeline::OutputDepth,
     ) -> Result<RenderJob, String> {
+        dac_pipeline::check_output_size(max_w, max_h)?;
         let mut job = self.render_job(id, max_w, max_h, false, true).ok_or("no such photo")?;
         // An export is made from the denoised picture its Denoise amount asks for, made when the job runs if the
         // background queue has not got to it. (A source kept in memory may lack the picture, so the job reads the file.)
diff --git a/crates/engine/src/tests_previews.rs b/crates/engine/src/tests_previews.rs
index cbc86ef34..7130077dd 100644
--- a/crates/engine/src/tests_previews.rs
+++ b/crates/engine/src/tests_previews.rs
@@ -183,3 +183,13 @@ fn pan_at_one_to_one_on_45_megapixels() {
         );
     }
 }
+
+/// An absurd export size is refused, not a multiply overflow (it panicked before the fix).
+#[test]
+fn huge_export_size_is_an_error() {
+    let mut s = Session::with_demo();
+    let id = s.catalog.photos().next().unwrap().id;
+    let huge = usize::MAX >> 8;
+    assert!(s.render_export(id, huge, huge, OutputSpace::Srgb, OutputDepth::U8).is_err());
+    assert!(s.render_export(id, 70_000, 10, OutputSpace::Srgb, OutputDepth::U8).is_err());
+}
diff --git a/crates/gpu/src/render.rs b/crates/gpu/src/render.rs
index ff6e23a30..3e601ae04 100644
--- a/crates/gpu/src/render.rs
+++ b/crates/gpu/src/render.rs
@@ -564,7 +564,10 @@ pub fn render(
     };
     let plan = dac_pipeline::plan(src, info, s, req);
     let (w, h) = (plan.w, plan.h);
-    let n = w * h;
+    let Some(n) = w.checked_mul(h).filter(|n| n.checked_mul(12).is_some()) else {
+        fail(FailKind::Limit, format!("{w}×{h} output is too large"));
+        return None;
+    };
     let _limit = match fault {
         Some(crate::Fault::Limit(bytes)) => Some(crate::ctx::LimitOverride::new(bytes)),
         _ => None,
diff --git a/crates/pipeline/src/lib.rs b/crates/pipeline/src/lib.rs
index 00b2ccae2..3b9f9be77 100644
--- a/crates/pipeline/src/lib.rs
+++ b/crates/pipeline/src/lib.rs
@@ -156,6 +156,21 @@ impl PixelWindow {
     }
 }
 
+/// The longest output side a render may be asked for (TIFF/PNG/JPEG all fit within it).
+pub const MAX_OUTPUT_SIDE: usize = 65_535;
+/// The most output pixels a render may be asked for (the box it fits into, ≈ 1.07 Gpx).
+pub const MAX_OUTPUT_PIXELS: usize = 1 << 30;
+
+/// Rejects an output box past [`MAX_OUTPUT_SIDE`] or [`MAX_OUTPUT_PIXELS`] before anything is
+/// sized from it.
+pub fn check_output_size(max_w: usize, max_h: usize) -> Result<(), String> {
+    let pixels = max_w.checked_mul(max_h);
+    if max_w > MAX_OUTPUT_SIDE || max_h > MAX_OUTPUT_SIDE || pixels.is_none_or(|n| n > MAX_OUTPUT_PIXELS) {
+        return Err(format!("output size {max_w}×{max_h} is too large (at most {MAX_OUTPUT_SIDE} px per side and {MAX_OUTPUT_PIXELS} pixels)"));
+    }
+    Ok(())
+}
+
 impl RenderRequest {
     pub fn fit(max_w: usize, max_h: usize) -> Self {
         Self {