diff --git a/crates/ui-egui/src/dialogs/png_options.rs b/crates/ui-egui/src/dialogs/png_options.rs
--- a/crates/ui-egui/src/dialogs/png_options.rs
+++ b/crates/ui-egui/src/dialogs/png_options.rs
@@ -128,7 +128,40 @@ fn heading(d: &Dialog) -> String {
     crate::i18n::fmt(tl!("{format} Options"), &[("format", format_of(d).map_or(tl!("Export"), |f| f.label))])
 }
 
-fn body(_: &mut VectorcraftApp, ui: &mut egui::Ui, d: &mut Dialog) -> bool {
+/// A byte count as the estimated-size row shows it (Save for Web uses the same bands).
+fn size_label(bytes: usize) -> String {
+    if bytes < 1024 {
+        format!("{bytes} bytes")
+    } else if bytes < 1 << 20 {
+        format!("{:.1}K", bytes as f64 / 1024.0)
+    } else {
+        format!("{:.2}M", bytes as f64 / f64::from(1 << 20))
+    }
+}
+
+/// Encode with the current options (no file written) and remember the size on the dialog.
+fn refresh_estimated_size(app: &mut VectorcraftApp, d: &mut Dialog) {
+    let raster = format_of(d).is_some_and(|f| matches!(f.id, "png" | "jpg" | "webp" | "gif" | "png8" | "tiff" | "bmp" | "tga" | "psd"));
+    if !raster {
+        d.fields.remove("__estimateKey");
+        d.fields.remove("__estimatedBytes");
+        return;
+    }
+    let mut params = form::params(d);
+    if let Some(o) = params.as_object_mut() {
+        o.remove("path");
+    }
+    let key = params.to_string();
+    if d.str("__estimateKey") == key && d.fields.contains_key("__estimatedBytes") {
+        return;
+    }
+    let bytes = app.run("document.export", params).ok().and_then(|r| r.get("bytes").and_then(Value::as_u64));
+    d.fields.insert("__estimateKey".into(), json!(key));
+    d.fields.insert("__estimatedBytes".into(), json!(bytes));
+}
+
+fn body(app: &mut VectorcraftApp, ui: &mut egui::Ui, d: &mut Dialog) -> bool {
+    refresh_estimated_size(app, d);
     let t = Tokens::get(ui.ctx());
     let id = format_of(d).map_or("png", |f| f.id);
     let label = |ui: &mut egui::Ui, text: &str| ui.label(egui::RichText::new(tl!(text)).color(t.text_dim));
@@ -161,6 +194,11 @@ fn body(_: &mut VectorcraftApp, ui: &mut egui::Ui, d: &mut Dialog) -> bool {
             ui.label(egui::RichText::new(format!("{pw} × {ph} px")).color(t.text));
             ui.end_row();
         }
+        if let Some(bytes) = d.fields.get("__estimatedBytes").and_then(Value::as_u64) {
+            label(ui, tl!("Estimated File Size:"));
+            ui.label(egui::RichText::new(size_label(bytes as usize)).color(t.text));
+            ui.end_row();
+        }
     });
     false
 }
diff --git a/crates/ui-egui/src/dialogs/tests_export.rs b/crates/ui-egui/src/dialogs/tests_export.rs
--- a/crates/ui-egui/src/dialogs/tests_export.rs
+++ b/crates/ui-egui/src/dialogs/tests_export.rs
@@ -67,6 +67,7 @@ fn png_options_draw_and_export_with_the_chosen_options() {
     assert_eq!((d.f64("ppi", 0.0), d.str("background"), d.str("antiAlias"), d.bool("interlaced")), (72.0, "transparent".into(), "art".into(), false));
     assert_eq!(d.fields["__size"], json!([60.0, 40.0]), "the chosen artboard");
     frame(&mut app);
+    assert!(app.ui.dialog.as_ref().unwrap().fields.get("__estimatedBytes").and_then(|v| v.as_u64()).is_some());
     // Other resolution and an Other background colour draw their own field and colour button.
     set(&mut app, "__otherPpi", json!(true));
     set(&mut app, "ppi", json!(200));
@@ -93,6 +94,18 @@ fn png_options_draw_and_export_with_the_chosen_options() {
     assert_eq!(&written.borrow()[1].1[..2], [0xFF, 0xD8]);
 }
 
+#[test]
+fn png_options_estimated_file_size_follows_resolution() {
+    let (mut app, _) = app(1);
+    png_options::open(&mut app, fileio::format("png").unwrap(), json!({"format": "png", "path": "/out/a.png"}));
+    frame(&mut app);
+    let small = app.ui.dialog.as_ref().unwrap().fields["__estimatedBytes"].as_u64().unwrap();
+    set(&mut app, "ppi", json!(300.0));
+    frame(&mut app);
+    let large = app.ui.dialog.as_ref().unwrap().fields["__estimatedBytes"].as_u64().unwrap();
+    assert!(large > small, "{large} vs {small}");
+}
+
 #[test]
 fn export_as_png_picks_the_file_then_shows_png_options() {
     let (mut app, written) = app(1);