diff --git a/src/application/routes/api/coffee/bags.rs b/src/application/routes/api/coffee/bags.rs
index 22c704c..e58aed4 100644
--- a/src/application/routes/api/coffee/bags.rs
+++ b/src/application/routes/api/coffee/bags.rs
@@ -170,7 +170,7 @@ pub(crate) async fn update_bag(
     let (submission, source) = payload.into_parts();
     let (body_update, image_data_url) = submission.into_parts();
 
-    let update = UpdateBag {
+    let mut update = UpdateBag {
         roast_id: body_update.roast_id.or(update_params.roast_id),
         roast_date: body_update.roast_date.or(update_params.roast_date),
         amount: body_update.amount.or(update_params.amount),
@@ -180,6 +180,15 @@ pub(crate) async fn update_bag(
         created_at: body_update.created_at,
     };
 
+    // When the bag amount changes, recompute remaining based on how much has been consumed.
+    if let Some(new_amount) = update.amount
+        && update.remaining.is_none()
+    {
+        let current = state.bag_repo.get(id).await.map_err(AppError::from)?;
+        let consumed = current.amount - current.remaining;
+        update.remaining = Some((new_amount - consumed).max(0.0));
+    }
+
     let bag = if let Some(true) = update.closed {
         state
             .bag_service
diff --git a/tests/server/bags_api.rs b/tests/server/bags_api.rs
index a661d6f..dd11705 100644
--- a/tests/server/bags_api.rs
+++ b/tests/server/bags_api.rs
@@ -1,5 +1,6 @@
 use crate::helpers::{
-    create_default_roast, create_default_roaster, spawn_app, spawn_app_with_auth,
+    create_default_bag, create_default_gear, create_default_roast, create_default_roaster,
+    spawn_app, spawn_app_with_auth,
 };
 use brewlog::domain::bags::{Bag, BagWithRoast, NewBag, UpdateBag};
 use chrono::NaiveDate;
@@ -302,3 +303,58 @@ async fn closing_a_bag_automatically_sets_finished_at() {
         chrono::Utc::now().date_naive()
     );
 }
+
+#[tokio::test]
+async fn updating_bag_amount_recomputes_remaining() {
+    // Arrange: create a bag and brew from it to consume some coffee
+    let app = spawn_app_with_auth().await;
+    let roaster = create_default_roaster(&app).await;
+    let roast = create_default_roast(&app, roaster.id).await;
+    let bag = create_default_bag(&app, roast.id).await;
+    let grinder = create_default_gear(&app, "grinder", "Comandante", "C40 MK4").await;
+    let brewer = create_default_gear(&app, "brewer", "Hario", "V60 02").await;
+    let client = reqwest::Client::new();
+
+    // Brew 15g from the 250g bag → remaining should be 235g
+    let new_brew = brewlog::domain::brews::NewBrew {
+        bag_id: bag.id,
+        coffee_weight: 15.0,
+        grinder_id: grinder.id,
+        grind_setting: 24.0,
+        brewer_id: brewer.id,
+        filter_paper_id: None,
+        water_volume: 250,
+        water_temp: 92.0,
+        quick_notes: Vec::new(),
+        brew_time: None,
+        created_at: None,
+    };
+
+    client
+        .post(app.api_url("/brews"))
+        .bearer_auth(app.auth_token.as_ref().unwrap())
+        .json(&new_brew)
+        .send()
+        .await
+        .expect("Failed to create brew");
+
+    // Act: update the bag amount from 250g to 500g
+    let update_payload = UpdateBag {
+        amount: Some(500.0),
+        ..Default::default()
+    };
+
+    let response = client
+        .put(app.api_url(&format!("/bags/{}", bag.id)))
+        .bearer_auth(app.auth_token.as_ref().unwrap())
+        .json(&update_payload)
+        .send()
+        .await
+        .expect("Failed to execute request");
+
+    // Assert: remaining should be 500 - 15 = 485 (recomputed from consumed)
+    assert_eq!(response.status(), 200);
+    let updated_bag: Bag = response.json().await.expect("Failed to parse response");
+    assert_eq!(updated_bag.amount, 500.0);
+    assert_eq!(updated_bag.remaining, 485.0);
+}