diff --git a/invokeai/frontend/web/src/features/controlLayers/konva/CanvasTool/CanvasBboxToolModule.ts b/invokeai/frontend/web/src/features/controlLayers/konva/CanvasTool/CanvasBboxToolModule.ts
index 933ce203533..0bd6bd063a7 100644
--- a/invokeai/frontend/web/src/features/controlLayers/konva/CanvasTool/CanvasBboxToolModule.ts
+++ b/invokeai/frontend/web/src/features/controlLayers/konva/CanvasTool/CanvasBboxToolModule.ts
@@ -1,13 +1,8 @@
-import {
-  roundDownToMultiple,
-  roundToMultiple,
-  roundToMultipleMin,
-  roundUpToMultiple,
-} from 'common/util/roundDownToMultiple';
+import { roundToMultiple, roundToMultipleMin } from 'common/util/roundDownToMultiple';
 import type { CanvasManager } from 'features/controlLayers/konva/CanvasManager';
 import { CanvasModuleBase } from 'features/controlLayers/konva/CanvasModuleBase';
 import type { CanvasToolModule } from 'features/controlLayers/konva/CanvasTool/CanvasToolModule';
-import { getKonvaNodeDebugAttrs, getPrefixedId } from 'features/controlLayers/konva/util';
+import { fitRectToGrid, getKonvaNodeDebugAttrs, getPrefixedId } from 'features/controlLayers/konva/util';
 import { selectBboxOverlay } from 'features/controlLayers/store/canvasSettingsSlice';
 import { selectBbox } from 'features/controlLayers/store/selectors';
 import type { Coordinate, Rect } from 'features/controlLayers/store/types';
@@ -398,18 +393,12 @@ export class CanvasBboxToolModule extends CanvasModuleBase {
     }
 
     // Determine the bbox size that fits within the visible rect. The bbox must be at least 64px in width and height,
-    // and its width and height must be multiples of 8px.
+    // and its width and height must be multiples of the bbox grid size.
     const gridSize = this.manager.stateApi.getBboxGridSize();
 
-    // To be conservative, we will round up the x and y to the nearest grid size, and round down the width and height.
-    // This ensures the bbox is never _larger_ than the visible rect. If the bbox is larger than the visible, we
-    // will always trigger the outpainting workflow, which is not what the user wants.
-    const x = roundUpToMultiple(visibleRect.x, gridSize);
-    const y = roundUpToMultiple(visibleRect.y, gridSize);
-    const width = roundDownToMultiple(visibleRect.width, gridSize);
-    const height = roundDownToMultiple(visibleRect.height, gridSize);
+    const rect = fitRectToGrid(visibleRect, gridSize);
 
-    this.manager.stateApi.setGenerationBbox({ x, y, width, height });
+    this.manager.stateApi.setGenerationBbox(rect);
   };
 
   /**
diff --git a/invokeai/frontend/web/src/features/controlLayers/konva/util.test.ts b/invokeai/frontend/web/src/features/controlLayers/konva/util.test.ts
index a7db030ae4a..f3c44821a06 100644
--- a/invokeai/frontend/web/src/features/controlLayers/konva/util.test.ts
+++ b/invokeai/frontend/web/src/features/controlLayers/konva/util.test.ts
@@ -1,4 +1,6 @@
-import { getPrefixedId, getRectUnion } from 'features/controlLayers/konva/util';
+import { roundUpToMultiple } from 'common/util/roundDownToMultiple';
+import { fitRectToGrid, getPrefixedId, getRectUnion } from 'features/controlLayers/konva/util';
+import type { Rect } from 'features/controlLayers/store/types';
 import { describe, expect, it } from 'vitest';
 
 describe('util', () => {
@@ -44,4 +46,74 @@ describe('util', () => {
       expect(union).toEqual({ x: 0, y: 0, width: 0, height: 0 });
     });
   });
+
+  describe('fitRectToGrid', () => {
+    it('should fit rect within grid without exceeding bounds', () => {
+      const rect: Rect = { x: 0, y: 0, width: 1047, height: 1758 };
+      const gridSize = 50;
+      const result = fitRectToGrid(rect, gridSize);
+
+      expect(result.x).toBe(roundUpToMultiple(rect.x, gridSize));
+      expect(result.y).toBe(roundUpToMultiple(rect.y, gridSize));
+      expect(result.width).toBeLessThanOrEqual(rect.width);
+      expect(result.height).toBeLessThanOrEqual(rect.height);
+      expect(result.width % gridSize).toBe(0);
+      expect(result.height % gridSize).toBe(0);
+    });
+
+    it('should handle small rect within grid bounds', () => {
+      const rect: Rect = { x: 20, y: 30, width: 80, height: 90 };
+      const gridSize = 25;
+      const result = fitRectToGrid(rect, gridSize);
+
+      expect(result.x).toBe(25);
+      expect(result.y).toBe(50);
+      expect(result.width % gridSize).toBe(0);
+      expect(result.height % gridSize).toBe(0);
+      expect(result.width).toBeLessThanOrEqual(rect.width);
+      expect(result.height).toBeLessThanOrEqual(rect.height);
+    });
+
+    it('should handle rect starting outside of grid alignment', () => {
+      const rect: Rect = { x: 13, y: 27, width: 94, height: 112 };
+      const gridSize = 20;
+      const result = fitRectToGrid(rect, gridSize);
+
+      expect(result.x).toBe(20);
+      expect(result.y).toBe(40);
+      expect(result.width % gridSize).toBe(0);
+      expect(result.height % gridSize).toBe(0);
+      expect(result.width).toBeLessThanOrEqual(rect.width);
+      expect(result.height).toBeLessThanOrEqual(rect.height);
+    });
+
+    it('should return the same rect if already aligned to grid', () => {
+      const rect: Rect = { x: 100, y: 100, width: 200, height: 300 };
+      const gridSize = 50;
+      const result = fitRectToGrid(rect, gridSize);
+
+      expect(result).toEqual(rect);
+    });
+
+    it('should handle large grid sizes relative to rect dimensions', () => {
+      const rect: Rect = { x: 250, y: 300, width: 400, height: 500 };
+      const gridSize = 100;
+      const result = fitRectToGrid(rect, gridSize);
+
+      expect(result.x).toBe(300);
+      expect(result.y).toBe(300);
+      expect(result.width % gridSize).toBe(0);
+      expect(result.height % gridSize).toBe(0);
+      expect(result.width).toBeLessThanOrEqual(rect.width);
+      expect(result.height).toBeLessThanOrEqual(rect.height);
+    });
+
+    it('should handle rect with zero width and height', () => {
+      const rect: Rect = { x: 40, y: 60, width: 100, height: 200 };
+      const gridSize = 20;
+      const result = fitRectToGrid(rect, gridSize);
+
+      expect(result).toEqual({ x: 40, y: 60, width: 100, height: 200 });
+    });
+  });
 });
diff --git a/invokeai/frontend/web/src/features/controlLayers/konva/util.ts b/invokeai/frontend/web/src/features/controlLayers/konva/util.ts
index 38ce6389cc2..a2d28b5ac0e 100644
--- a/invokeai/frontend/web/src/features/controlLayers/konva/util.ts
+++ b/invokeai/frontend/web/src/features/controlLayers/konva/util.ts
@@ -1,5 +1,6 @@
 import type { Selector, Store } from '@reduxjs/toolkit';
 import { $authToken } from 'app/store/nanostores/authToken';
+import { roundDownToMultiple, roundUpToMultiple } from 'common/util/roundDownToMultiple';
 import type {
   CanvasEntityIdentifier,
   CanvasObjectState,
@@ -560,6 +561,33 @@ export const getRectIntersection = (...rects: Rect[]): Rect => {
   return rect || getEmptyRect();
 };
 
+/**
+ * Fits a rect to the nearest multiple of the grid size, rounding down. The returned rect will be smaller than or equal
+ * to the input rect, and will be aligned to the grid.
+ *
+ * In other words, shrink the rect inwards on each size until it fits within the visible rect and aligns to the grid.
+ *
+ * @param rect The rect to fit
+ * @param gridSize The size of the grid
+ * @returns The fitted rect
+ */
+export const fitRectToGrid = (rect: Rect, gridSize: number): Rect => {
+  // Rounding x and y up effectively shrinks the left and top edges of the rect, and rounding width and height down
+  // effectively shrinks the right and bottom edges.
+  const x = roundUpToMultiple(rect.x, gridSize);
+  const y = roundUpToMultiple(rect.y, gridSize);
+
+  // Because we've just shifted the rect's x and y, we need to adjust the width and height by the same amount before
+  // we round those values down.
+  const offsetX = x - rect.x;
+  const offsetY = y - rect.y;
+
+  const width = roundDownToMultiple(rect.width - offsetX, gridSize);
+  const height = roundDownToMultiple(rect.height - offsetY, gridSize);
+
+  return { x, y, width, height };
+};
+
 /**
  * Asserts that the value is never reached. Used for exhaustive checks in switch statements or conditional logic to ensure
  * that all possible values are handled.