diff --git a/src/hooks/useCanvasInteraction.ts b/src/hooks/useCanvasInteraction.ts
index 0b0ab44..df56ab6 100644
--- a/src/hooks/useCanvasInteraction.ts
+++ b/src/hooks/useCanvasInteraction.ts
@@ -2,7 +2,13 @@ import { useEffect, useRef, useCallback } from 'react';
 import type { RefObject } from 'react';
 import { useAppStore } from '../store/useAppStore';
 import { screenToImage, clamp, computeFitZoom, pointInRect } from '../lib/geometry';
-import { MIN_ZOOM, MAX_ZOOM, ZOOM_STEP, MIN_BOX_SIZE, HANDLE_HALFSIZE_PX } from '../lib/constants';
+import {
+  MIN_ZOOM,
+  MAX_ZOOM,
+  ZOOM_FACTOR,
+  MIN_BOX_SIZE,
+  HANDLE_HALFSIZE_PX,
+} from '../lib/constants';
 import type { DragHandle, DragState } from '../lib/types';
 
 export interface DrawingState {
@@ -200,8 +206,8 @@ export function useCanvasInteraction(
       if (e.ctrlKey || e.metaKey) {
         // Zoom toward cursor — must prevent browser zoom
         e.preventDefault();
-        const delta = e.deltaY < 0 ? ZOOM_STEP : -ZOOM_STEP;
-        applyZoom(zoom + delta, e.offsetX, e.offsetY);
+        const factor = e.deltaY < 0 ? ZOOM_FACTOR : 1 / ZOOM_FACTOR;
+        applyZoom(zoom * factor, e.offsetX, e.offsetY);
       } else if (e.shiftKey) {
         // Horizontal pan
         const d = e.deltaX || e.deltaY;
diff --git a/src/lib/constants.ts b/src/lib/constants.ts
index 54e5634..9e5b23c 100644
--- a/src/lib/constants.ts
+++ b/src/lib/constants.ts
@@ -8,6 +8,6 @@ export const CLASSES: AnnotationClass[] = [
 
 export const MIN_ZOOM = 0.25;
 export const MAX_ZOOM = 5.0;
-export const ZOOM_STEP = 0.1;
+export const ZOOM_FACTOR = 1.1; // multiplicative zoom instead of additive
 export const MIN_BOX_SIZE = 4; // px in image space, ignore smaller drags
 export const HANDLE_HALFSIZE_PX = 4; // half-size of resize handles in screen pixels (8px total)
diff --git a/src/store/useAppStore.ts b/src/store/useAppStore.ts
index 240454b..00d2e38 100644
--- a/src/store/useAppStore.ts
+++ b/src/store/useAppStore.ts
@@ -1,6 +1,6 @@
 import { create } from 'zustand';
 import type { Annotation, EditMode } from '../lib/types';
-import { CLASSES, MIN_ZOOM, MAX_ZOOM, ZOOM_STEP } from '../lib/constants';
+import { CLASSES, MIN_ZOOM, MAX_ZOOM, ZOOM_FACTOR } from '../lib/constants';
 import { clamp, computeFitZoom } from '../lib/geometry';
 
 interface AppState {
@@ -175,7 +175,7 @@ export const useAppStore = create<AppState>((set) => ({
       canvasWidth && imageWidth
         ? computeFitZoom(canvasWidth, canvasHeight, imageWidth, imageHeight)
         : MIN_ZOOM;
-    const newZoom = clamp(zoom + ZOOM_STEP, minZoom, MAX_ZOOM);
+    const newZoom = clamp(zoom * ZOOM_FACTOR, minZoom, MAX_ZOOM);
     if (newZoom === zoom) return;
     const cx = canvasWidth / 2;
     const cy = canvasHeight / 2;
@@ -191,7 +191,7 @@ export const useAppStore = create<AppState>((set) => ({
       canvasWidth && imageWidth
         ? computeFitZoom(canvasWidth, canvasHeight, imageWidth, imageHeight)
         : MIN_ZOOM;
-    const newZoom = clamp(zoom - ZOOM_STEP, minZoom, MAX_ZOOM);
+    const newZoom = clamp(zoom / ZOOM_FACTOR, minZoom, MAX_ZOOM);
     if (newZoom === zoom) return;
     const cx = canvasWidth / 2;
     const cy = canvasHeight / 2;