diff --git a/packages/gallery/src/interface/focus.js b/packages/gallery/src/interface/focus.js
index 6c60c3d..a5e3814 100644
--- a/packages/gallery/src/interface/focus.js
+++ b/packages/gallery/src/interface/focus.js
@@ -4,7 +4,7 @@ import {
 	subscribeToRender,
 	defineView,
 	nodeParentWithType,
-	viewportSize,
+	availableViewportSize,
 	defineElement as el
 } from '../utils/domvm.js';
 
@@ -42,7 +42,7 @@ export function FocusView(vm, params, key, { appbar }) {
 				};
 			}
 		},
-		[doc, viewportSize]
+		[doc, availableViewportSize]
 	);
 
 	function navBack() {
diff --git a/packages/gallery/src/interface/sectionView.js b/packages/gallery/src/interface/sectionView.js
index 3c7c209..805c716 100644
--- a/packages/gallery/src/interface/sectionView.js
+++ b/packages/gallery/src/interface/sectionView.js
@@ -6,7 +6,7 @@ import {
 	patchRefStyle,
 	patchNodeStyle,
 	subscribeToRender,
-	viewportSize
+	availableViewportSize
 } from '../utils/domvm.js';
 import { injectStyle, styled } from '../services/style.js';
 import { DEFAULT_TRANSITION, CLICKABLE, IMAGE_MARGIN, CONTENT_MARGIN } from './styles.js';
@@ -18,8 +18,9 @@ const OPTIMAL_IMAGE_HEIGHT = 140;
 const ROW_HEIGHT_CUTOFF_MODIFIER = 2;
 
 const IMAGE_MARGIN_WIDTH = 2 * IMAGE_MARGIN;
+const CONTENT_MARGIN_WIDTH = 2 * CONTENT_MARGIN;
 
-const aspectRatio = (img, margin = 0) => (img.width + margin) / (img.height + margin);
+const aspectRatio = img => img.width / img.height;
 
 export function SectionView(vm, params, key, context) {
 	const { appbar } = context;
@@ -27,12 +28,9 @@ export function SectionView(vm, params, key, context) {
 	const sectionSelectButtonRef = `secSel${key}`;
 
 	function calculateSections(photos) {
-		const { width: vw } = viewportSize();
-		const availableWidth = vw - CONTENT_MARGIN;
-		const totalImageRatio = photos.reduce(
-			(acc, img) => acc + aspectRatio(img, IMAGE_MARGIN_WIDTH),
-			0
-		);
+		const { width: vw } = availableViewportSize();
+		const availableWidth = vw - CONTENT_MARGIN_WIDTH;
+		const totalImageRatio = photos.reduce((acc, img) => acc + aspectRatio(img), 0);
 		const rowCount = Math.ceil(totalImageRatio * OPTIMAL_IMAGE_HEIGHT / availableWidth);
 		const rowRatios = partition(photos.map(aspectRatio), rowCount);
 
@@ -59,7 +57,7 @@ export function SectionView(vm, params, key, context) {
 		return result;
 	}
 
-	subscribeToRender(vm, [viewportSize]);
+	subscribeToRender(vm, [availableViewportSize]);
 
 	return function render(vm, params) {
 		const { selectedIds, selectMode } = params;
diff --git a/packages/gallery/src/utils/domvm.js b/packages/gallery/src/utils/domvm.js
index 5b1709c..cd484cb 100644
--- a/packages/gallery/src/utils/domvm.js
+++ b/packages/gallery/src/utils/domvm.js
@@ -1,7 +1,7 @@
 // export * from 'domvm/dist/dev/domvm.dev.js';
 export * from 'domvm/dist/mini/domvm.mini.js';
 import { defineView } from 'domvm/dist/mini/domvm.mini.js';
-import { prop, call } from 'frptools';
+import { prop, computed, call } from 'frptools';
 import { deepAssign } from './conversion.js';
 import { error } from '../services/console.js';
 
@@ -52,13 +52,46 @@ export function renderSwitch(renderMap, switchValue) {
 }
 
 // Expose viewport size in a subscribable.
-const SCROLLBAR_SIZE = 20;
-export const viewportSize = prop({}, o => (o ? `${o.width}x${o.height}` : ''));
+export const scrollbarSize = prop(0);
+export const fullViewportSize = prop(
+	{ width: window.innerWidth, height: window.innerHeight },
+	o => (o ? `${o.width}x${o.height}` : '')
+);
+export const availableViewportSize = computed(
+	(ss, vs) => ({
+		width: vs.width - ss,
+		height: vs.height - ss
+	}),
+	[scrollbarSize, fullViewportSize]
+);
+
+(function getScrollbarSize() {
+	const outer = document.createElement('div');
+	const inner = document.createElement('div');
+	outer.style.visibility = 'hidden';
+	outer.style.width = '100px';
+	outer.style.msOverflowStyle = 'scrollbar'; // needed for WinJS apps
+
+	document.body.appendChild(outer);
+
+	const widthNoScroll = outer.offsetWidth;
+	// force scrollbars
+	outer.style.overflow = 'scroll';
+
+	// add innerdiv
+	inner.style.width = '100%';
+	outer.appendChild(inner);
+
+	const widthWithScroll = inner.offsetWidth;
+
+	// remove divs
+	outer.parentNode.removeChild(outer);
+
+	scrollbarSize(widthNoScroll - widthWithScroll);
+})();
+
 const extractWindowSize = () =>
-	viewportSize({
-		width: window.innerWidth - SCROLLBAR_SIZE,
-		height: window.innerHeight - SCROLLBAR_SIZE
-	});
+	fullViewportSize({ width: window.innerWidth, height: window.innerHeight });
 window.addEventListener('resize', extractWindowSize);
 // Prime our window size
 extractWindowSize();