diff --git a/src/board/UBBoardController.cpp b/src/board/UBBoardController.cpp
index 46ebf85b..0c31fc63 100644
--- a/src/board/UBBoardController.cpp
+++ b/src/board/UBBoardController.cpp
@@ -1519,12 +1519,17 @@ void UBBoardController::updateSystemScaleFactor()
     if (mActiveScene)
     {
         QSize pageNominalSize = mActiveScene->nominalSize();
-        QSize controlSize = controlViewport();
+		//we're going to keep scale factor untouched if the size is custom
+		QMap<DocumentSizeRatio::Enum, QSize> sizesMap = UBSettings::settings()->documentSizes;
+		if(pageNominalSize == sizesMap.value(DocumentSizeRatio::Ratio16_9) || pageNominalSize == sizesMap.value(DocumentSizeRatio::Ratio4_3))
+		{
+			QSize controlSize = controlViewport();
 
-        qreal hFactor = ((qreal)controlSize.width()) / ((qreal)pageNominalSize.width());
-        qreal vFactor = ((qreal)controlSize.height()) / ((qreal)pageNominalSize.height());
+			qreal hFactor = ((qreal)controlSize.width()) / ((qreal)pageNominalSize.width());
+			qreal vFactor = ((qreal)controlSize.height()) / ((qreal)pageNominalSize.height());
 
-        newScaleFactor = qMin(hFactor, vFactor);
+			newScaleFactor = qMin(hFactor, vFactor);
+		}
     }
 
     if (mSystemScaleFactor != newScaleFactor)
@@ -1543,7 +1548,6 @@ void UBBoardController::updateSystemScaleFactor()
     mControlView->setTransform(scalingTransform);
     mControlView->horizontalScrollBar()->setValue(viewState.horizontalPosition);
     mControlView->verticalScrollBar()->setValue(viewState.verticalPostition);
-
 }