Index: scribus/prefsmanager.cpp
===================================================================
--- scribus/prefsmanager.cpp	(revision 27777)
+++ scribus/prefsmanager.cpp	(working copy)
@@ -118,6 +118,41 @@
 	//>>CB
 }
 
+MarginStruct PrefsManager::defaultPageMarginsForUnit(int unitIndex)
+{
+	double margin = 36.0;
+	switch (unitIndex)
+	{
+		case SC_PT:
+		case SC_P:
+			break;
+		case SC_MM:
+			margin = mm2pts(10.0);
+			break;
+		case SC_IN:
+			margin = in2pts(0.5);
+			break;
+		case SC_CM:
+			margin = cm2pts(1.0);
+			break;
+		case SC_C:
+			margin = c2pts(3.0);
+			break;
+		default:
+			break;
+	}
+	return MarginStruct(margin, margin, margin, margin);
+}
+
+bool PrefsManager::hasDefaultPageMarginsForUnit(const MarginStruct& margins, int unitIndex)
+{
+	const MarginStruct defaults = defaultPageMarginsForUnit(unitIndex);
+	return qFuzzyCompare(margins.top(), defaults.top())
+		&& qFuzzyCompare(margins.left(), defaults.left())
+		&& qFuzzyCompare(margins.bottom(), defaults.bottom())
+		&& qFuzzyCompare(margins.right(), defaults.right());
+}
+
 void PrefsManager::initDefaults()
 {
 	/** Default font and size **/
@@ -332,7 +367,7 @@
 	PageSizeInfo psi = PagePresetManager::instance().pageInfoByName(appPrefs.docSetupPrefs.pageSize);
 	appPrefs.docSetupPrefs.pageWidth = psi.width;
 	appPrefs.docSetupPrefs.pageHeight = psi.height;
-	appPrefs.docSetupPrefs.margins.set(40, 40, 40, 40);
+	appPrefs.docSetupPrefs.margins = defaultPageMarginsForUnit(appPrefs.docSetupPrefs.docUnitIndex);
 	appPrefs.docSetupPrefs.marginPreset = 0;
 	appPrefs.docSetupPrefs.bleeds.set(0, 0, 0, 0);
 	appPrefs.hyphPrefs.specialWords.clear();
Index: scribus/prefsmanager.h
===================================================================
--- scribus/prefsmanager.h	(revision 27777)
+++ scribus/prefsmanager.h	(working copy)
@@ -60,6 +60,8 @@
 	Must be run after: PrefsManager::GetAllFonts()
 	*/
 	void initDefaults();
+	static MarginStruct defaultPageMarginsForUnit(int unitIndex);
+	static bool hasDefaultPageMarginsForUnit(const MarginStruct& margins, int unitIndex);
 	void initDefaultGUIFont(const QFont&);
 	void initDefaultCheckerPrefs(CheckerPrefsList& cp);
 	void initDefaultActionKeys();
Index: scribus/ui/fsw/firststartwizard.cpp
===================================================================
--- scribus/ui/fsw/firststartwizard.cpp	(revision 27777)
+++ scribus/ui/fsw/firststartwizard.cpp	(working copy)
@@ -168,7 +168,11 @@
 	// --- Language (FSW_Language) ---
 	p.uiPrefs.language           = m_language->uiLanguage();        // priAbbrev
 	p.docSetupPrefs.language     = m_language->documentLanguage();  // normalised short abbrev
-	p.docSetupPrefs.docUnitIndex = m_language->unitIndex();
+	const int oldUnitIndex = p.docSetupPrefs.docUnitIndex;
+	const int newUnitIndex = m_language->unitIndex();
+	if (PrefsManager::hasDefaultPageMarginsForUnit(p.docSetupPrefs.margins, oldUnitIndex))
+		p.docSetupPrefs.margins = PrefsManager::defaultPageMarginsForUnit(newUnitIndex);
+	p.docSetupPrefs.docUnitIndex = newUnitIndex;
 
 	// --- Appearance (FSW_Appearance) ---
 	switch (m_appearance->themeMode())
Index: scribus/ui/newdocdialog.cpp
===================================================================
--- scribus/ui/newdocdialog.cpp	(revision 27777)
+++ scribus/ui/newdocdialog.cpp	(working copy)
@@ -45,6 +45,7 @@
 #include "scpaths.h"
 #include "scrspinbox.h"
 #include "units.h"
+#include "ui/marginpresetlayout.h"
 #include "ui/widgets/pagesizelist.h"
 
 
@@ -570,6 +571,9 @@
 
 void NewDocDialog::setUnit(int newUnitIndex)
 {
+	const bool useDefaultMargins = marginGroup->marginPreset() == PresetLayout::none
+		&& PrefsManager::hasDefaultPageMarginsForUnit(marginGroup->margins(), m_unitIndex);
+
 	QSignalBlocker sig(unitOfMeasureComboBox);
 	unitOfMeasureComboBox->setCurrentIndex(newUnitIndex);
 
@@ -584,6 +588,8 @@
 	heightSpinBox->setValue(m_pageHeight * m_unitRatio);
 
 	marginGroup->setNewUnit(m_unitIndex);
+	if (useDefaultMargins)
+		marginGroup->setNewValues(PrefsManager::defaultPageMarginsForUnit(m_unitIndex));
 	marginGroup->setPageHeight(m_pageHeight);
 	marginGroup->setPageWidth(m_pageWidth);
 	bleedGroup->setNewUnit(m_unitIndex);
Index: scribus/ui/preferences/prefs_documentsetup.cpp
===================================================================
--- scribus/ui/preferences/prefs_documentsetup.cpp	(revision 27777)
+++ scribus/ui/preferences/prefs_documentsetup.cpp	(working copy)
@@ -15,6 +15,7 @@
 #include "prefsfile.h"
 #include "prefsmanager.h"
 #include "prefsstructs.h"
+#include "ui/marginpresetlayout.h"
 #include "ui/newmarginwidget.h"
 #include "ui/preferences/prefs_documentsetup.h"
 #include "undomanager.h"
@@ -83,16 +84,21 @@
 
 void Prefs_DocumentSetup::unitChange()
 {
+	const int docUnitIndex = pageUnitsComboBox->currentIndex();
+	const bool useDefaultMargins = !m_doc && marginsWidget->marginPreset() == PresetLayout::none
+		&& PrefsManager::hasDefaultPageMarginsForUnit(marginsWidget->margins(), m_unitIndex);
+
 	pageWidthSpinBox->blockSignals(true);
 	pageHeightSpinBox->blockSignals(true);
 
-	int docUnitIndex = pageUnitsComboBox->currentIndex();
 	pageWidthSpinBox->setNewUnit(docUnitIndex);
 	pageHeightSpinBox->setNewUnit(docUnitIndex);
 	unitRatio = unitGetRatioFromIndex(docUnitIndex);
 	pageWidthSpinBox->setValue(pageW * unitRatio);
 	pageHeightSpinBox->setValue(pageH * unitRatio);
 	marginsWidget->setNewUnit(docUnitIndex);
+	if (useDefaultMargins)
+		marginsWidget->setNewValues(PrefsManager::defaultPageMarginsForUnit(docUnitIndex));
 	marginsWidget->setPageHeight(pageH);
 	marginsWidget->setPageWidth(pageW);
 	bleedsWidget->setNewUnit(docUnitIndex);
@@ -102,6 +108,7 @@
 	pageWidthSpinBox->blockSignals(false);
 	pageHeightSpinBox->blockSignals(false);
 
+	m_unitIndex = docUnitIndex;
 	emit prefsChangeUnits(docUnitIndex);
 }
 
@@ -145,7 +152,8 @@
 
 	setCurrentComboItem(languageComboBox, LanguageManager::instance()->getLangFromAbbrev(prefsData->docSetupPrefs.language, true));
 
-	unitRatio = unitGetRatioFromIndex(prefsData->docSetupPrefs.docUnitIndex);
+	m_unitIndex = prefsData->docSetupPrefs.docUnitIndex;
+	unitRatio = unitGetRatioFromIndex(m_unitIndex);
 	setupPageSizes(prefsData);
 
 	pageOrientationComboBox->setCurrentIndex(prefsData->docSetupPrefs.pageOrientation);
Index: scribus/ui/preferences/prefs_documentsetup.h
===================================================================
--- scribus/ui/preferences/prefs_documentsetup.h	(revision 27777)
+++ scribus/ui/preferences/prefs_documentsetup.h	(working copy)
@@ -64,6 +64,7 @@
 		ScribusDoc* m_doc { nullptr };
 
 		double unitRatio { 1.0 };
+		int m_unitIndex { 0 };
 		double pageW { 1.0 };
 		double pageH { 1.0 };
 		QString prefsPageSizeName;