diff --git a/kcalc.cpp b/kcalc.cpp
--- a/kcalc.cpp
+++ b/kcalc.cpp
@@ -1541,19 +1541,30 @@
 void KCalculator::slotConstclicked(int button) {
 
 	if(KCalcConstButton *btn = qobject_cast<KCalcConstButton*>(const_buttons_[button])) {
+		// Constants are stored in a C locale way to ensure they can be read correctly
+		// by instances of KCalc with different locale settings.
+		const QChar localeCDecimalPoint(QLatin1Char('.'));
+		const QChar localeCGroupSeparator(QLatin1Char(','));
+		const bool groupSeparatorEnabled = KCalcSettings::groupDigits() &&
+				!(QLocale().numberOptions() & QLocale::OmitGroupSeparator);
+
 		if (!shift_mode_) {
 			// set the display to the configured value of constant button
-			// internally, we deal with C locale style numbers, we need to convert
 			QString val = btn->constant();
-			val.replace(QLatin1Char('.'), KNumber::decimalSeparator());
+			if (groupSeparatorEnabled)
+				val.remove(localeCGroupSeparator);
+			val.replace(localeCDecimalPoint, KNumber::decimalSeparator());
 			calc_display->setAmount(KNumber(val));
-
 		} else {
 			pbShift->setChecked(false);
 
-			// internally, we deal with C locale style numbers, we need to convert
 			QString val = calc_display->text();
-			val.replace(KNumber::decimalSeparator(), QLatin1String("."));
+			for (QChar &c : val) {
+				if (c == KNumber::decimalSeparator())
+					c = localeCDecimalPoint;
+				else if (groupSeparatorEnabled && (c == KNumber::groupSeparator()))
+					c = localeCGroupSeparator;
+			}
 			KCalcSettings::setValueConstant(button, val);
 
 			// below set new tooltip