diff --git a/src/corelib/text/qstring.h b/src/corelib/text/qstring.h
index 7daf86a366c..d86d659e754 100644
--- a/src/corelib/text/qstring.h
+++ b/src/corelib/text/qstring.h
@@ -1167,7 +1167,7 @@ inline QString::~QString() {}
 
 inline void QString::reserve(qsizetype asize)
 {
-    if (d->needsDetach() || asize >= capacity())
+    if (d->needsDetach() || asize >= capacity() - d.freeSpaceAtBegin())
         reallocData(uint(qMax(asize, size())) + 1u, d->detachFlags());
 
     // we're not shared anymore, for sure
diff --git a/src/corelib/tools/qlist.h b/src/corelib/tools/qlist.h
index bd593bae55c..e94c108f0ca 100644
--- a/src/corelib/tools/qlist.h
+++ b/src/corelib/tools/qlist.h
@@ -467,7 +467,7 @@ inline void QList<T>::resize_internal(qsizetype newSize, Qt::Initialization)
 {
     Q_ASSERT(newSize >= 0);
 
-    if (d->needsDetach() || newSize > capacity()) {
+    if (d->needsDetach() || newSize > capacity() - d.freeSpaceAtBegin()) {
         // must allocate memory
         DataPointer detached(Data::allocate(d->detachCapacity(newSize),
                                             d->detachFlags()));
@@ -485,7 +485,7 @@ template <typename T>
 void QList<T>::reserve(qsizetype asize)
 {
     // capacity() == 0 for immutable data, so this will force a detaching below
-    if (asize <= capacity()) {
+    if (asize <= capacity() - d.freeSpaceAtBegin()) {
         if (d->flags() & Data::CapacityReserved)
             return;  // already reserved, don't shrink
         if (!d->isShared()) {