diff --git a/instances/templates/instance.html b/instances/templates/instance.html
index 9f87dc3c..98fa3e07 100644
--- a/instances/templates/instance.html
+++ b/instances/templates/instance.html
@@ -320,6 +320,8 @@
             var container  = $(this).closest('.js-custom__container');
             var toggles    = container.find('.js-custom__toggle');
             toggles.toggle();
+            // a custom value is sent only while its box is ticked
+            container.find('input.js-custom__toggle').prop('disabled', !this.checked);
         });
     });
 </script>
diff --git a/instances/templates/instances/resize_tab.html b/instances/templates/instances/resize_tab.html
index cf0321e8..b95b293a 100644
--- a/instances/templates/instances/resize_tab.html
+++ b/instances/templates/instances/resize_tab.html
@@ -97,7 +97,7 @@
                                 <option value="{{ mem }}" {% if mem == instance.cur_memory %}selected{% endif %}>{{ mem }}</option>
                             {% endfor %}
                             </select>
-                            <input type="text" name="cur_memory_custom" class="form-select js-custom__toggle" style="display: none" />
+                            <input type="text" name="cur_memory_custom" class="form-select js-custom__toggle" style="display: none" disabled />
                             <small><input type="checkbox" class="js-custom__checkbox" /> {% trans "Custom value" %}</small>
                         </div>
                     </div>
@@ -106,13 +106,18 @@
                             {% trans "Maximum Allocation" %} ({% trans "MB" %})
                         </label>
                         <div class="col-sm-4 js-custom__container">
-                            <select name="memory" class="form-select js-custom__toggle">
+                            {# a running VM keeps its maximum: only the current memory changes #}
+                            <select name="memory" class="form-select js-custom__toggle"{% if instance.status != 5 %} disabled{% endif %}>
                             {% for mem in memory_range %}
                                 <option value="{{ mem }}" {% if mem == instance.memory %}selected{% endif %}>{{ mem }}</option>
                             {% endfor %}
                             </select>
-                            <input type="text" name="memory_custom" class="form-select js-custom__toggle" style="display: none" />
+                            {% if instance.status == 5 %}
+                            <input type="text" name="memory_custom" class="form-select js-custom__toggle" style="display: none" disabled />
                             <small><input type="checkbox" class="js-custom__checkbox" /> {% trans "Custom value" %}</small>
+                            {% else %}
+                            <small class="text-body-secondary">{% trans "Shut off the VM to change the maximum" %}</small>
+                            {% endif %}
                         </div>
                     </div>
                     <button type="submit" class="btn btn-lg btn-success float-end" name="resizevm_mem">{% trans "Resize" %}</button>
@@ -127,7 +132,7 @@
             {% if can_change %}
                 <form action="{% url 'instances:resize_disk' instance.id %}" method="post" role="form" aria-label="Resize instance disk form">
                     {% csrf_token %}
-                    <p class="fw-bold">{% trans "Disk allocation (GB)" %}:</p>
+                    <p class="fw-bold">{% trans "New disk size (GiB); leave empty to keep a disk as it is" %}:</p>
                     {% for disk in instance.disks %}
                     <div class="row">
                         <label class="col-sm-4 col-form-label">{% trans "Current Allocation" %} ({{ disk.dev }})</label>
@@ -139,7 +144,9 @@
                         </div>
                         {% else %}
                         <div class="col-sm-4 js-custom__container">
-                            <input type="number" name="disk_size_{{ disk.dev }}" class="form-control" value="{% widthratio disk.size 1073741824 1 %}" />
+                            {# empty: the disk stays as it is #}
+                            <input type="number" name="disk_size_{{ disk.dev }}" class="form-control" min="1" step="1"
+                                   placeholder="{{ disk.size|filesizeformat }}" aria-label="{% trans "New size (GiB)" %} {{ disk.dev }}" />
                         </div>
                         {% endif %}
                     </div>
diff --git a/instances/tests/test_quota.py b/instances/tests/test_quota.py
index cd8cebff..c7322526 100644
--- a/instances/tests/test_quota.py
+++ b/instances/tests/test_quota.py
@@ -163,7 +163,8 @@ class QuotaViewTests(TestCase):
         return messages, check, wvm.return_value
 
     def test_unverified_quota_has_its_own_message(self):
-        messages, _, proxy = self.post("instances:resizevm_cpu", {"vcpu": 2, "cur_vcpu": 2}, quota=QUOTA_UNVERIFIED)
+        messages, _, proxy = self.post("instances:resizevm_cpu", {"vcpu": 2, "cur_vcpu": 2}, quota=QUOTA_UNVERIFIED,
+                                         status=5)
         self.assertEqual(len(messages), 1)
         self.assertIn("a host of your virtual machines cannot be reached", messages[0])
         self.assertNotIn("quota reached", messages[0])
diff --git a/instances/tests/test_quota_lock.py b/instances/tests/test_quota_lock.py
index 214a718d..5ac608f5 100644
--- a/instances/tests/test_quota_lock.py
+++ b/instances/tests/test_quota_lock.py
@@ -52,6 +52,7 @@ class QuotaRaceTestCase(TransactionTestCase):
             "instances.models.wvmInstance"
         ) as wvm:
             wvm.return_value.get_vcpu.return_value = 1
+            wvm.return_value.instance.isActive.return_value = False  # vCPUs change on a shut-off VM
             wvm.return_value.resize_cpu.side_effect = self.fake_resize
             threads = [threading.Thread(target=self.resize, args=(vm,)) for vm in self.vms]
             for t in threads:
diff --git a/instances/tests/test_resize_input.py b/instances/tests/test_resize_input.py
new file mode 100644
index 00000000..adc7bf07
--- /dev/null
+++ b/instances/tests/test_resize_input.py
@@ -0,0 +1,78 @@
+"""The Resize tab changes only what was asked, and refuses what libvirt would
+not apply as asked."""
+
+from unittest.mock import patch
+
+from accounts.models import UserInstance
+from appsettings.settings import get_settings
+from computes.models import Compute
+from django.contrib.auth import get_user_model
+from django.contrib.messages import get_messages
+from django.test import TestCase
+from django.urls import reverse
+from instances.models import Instance
+from libvirt import VIR_DOMAIN_RUNNING
+
+GIB = 1 << 30
+
+
+class ResizeInputTests(TestCase):
+    def setUp(self):
+        get_settings()
+        self.user = get_user_model().objects.create_user("resize-user", password="pw")
+        self.client.force_login(self.user)
+        compute = Compute.objects.create(name="rs", hostname="192.0.2.1", login="u", password="p", type=1)
+        self.vm = Instance.objects.create(compute=compute, name="rs-vm", uuid="13131313-0000-0000-0000-000000000000")
+        UserInstance.objects.create(user=self.user, instance=self.vm, is_change=True)
+
+    def post(self, view, data, status=5, disks=()):
+        with patch("instances.views.utils.check_user_quota", return_value=""), patch(
+            "instances.models.wvmInstance"
+        ) as wvm:
+            proxy = wvm.return_value
+            proxy.get_vcpu.return_value = 2
+            proxy.get_memory.return_value = 2048
+            proxy.get_cur_memory.return_value = 2048
+            proxy.get_status.return_value = status
+            proxy.instance.isActive.return_value = status != 5
+            proxy.get_disk_devices.return_value = [dict(disk) for disk in disks]
+            response = self.client.post(reverse(view, args=[self.vm.id]), data)
+        self.assertEqual(response.status_code, 302)
+        return [str(m) for m in get_messages(response.wsgi_request)], proxy
+
+    def test_only_a_disk_given_a_larger_size_grows(self):
+        # a 3.5 GiB disk left as it was is not grown; a disk outside a pool has no size
+        disks = [{"dev": "vda", "size": 3 * GIB + GIB // 2}, {"dev": "vdb", "size": 20 * GIB}, {"dev": "vdc", "size": None}]
+        messages, proxy = self.post("instances:resize_disk", {"disk_size_vda": "", "disk_size_vdb": "30"}, disks=disks)
+        proxy.resize_disk.assert_called_once()
+        self.assertEqual([(d["dev"], d["size_new"]) for d in proxy.resize_disk.call_args.args[0]], [("vdb", 30 * GIB)])
+        self.assertEqual(messages, ["Disk is resized: vdb"])
+
+    def test_a_disk_size_that_is_not_a_whole_number_is_refused(self):
+        messages, proxy = self.post("instances:resize_disk", {"disk_size_vda": "4.5"}, disks=[{"dev": "vda", "size": GIB}])
+        proxy.resize_disk.assert_not_called()
+        self.assertEqual(messages, ["Enter disk sizes as whole GiB"])
+
+    def test_vcpus_change_only_on_a_shut_off_vm(self):
+        messages, proxy = self.post("instances:resizevm_cpu", {"vcpu": "4", "cur_vcpu": "4"}, status=VIR_DOMAIN_RUNNING)
+        proxy.resize_cpu.assert_not_called()
+        self.assertEqual(messages, ["Shut off the VM to change its vCPUs"])
+
+    def test_current_vcpus_above_the_maximum_are_refused(self):
+        messages, proxy = self.post("instances:resizevm_cpu", {"vcpu": "2", "cur_vcpu": "4"})
+        proxy.resize_cpu.assert_not_called()
+        self.assertEqual(messages, ["The current vCPUs cannot be more than the maximum"])
+
+    def test_current_memory_above_the_maximum_is_refused(self):
+        messages, proxy = self.post("instances:resize_memory", {"memory": "4096", "cur_memory": "8192"})
+        proxy.resize_mem.assert_not_called()
+        self.assertEqual(messages, ["The current memory cannot be more than the maximum"])
+
+    def test_a_running_vm_offers_no_maximum_memory(self):
+        with patch("instances.models.wvmInstance"):
+            self.vm.__dict__["status"] = VIR_DOMAIN_RUNNING
+            from django.template.loader import render_to_string
+            html = render_to_string("instances/resize_tab.html", {"instance": self.vm, "can_change": True,
+                                                                   "memory_range": [1024, 2048]})
+        self.assertRegex(html, r'<select name="memory" class="form-select js-custom__toggle" disabled>')
+        self.assertNotIn('name="memory_custom"', html)
diff --git a/instances/views.py b/instances/views.py
index 9cd5e7af..6be749e2 100755
--- a/instances/views.py
+++ b/instances/views.py
@@ -966,6 +966,13 @@ def resizevm_cpu(request, pk):
     if not _is_int(new_vcpu) or not _is_int(new_cur_vcpu):
         messages.error(request, _("Enter a whole number of vCPUs"))
         return redirect(reverse("instances:instance", args=[instance.id]) + "#resize")
+    # a running VM would get them only at its next start, past the quota counted now
+    if instance.proxy.instance.isActive():
+        messages.error(request, _("Shut off the VM to change its vCPUs"))
+        return redirect(reverse("instances:instance", args=[instance.id]) + "#resize")
+    if int(new_cur_vcpu) > int(new_vcpu):
+        messages.error(request, _("The current vCPUs cannot be more than the maximum"))
+        return redirect(reverse("instances:instance", args=[instance.id]) + "#resize")
 
     quota_msg = utils.check_user_quota(
         request.user, 0, int(new_vcpu) - vcpu, 0, 0
@@ -1015,6 +1022,9 @@ def resize_memory(request, pk):
     if not _is_int(new_memory) or not _is_int(new_cur_memory):
         messages.error(request, _("Enter a whole number for memory"))
         return redirect(reverse("instances:instance", args=[instance.id]) + "#resize")
+    if int(new_cur_memory) > int(new_memory):
+        messages.error(request, _("The current memory cannot be more than the maximum"))
+        return redirect(reverse("instances:instance", args=[instance.id]) + "#resize")
     quota_msg = utils.check_user_quota(request.user, 0, 0, int(new_memory) - memory, 0)
     if not request.user.is_superuser and quota_msg:
         msg = quota_refused(quota_msg, _(
@@ -1049,11 +1059,15 @@ def resize_disk(request, pk):
 
     disks_new = list()
     for disk in disks:
-        input_disk_size = (
-            int(request.POST.get("disk_size_" + disk["dev"], "0")) * 1073741824
-        )
-        if input_disk_size > disk["size"] + (64 << 20):
-            disk["size_new"] = input_disk_size
+        # an empty field leaves the disk as it is; a disk outside a pool has no size and no field
+        size = request.POST.get("disk_size_" + disk["dev"], "")
+        if not size or disk["size"] is None:
+            continue
+        if not _is_int(size):
+            messages.error(request, _("Enter disk sizes as whole GiB"))
+            return redirect(reverse("instances:instance", args=[instance.id]) + "#resize")
+        if int(size) << 30 > disk["size"]:
+            disk["size_new"] = int(size) << 30
             disks_new.append(disk)
 
     if not disks_new: