diff --git a/controller/internal/stacks/deploy.go b/controller/internal/stacks/deploy.go
index ffdb057..99768c6 100644
--- a/controller/internal/stacks/deploy.go
+++ b/controller/internal/stacks/deploy.go
@@ -1517,15 +1517,20 @@ func randomAlphanumeric(length int) (string, error) {
 // household reads it in its own language). One value where there used to be a sentence plus a
 // wrapper at each call site. An unreadable memory reading admits with a WARN, exactly as the deploy
 // always has.
-func (m *Manager) memoryVerdict(newReqMB, newLimitMB, releasedReqMB, releasedLimitMB int) (refusal error, warningKey string) {
-	reservedMB := m.cfg.System.ReservedMemoryMB
+// InstallMemory is the ONE reading of the memory the install check judges: the guest's total, the reserve, and the
+// installed apps' committed requests („used"). The install page (web deploy handler) shows exactly these, so the page
+// and the check can never disagree (R-1125, 2026-10-17: the page used /proc's host-wide figures and blocked an app the
+// check would accept, or the other way round). ok=false when the memory cannot be read. Pinned by
+// TestR1125_InstallMemoryIsTheChecksReading.
+func (m *Manager) InstallMemory() (totalMB, reservedMB, usedMB int, ok bool) {
+	reservedMB = m.cfg.System.ReservedMemoryMB
 	totalMB, usedMB, memErr := system.GetMemoryMB()
 	// F1: the controller container cannot read the guest's RAM cap from /proc (no lxcfs) or its own
 	// cgroup (the cap is on the LXC ancestor). Prefer the guest cap from the Docker daemon (runs in the
 	// LXC). And use the controller's OWN committed-memory accounting for "used" — accurate and cheap —
 	// rather than host /proc RSS, which is unobservable-per-guest and would otherwise make this guard
 	// either never fire (host total) or always fire (host used > guest cap).
-	if gt, ok := system.GuestMemTotalMB(); ok && gt > 0 {
+	if gt, gok := system.GuestMemTotalMB(); gok && gt > 0 {
 		totalMB = gt
 		memErr = nil
 	}
@@ -1534,6 +1539,14 @@ func (m *Manager) memoryVerdict(newReqMB, newLimitMB, releasedReqMB, releasedLim
 	}
 	if memErr != nil {
 		m.logger.Printf("[WARN] [stacks] Cannot read system memory: %v — skipping memory check", memErr)
+		return 0, 0, 0, false
+	}
+	return totalMB, reservedMB, usedMB, true
+}
+
+func (m *Manager) memoryVerdict(newReqMB, newLimitMB, releasedReqMB, releasedLimitMB int) (refusal error, warningKey string) {
+	totalMB, reservedMB, usedMB, ok := m.InstallMemory()
+	if !ok {
 		return nil, ""
 	}
 	usedMB -= releasedReqMB
diff --git a/controller/internal/stacks/r1125_install_memory_test.go b/controller/internal/stacks/r1125_install_memory_test.go
new file mode 100644
index 0000000..70c13ef
--- /dev/null
+++ b/controller/internal/stacks/r1125_install_memory_test.go
@@ -0,0 +1,48 @@
+package stacks
+
+import (
+	"io"
+	"log"
+	"os"
+	"strings"
+	"testing"
+
+	"gitea.dooplex.hu/admin/felhom-controller/internal/config"
+)
+
+// R-1125: the install page and the install check read memory the SAME way. „Used" is the installed (running) apps'
+// committed requests — never /proc's host-wide figure, which the page used before 2026-10-17 (on scratch 9202 it
+// showed 12,637 MB „needed" while the check counted 2,648 MB).
+// RED-PROOF (2026-10-17): the deploy handler back on system.GetMemoryMB fails the source half.
+func TestR1125_InstallMemoryIsTheChecksReading(t *testing.T) {
+	cfg := config.Default()
+	m := &Manager{cfg: cfg, logger: log.New(io.Discard, "", 0), stacks: map[string]*Stack{
+		"a": {Deployed: true, State: StateRunning, Meta: Metadata{Resources: ResourceHints{MemRequest: "600M", MemLimit: "1664M"}}},
+		"b": {Deployed: true, State: StateRunning, Meta: Metadata{Resources: ResourceHints{MemRequest: "2048M", MemLimit: "4096M"}}},
+		"c": {Deployed: true, State: StateStopped, Meta: Metadata{Resources: ResourceHints{MemRequest: "900M"}}},
+		"d": {Deployed: false, Meta: Metadata{Resources: ResourceHints{MemRequest: "500M"}}},
+	}}
+	_, reserved, used, ok := m.InstallMemory()
+	if !ok {
+		t.Skip("no memory reading on this platform")
+	}
+	if used != 2648 {
+		t.Fatalf("„used\" must be the running installed apps' requests (600+2048), got %d", used)
+	}
+	if reserved != cfg.System.ReservedMemoryMB {
+		t.Fatalf("reserve %d, want the configured %d", reserved, cfg.System.ReservedMemoryMB)
+	}
+	src, err := os.ReadFile("../web/handlers.go")
+	if err != nil {
+		t.Fatal(err)
+	}
+	body := string(src)
+	i := strings.Index(body, "// Memory info for deploy page")
+	if i < 0 {
+		t.Fatal("the deploy page's memory block moved — re-point this test")
+	}
+	block := body[i : i+1200]
+	if !strings.Contains(block, "s.stackMgr.InstallMemory()") || strings.Contains(block, "system.GetMemoryMB()") {
+		t.Fatalf("the install page must read memory through InstallMemory, the check's own reading:\n%s", block)
+	}
+}
diff --git a/controller/internal/web/handlers.go b/controller/internal/web/handlers.go
index ef20414..7c24179 100644
--- a/controller/internal/web/handlers.go
+++ b/controller/internal/web/handlers.go
@@ -688,9 +688,10 @@ func (s *Server) deployPageData(w http.ResponseWriter, r *http.Request, name str
 	// Memory info for deploy page (only for non-deployed apps)
 	if !alreadyDeployed {
 		memInfo := map[string]interface{}{"Available": false}
-		totalMB, usedMB, memErr := system.GetMemoryMB()
-		if memErr == nil {
-			reservedMB := s.cfg.System.ReservedMemoryMB
+		// R-1125: the SAME reading the install check judges (guest total, reserve, installed apps' requests) — never
+		// /proc's host-wide figures, which made this box and the check disagree.
+		totalMB, reservedMB, usedMB, memOK := s.stackMgr.InstallMemory()
+		if memOK {
 			usableMB := totalMB - reservedMB
 			newReqMB := stacks.ParseMemoryMB(meta.Resources.MemRequest)
 			afterMB := usedMB + newReqMB