diff --git a/cmd/metal-api/internal/metal/machine.go b/cmd/metal-api/internal/metal/machine.go
index ecd246604..339e9cf85 100644
--- a/cmd/metal-api/internal/metal/machine.go
+++ b/cmd/metal-api/internal/metal/machine.go
@@ -187,6 +187,22 @@ func (ms Machines) WithSize(id string) Machines {
 	return res
 }
 
+func (ms Machines) WithPartition(id string) Machines {
+	var res Machines
+
+	for _, m := range ms {
+		m := m
+
+		if m.PartitionID != id {
+			continue
+		}
+
+		res = append(res, m)
+	}
+
+	return res
+}
+
 // MachineNetwork stores the Network details of the machine
 type MachineNetwork struct {
 	NetworkID           string   `rethinkdb:"networkid" json:"networkid"`
diff --git a/cmd/metal-api/internal/service/partition-service.go b/cmd/metal-api/internal/service/partition-service.go
index 615dc95b8..ddd397f01 100644
--- a/cmd/metal-api/internal/service/partition-service.go
+++ b/cmd/metal-api/internal/service/partition-service.go
@@ -461,7 +461,7 @@ func (r *partitionResource) calcPartitionCapacity(pcr *v1.PartitionCapacityReque
 				reservation := reservation
 
 				cap.Reservations += reservation.Amount
-				cap.UsedReservations += min(len(machinesByProject[reservation.ProjectID]), reservation.Amount)
+				cap.UsedReservations += min(len(machinesByProject[reservation.ProjectID].WithSize(size.ID).WithPartition(pc.ID)), reservation.Amount)
 			}
 		}
 
diff --git a/cmd/metal-api/internal/service/size-service.go b/cmd/metal-api/internal/service/size-service.go
index b0a1736d0..8afb0beb5 100644
--- a/cmd/metal-api/internal/service/size-service.go
+++ b/cmd/metal-api/internal/service/size-service.go
@@ -70,6 +70,7 @@ func (r *sizeResource) webService() *restful.WebService {
 		Doc("get all size reservations").
 		Metadata(restfulspec.KeyOpenAPITags, tags).
 		Metadata(auditing.Exclude, true).
+		Reads(v1.EmptyBody{}).
 		Writes([]v1.SizeReservationResponse{}).
 		Returns(http.StatusOK, "OK", []v1.SizeReservationResponse{}).
 		DefaultReturns("Error", httperrors.HTTPErrorResponse{}))
@@ -454,7 +455,7 @@ func (r *sizeResource) listSizeReservations(request *restful.Request, response *
 
 			for _, partitionID := range reservation.PartitionIDs {
 				project := pointer.SafeDeref(projectsByID[reservation.ProjectID])
-				allocations := len(machinesByProjectID[reservation.ProjectID].WithSize(size.ID))
+				allocations := len(machinesByProjectID[reservation.ProjectID].WithPartition(partitionID).WithSize(size.ID))
 
 				result = append(result, &v1.SizeReservationResponse{
 					SizeID:             size.ID,
diff --git a/spec/metal-api.json b/spec/metal-api.json
index eba6d34a2..2f26cb4ca 100644
--- a/spec/metal-api.json
+++ b/spec/metal-api.json
@@ -8939,6 +8939,16 @@
           "application/json"
         ],
         "operationId": "listSizeReservations",
+        "parameters": [
+          {
+            "in": "body",
+            "name": "body",
+            "required": true,
+            "schema": {
+              "$ref": "#/definitions/v1.EmptyBody"
+            }
+          }
+        ],
         "produces": [
           "application/json"
         ],