diff --git a/PTC/management_dashboard_preview.html b/PTC/management_dashboard_preview.html
index 4b80db9..af949d3 100644
--- a/PTC/management_dashboard_preview.html
+++ b/PTC/management_dashboard_preview.html
@@ -1924,6 +1924,7 @@
             const [lifecycleAccountDetailModalLoading, setLifecycleAccountDetailModalLoading] = useState(false);
             const [lifecycleAllocationModal, setLifecycleAllocationModal] = useState(null);
             const [lifecycleAllocationSaving, setLifecycleAllocationSaving] = useState(false);
+            const [lifecycleCommonAllocationSaving, setLifecycleCommonAllocationSaving] = useState(false);
             const [projectEditModalOpen, setProjectEditModalOpen] = useState(false);
             const [relatedProjectSearch, setRelatedProjectSearch] = useState("");
             const [projectTxnDateFrom, setProjectTxnDateFrom] = useState("");
@@ -3605,6 +3606,34 @@
                 }
             }
 
+            async function saveLifecycleCommonAllocationMode(nextMode) {
+                if (!selectedProjectCode) return;
+                if (!["expense_ratio", "income_ratio"].includes(nextMode || "")) return;
+                setLifecycleCommonAllocationSaving(true);
+                setError("");
+                try {
+                    const res = await fetch(`${API_BASE}/api/lifecycle-common-allocation/upsert`, {
+                        method: "POST",
+                        headers: { "Content-Type": "application/json" },
+                        body: JSON.stringify({
+                            base_project_code: selectedProjectCode,
+                            allocation_mode: nextMode,
+                        }),
+                    });
+                    if (!res.ok) throw new Error("common allocation save failed");
+                    await res.json();
+                    const detailRes = await fetch(`${API_BASE}/api/project-detail?${detailQuery}`);
+                    if (detailRes.ok) {
+                        const nextDetail = await detailRes.json();
+                        setDetail(nextDetail);
+                    }
+                } catch (err) {
+                    setError("공통배분 기준 저장에 실패했습니다.");
+                } finally {
+                    setLifecycleCommonAllocationSaving(false);
+                }
+            }
+
             async function saveProjectMaster() {
                 if (!selectedProjectCode) return;
                 setSaving(true);
@@ -5182,6 +5211,19 @@
                                                 <div>
                                                     <div style={{ fontSize: 18, fontWeight: 700 }}>프로젝트 생애주기 원가</div>
                                                     <div className="subtle" style={{ marginTop: 6 }}>현재 시공 프로젝트를 포함해 연결된 전체 비용을 시공비, 인건비, 관리비로 나눠 봅니다.</div>
+                                                    <div style={{ display: "flex", alignItems: "center", gap: 8, marginTop: 10 }}>
+                                                        <span className="subtle">공통배분 기준</span>
+                                                        <select
+                                                            value={detail?.lifecycle_cost?.summary?.common_allocation_mode || "expense_ratio"}
+                                                            onChange={(e) => saveLifecycleCommonAllocationMode(e.target.value)}
+                                                            disabled={lifecycleCommonAllocationSaving}
+                                                            style={{ minWidth: 220 }}
+                                                        >
+                                                            <option value="expense_ratio">프로젝트 지출 / 전체지출 비율</option>
+                                                            <option value="income_ratio">프로젝트 입금 / 전체입금 비율</option>
+                                                        </select>
+                                                        <span className="subtle">{lifecycleCommonAllocationSaving ? "저장 중..." : ""}</span>
+                                                    </div>
                                                 </div>
                                                 <div style={{ display: "grid", gridTemplateColumns: "repeat(4, minmax(120px, 1fr))", gap: 24, minWidth: 560 }}>
                                                     <div>
diff --git a/server/ptc_api_server.py b/server/ptc_api_server.py
index a1fabe2..4b008bf 100644
--- a/server/ptc_api_server.py
+++ b/server/ptc_api_server.py
@@ -623,6 +623,15 @@ def init_db() -> None:
         )
         """
     )
+    cur.execute(
+        """
+        create table if not exists project_lifecycle_common_allocation (
+            base_project_code text primary key,
+            allocation_mode text not null default 'expense_ratio',
+            updated_at text not null
+        )
+        """
+    )
     cur.execute(
         """
         create table if not exists project_budget_lines (
@@ -1006,6 +1015,23 @@ def fetch_lifecycle_allocation_map(conn: sqlite3.Connection, base_project_code:
     return allocation_map
 
 
+def fetch_lifecycle_common_allocation_mode(conn: sqlite3.Connection, base_project_code: str) -> str:
+    if not base_project_code:
+        return "expense_ratio"
+    row = conn.execute(
+        """
+        select allocation_mode
+        from project_lifecycle_common_allocation
+        where base_project_code = ?
+        """,
+        (base_project_code,),
+    ).fetchone()
+    mode = (row["allocation_mode"] or "").strip() if row else ""
+    if mode not in {"expense_ratio", "income_ratio"}:
+        return "expense_ratio"
+    return mode
+
+
 def resolve_lifecycle_allocation(project_type: str, allocation_item: dict | None) -> tuple[int, int, float]:
     if project_type in {"영업", "설계"}:
         if allocation_item:
@@ -1052,7 +1078,9 @@ def _iter_year_months(start_ym: str, end_ym: str):
             m = 1
 
 
-def calculate_monthly_shared_distribution(conn: sqlite3.Connection, base_project_code: str) -> dict:
+def calculate_monthly_shared_distribution(
+    conn: sqlite3.Connection, base_project_code: str, allocation_mode: str = "expense_ratio"
+) -> dict:
     project_rows = conn.execute(
         """
         select
@@ -1117,10 +1145,33 @@ def calculate_monthly_shared_distribution(conn: sqlite3.Connection, base_project
     if not candidate_months:
         return {"labor_shared": 0.0, "common_shared": 0.0}
 
-    month_active_counts: dict[str, int] = defaultdict(int)
-    for start_ym, end_ym in project_ranges.values():
+    month_active_projects: dict[str, set[str]] = defaultdict(set)
+    for project_code, (start_ym, end_ym) in project_ranges.items():
         for ym in _iter_year_months(start_ym, end_ym):
-            month_active_counts[ym] += 1
+            month_active_projects[ym].add(project_code)
+
+    monthly_project_amount_rows = conn.execute(
+        """
+        select
+            substr(coalesce(transaction_date, ''), 1, 7) as ym,
+            coalesce(project_code, '') as project_code,
+            coalesce(sum(case when in_out = '출금' then supply_amount else 0 end), 0) as expense_supply,
+            coalesce(sum(case when in_out = '입금' then supply_amount else 0 end), 0) as income_supply
+        from ptc_transactions
+        where coalesce(transaction_date, '') <> ''
+        group by substr(coalesce(transaction_date, ''), 1, 7), coalesce(project_code, '')
+        having ym <> ''
+        """
+    ).fetchall()
+    project_expense_by_month: dict[tuple[str, str], float] = defaultdict(float)
+    project_income_by_month: dict[tuple[str, str], float] = defaultdict(float)
+    for row in monthly_project_amount_rows:
+        ym = (row["ym"] or "").strip()
+        project_code = (row["project_code"] or "").strip()
+        if not ym or not project_code:
+            continue
+        project_expense_by_month[(ym, project_code)] += float(row["expense_supply"] or 0)
+        project_income_by_month[(ym, project_code)] += float(row["income_supply"] or 0)
 
     base_start_ym, base_end_ym = base_range
     labor_shared = 0.0
@@ -1130,11 +1181,26 @@ def calculate_monthly_shared_distribution(conn: sqlite3.Connection, base_project
             continue
         if base_end_ym and ym > base_end_ym:
             continue
-        active_count = int(month_active_counts.get(ym) or 0)
-        if active_count <= 0:
+        active_projects = sorted(month_active_projects.get(ym) or [])
+        if not active_projects:
             continue
-        labor_shared += float(labor_pool_by_month.get(ym) or 0) / active_count
-        common_shared += float(common_pool_by_month.get(ym) or 0) / active_count
+        if base_project_code not in active_projects:
+            continue
+
+        if allocation_mode == "income_ratio":
+            base_value = float(project_income_by_month.get((ym, base_project_code)) or 0.0)
+            total_value = sum(float(project_income_by_month.get((ym, code)) or 0.0) for code in active_projects)
+        else:
+            base_value = float(project_expense_by_month.get((ym, base_project_code)) or 0.0)
+            total_value = sum(float(project_expense_by_month.get((ym, code)) or 0.0) for code in active_projects)
+
+        if total_value > 0:
+            ratio = max(0.0, min(1.0, base_value / total_value))
+        else:
+            ratio = 1.0 / len(active_projects)
+
+        labor_shared += float(labor_pool_by_month.get(ym) or 0) * ratio
+        common_shared += float(common_pool_by_month.get(ym) or 0) * ratio
 
     return {"labor_shared": labor_shared, "common_shared": common_shared}
 
@@ -1369,7 +1435,8 @@ def build_project_lifecycle_cost(
             account_entry["shared_expense_supply"] += shared_expense_supply
             account_entry["expense_supply"] += base_expense_supply
 
-    monthly_shared = calculate_monthly_shared_distribution(conn, base_project_code)
+    common_allocation_mode = fetch_lifecycle_common_allocation_mode(conn, base_project_code)
+    monthly_shared = calculate_monthly_shared_distribution(conn, base_project_code, common_allocation_mode)
     labor_shared = float(monthly_shared.get("labor_shared") or 0.0)
     common_shared = float(monthly_shared.get("common_shared") or 0.0)
     base_project_info = project_lookup.get(base_project_code) or {}
@@ -1502,6 +1569,7 @@ def build_project_lifecycle_cost(
             "income_supply": total_income,
             "expense_supply": total_expense,
             "profit_supply": total_income - total_expense,
+            "common_allocation_mode": common_allocation_mode,
         },
     }
 
@@ -2199,6 +2267,42 @@ class Handler(BaseHTTPRequestHandler):
                 self._send(200, {"ok": True})
                 return
 
+            if parsed.path == "/api/lifecycle-common-allocation/upsert":
+                payload = self._read_json()
+                base_project_code = str(payload.get("base_project_code", "")).strip()
+                allocation_mode = str(payload.get("allocation_mode", "")).strip()
+                if not base_project_code:
+                    self._send(400, {"ok": False, "message": "base_project_code is required"})
+                    return
+                if allocation_mode not in {"expense_ratio", "income_ratio"}:
+                    self._send(400, {"ok": False, "message": "allocation_mode must be expense_ratio or income_ratio"})
+                    return
+                updated_at = datetime.now().isoformat()
+                conn.execute(
+                    """
+                    insert into project_lifecycle_common_allocation (
+                        base_project_code, allocation_mode, updated_at
+                    ) values (?, ?, ?)
+                    on conflict(base_project_code) do update set
+                        allocation_mode = excluded.allocation_mode,
+                        updated_at = excluded.updated_at
+                    """,
+                    (base_project_code, allocation_mode, updated_at),
+                )
+                conn.commit()
+                self._send(
+                    200,
+                    {
+                        "ok": True,
+                        "item": {
+                            "base_project_code": base_project_code,
+                            "allocation_mode": allocation_mode,
+                            "updated_at": updated_at,
+                        },
+                    },
+                )
+                return
+
             if parsed.path == "/api/project-master/batch-update-method":
                 payload = self._read_json()
                 project_codes = payload.get("project_codes", [])