diff --git a/saju-lab/app/calculator/compatibility.py b/saju-lab/app/calculator/compatibility.py
new file mode 100644
index 0000000..5889418
--- /dev/null
+++ b/saju-lab/app/calculator/compatibility.py
@@ -0,0 +1,170 @@
+"""사주 궁합 — 두 사주 일주(일간 오행 + 일지) 매칭 → 점수 + breakdown.
+
+saju-web/app/compatibility/result/page.tsx의 `calculateCompatibility()` 로직과 1:1 매핑.
+
+알고리즘 (saju-web TS):
+    score = 50 (base)
+    # 일간 오행 관계
+    if element1 == element2:               score += 10  # same
+    elif relation in (produce, produced):  score += 25  # 상생
+    else:                                  score -= 10  # 상극 (overcome/overcomed)
+    # 일지 관계
+    if 六合 (sixHarmony):                  score += 20
+    if 三合 (threeHarmony):                score += 15
+    if 沖 (conflict):                      score -= 20
+    score = clamp(score, 0, 100)
+
+breakdown은 saju-lab 자체 확장 — 각 카테고리별 (score_delta, description) 제공.
+"""
+from __future__ import annotations
+
+from typing import Any, Dict
+
+from .constants import FIVE_ELEMENTS
+
+
+# ─── 오행 상생/상극 맵 (saju-web과 동일) ──────────────────────────────
+_PRODUCE_MAP = {"木": "火", "火": "土", "土": "金", "金": "水", "水": "木"}
+_OVERCOME_MAP = {"木": "土", "火": "金", "土": "水", "金": "木", "水": "火"}
+
+
+# ─── 지지 6합/6충 (saju-web과 동일) ───────────────────────────────────
+_SIX_HARMONY: dict[str, str] = {
+    "子": "丑", "丑": "子",
+    "寅": "亥", "亥": "寅",
+    "卯": "戌", "戌": "卯",
+    "辰": "酉", "酉": "辰",
+    "巳": "申", "申": "巳",
+    "午": "未", "未": "午",
+}
+
+# 삼합 그룹 (saju-web threeHarmonyGroups)
+_THREE_HARMONY_GROUPS = [
+    {"申", "子", "辰"},
+    {"寅", "午", "戌"},
+    {"亥", "卯", "未"},
+    {"巳", "酉", "丑"},
+]
+
+_CONFLICT: dict[str, str] = {
+    "子": "午", "午": "子",
+    "丑": "未", "未": "丑",
+    "寅": "申", "申": "寅",
+    "卯": "酉", "酉": "卯",
+    "辰": "戌", "戌": "辰",
+    "巳": "亥", "亥": "巳",
+}
+
+
+# ─── 헬퍼 ─────────────────────────────────────────────────────────
+def _get_element_relation(el1: str, el2: str) -> str:
+    """오행 관계 판별 — 'same' | 'produce' | 'produced' | 'overcome' | 'overcomed'.
+
+    saju-web getElementRelation()과 1:1 매핑.
+    """
+    if el1 == el2:
+        return "same"
+    if _PRODUCE_MAP.get(el1) == el2:
+        return "produce"      # el1 → el2 (el1이 el2를 생함)
+    if _PRODUCE_MAP.get(el2) == el1:
+        return "produced"     # el2 → el1 (el1이 el2로부터 생을 받음)
+    if _OVERCOME_MAP.get(el1) == el2:
+        return "overcome"     # el1 → el2 (el1이 el2를 극함)
+    return "overcomed"        # el2 → el1 (el1이 el2로부터 극을 받음)
+
+
+def _get_branch_relation(b1: str, b2: str) -> dict:
+    """지지 관계 — {six_harmony, three_harmony, conflict}.
+
+    saju-web getBranchRelation()과 1:1 매핑.
+    """
+    is_three = any(b1 in g and b2 in g and b1 != b2 for g in _THREE_HARMONY_GROUPS)
+    return {
+        "six_harmony": _SIX_HARMONY.get(b1) == b2,
+        "three_harmony": is_three,
+        "conflict": _CONFLICT.get(b1) == b2,
+    }
+
+
+def _day_master_element_delta(el1: str, el2: str) -> tuple[int, str, str]:
+    """일간 오행 매칭 delta + relation tag + description."""
+    relation = _get_element_relation(el1, el2)
+    if relation == "same":
+        return 10, "same", f"일간 동일 오행 ({el1}={el2}) — 안정적이나 자극은 약함"
+    if relation in ("produce", "produced"):
+        return 25, relation, f"일간 상생 관계 ({el1}↔{el2}) — 서로 도움"
+    # overcome / overcomed
+    return -10, relation, f"일간 상극 관계 ({el1}↔{el2}) — 갈등 가능"
+
+
+def _branch_interaction_delta(b1: str, b2: str) -> tuple[int, dict, str]:
+    """일지 6합/3합/충 delta + flags + description."""
+    rel = _get_branch_relation(b1, b2)
+    delta = 0
+    parts: list[str] = []
+    if rel["six_harmony"]:
+        delta += 20
+        parts.append(f"일지 6합 ({b1}+{b2})")
+    if rel["three_harmony"]:
+        delta += 15
+        parts.append(f"일지 3합 ({b1}+{b2})")
+    if rel["conflict"]:
+        delta -= 20
+        parts.append(f"일지 충 ({b1}↔{b2})")
+    if not parts:
+        parts.append(f"일지 중립 ({b1}, {b2})")
+    return delta, rel, " · ".join(parts)
+
+
+# ─── 메인 ─────────────────────────────────────────────────────────
+def calculate_compatibility(saju_a: Dict[str, Any], saju_b: Dict[str, Any]) -> Dict[str, Any]:
+    """두 사주 궁합 점수 (0~100) + breakdown.
+
+    Args:
+        saju_a: calculate_saju() 결과 — `saju["day"]["stem"|"branch"|"element"]` 사용
+        saju_b: 두 번째 사주
+
+    Returns:
+        {
+            "score": int (0~100),
+            "breakdown": {
+                "day_master_element": {"score": int, "relation": str, "description": str},
+                "branch_interaction": {"score": int, "flags": dict, "description": str},
+                "base": 50,
+            }
+        }
+    """
+    day_a = saju_a["day"]
+    day_b = saju_b["day"]
+
+    el1 = day_a.get("element") or FIVE_ELEMENTS[day_a["stem"]]
+    el2 = day_b.get("element") or FIVE_ELEMENTS[day_b["stem"]]
+    b1 = day_a["branch"]
+    b2 = day_b["branch"]
+
+    elem_delta, elem_relation, elem_desc = _day_master_element_delta(el1, el2)
+    branch_delta, branch_flags, branch_desc = _branch_interaction_delta(b1, b2)
+
+    raw = 50 + elem_delta + branch_delta
+    score = max(0, min(100, raw))
+
+    return {
+        "score": score,
+        "breakdown": {
+            "base": 50,
+            "day_master_element": {
+                "score": elem_delta,
+                "relation": elem_relation,
+                "element_a": el1,
+                "element_b": el2,
+                "description": elem_desc,
+            },
+            "branch_interaction": {
+                "score": branch_delta,
+                "flags": branch_flags,
+                "branch_a": b1,
+                "branch_b": b2,
+                "description": branch_desc,
+            },
+        },
+    }
diff --git a/saju-lab/tests/test_compatibility.py b/saju-lab/tests/test_compatibility.py
new file mode 100644
index 0000000..0f68cce
--- /dev/null
+++ b/saju-lab/tests/test_compatibility.py
@@ -0,0 +1,174 @@
+"""compatibility.py — 두 사주 궁합 점수 + breakdown 검증.
+
+saju-web/app/compatibility/result/page.tsx의 calculateCompatibility() 로직과 1:1 매핑.
+알고리즘:
+    score = 50 (base)
+    + (same: +10 / produce|produced: +25 / overcome|overcomed: -10)
+    + (six_harmony: +20)
+    + (three_harmony: +15)
+    + (conflict: -20)
+    clamp(0, 100)
+"""
+from app.calculator.core import calculate_saju
+from app.calculator.compatibility import (
+    calculate_compatibility,
+    _get_element_relation,
+    _get_branch_relation,
+)
+
+
+# ─── 단위 헬퍼 테스트 ─────────────────────────────────────────────
+def test_element_relation_same():
+    assert _get_element_relation("金", "金") == "same"
+
+
+def test_element_relation_produce():
+    # 木 → 火 (목생화): el1 produces el2
+    assert _get_element_relation("木", "火") == "produce"
+
+
+def test_element_relation_produced():
+    # 火 ← 木: el1 is produced by el2
+    assert _get_element_relation("火", "木") == "produced"
+
+
+def test_element_relation_overcome():
+    # 金 → 木 (금극목): el1 overcomes el2
+    assert _get_element_relation("金", "木") == "overcome"
+
+
+def test_element_relation_overcomed():
+    # 木 ← 金: el1 is overcomed by el2
+    assert _get_element_relation("木", "金") == "overcomed"
+
+
+def test_branch_six_harmony():
+    # 寅亥 6합
+    rel = _get_branch_relation("寅", "亥")
+    assert rel["six_harmony"] is True
+    assert rel["conflict"] is False
+
+
+def test_branch_three_harmony():
+    # 亥卯未 3합 — 亥+卯
+    rel = _get_branch_relation("亥", "卯")
+    assert rel["three_harmony"] is True
+
+
+def test_branch_conflict():
+    # 子午 충
+    rel = _get_branch_relation("子", "午")
+    assert rel["conflict"] is True
+    assert rel["six_harmony"] is False
+
+
+def test_branch_same_not_three_harmony():
+    # 같은 지지는 3합 아님 (서로 다른 지지여야 합)
+    rel = _get_branch_relation("未", "未")
+    assert rel["three_harmony"] is False
+    assert rel["six_harmony"] is False
+    assert rel["conflict"] is False
+
+
+# ─── 통합: calculate_compatibility ────────────────────────────────
+def test_score_range_0_to_100():
+    """모든 결과가 0~100 범위 + breakdown 키 존재."""
+    a = calculate_saju(1990, 5, 15, 14, "male")
+    b = calculate_saju(1992, 8, 8, 18, "female")
+    result = calculate_compatibility(a, b)
+    assert 0 <= result["score"] <= 100
+    assert "breakdown" in result
+    bd = result["breakdown"]
+    assert "day_master_element" in bd
+    assert "branch_interaction" in bd
+    assert "base" in bd
+    assert bd["base"] == 50
+
+
+def test_same_saju_same_element_no_branch_interaction():
+    """1990-5-15 male+female (둘 다 辛未/金) — same element, 같은 일지(중립) → 50+10+0 = 60."""
+    a = calculate_saju(1990, 5, 15, 14, "male")
+    b = calculate_saju(1990, 5, 15, 14, "female")
+    result = calculate_compatibility(a, b)
+    assert result["score"] == 60
+    assert result["breakdown"]["day_master_element"]["relation"] == "same"
+    assert result["breakdown"]["day_master_element"]["score"] == 10
+    assert result["breakdown"]["branch_interaction"]["score"] == 0
+
+
+def test_six_harmony_plus_produce_high_score():
+    """1985-1-1 (辛卯/金) + 1989-7-10 (壬戌/水)
+       → 金生水 (produce) +25, 卯戌 6합 +20 → 50+25+20 = 95.
+    """
+    a = calculate_saju(1985, 1, 1, 12, "male")
+    b = calculate_saju(1989, 7, 10, 12, "male")
+    result = calculate_compatibility(a, b)
+    assert result["score"] == 95
+    bd = result["breakdown"]
+    assert bd["day_master_element"]["relation"] == "produce"
+    assert bd["day_master_element"]["score"] == 25
+    assert bd["branch_interaction"]["flags"]["six_harmony"] is True
+    assert bd["branch_interaction"]["score"] == 20
+
+
+def test_conflict_plus_overcome_low_score():
+    """1988-12-1 (辛巳/金) + 1992-3-21 (丁亥/火)
+       → 火→金 (overcomed) -10, 巳亥 충 -20 → 50-10-20 = 20.
+    """
+    a = calculate_saju(1988, 12, 1, 8, "male")
+    b = calculate_saju(1992, 3, 21, 12, "female")
+    result = calculate_compatibility(a, b)
+    assert result["score"] == 20
+    bd = result["breakdown"]
+    assert bd["day_master_element"]["relation"] in ("overcome", "overcomed")
+    assert bd["day_master_element"]["score"] == -10
+    assert bd["branch_interaction"]["flags"]["conflict"] is True
+    assert bd["branch_interaction"]["score"] == -20
+
+
+def test_three_harmony_plus_overcome():
+    """1990-5-15 (辛未/金) + 1992-3-21 (丁亥/火)
+       → 火→金 (overcomed) -10, 亥未 3합 +15 → 50-10+15 = 55.
+    """
+    a = calculate_saju(1990, 5, 15, 14, "male")
+    b = calculate_saju(1992, 3, 21, 12, "female")
+    result = calculate_compatibility(a, b)
+    assert result["score"] == 55
+    bd = result["breakdown"]
+    assert bd["day_master_element"]["score"] == -10
+    assert bd["branch_interaction"]["flags"]["three_harmony"] is True
+    assert bd["branch_interaction"]["score"] == 15
+
+
+def test_score_clamp_at_zero():
+    """극단 케이스: 상극(-10) + 충(-20) = 50-30 = 20. clamp 동작 검증을 위한 sanity."""
+    a = calculate_saju(1988, 12, 1, 8, "male")
+    b = calculate_saju(1992, 3, 21, 12, "female")
+    result = calculate_compatibility(a, b)
+    assert result["score"] >= 0
+    assert result["score"] <= 100
+
+
+def test_breakdown_descriptions_non_empty():
+    """breakdown의 description 필드가 비어있지 않음."""
+    a = calculate_saju(1990, 5, 15, 14, "male")
+    b = calculate_saju(1995, 8, 20, 12, "female")
+    result = calculate_compatibility(a, b)
+    bd = result["breakdown"]
+    assert isinstance(bd["day_master_element"]["description"], str)
+    assert len(bd["day_master_element"]["description"]) > 0
+    assert isinstance(bd["branch_interaction"]["description"], str)
+    assert len(bd["branch_interaction"]["description"]) > 0
+
+
+def test_symmetry_score_only():
+    """A vs B와 B vs A의 score는 동일 (saju-web과 동일하게 대칭).
+
+    note: relation 태그(produce vs produced)는 비대칭일 수 있으나
+    delta 점수는 동일하므로 최종 score는 항상 같다.
+    """
+    a = calculate_saju(1985, 1, 1, 12, "male")
+    b = calculate_saju(1989, 7, 10, 12, "male")
+    r_ab = calculate_compatibility(a, b)
+    r_ba = calculate_compatibility(b, a)
+    assert r_ab["score"] == r_ba["score"]