diff --git a/saju-lab/app/calculator/lunar.py b/saju-lab/app/calculator/lunar.py
new file mode 100644
index 0000000..83ae3d1
--- /dev/null
+++ b/saju-lab/app/calculator/lunar.py
@@ -0,0 +1,23 @@
+"""음력↔양력 변환 — sxtwl 기반."""
+import sxtwl
+
+
+def solar_to_lunar(year: int, month: int, day: int) -> dict:
+    """양력 → 음력 변환. {'year', 'month', 'day', 'is_leap'} 반환."""
+    day_obj = sxtwl.fromSolar(year, month, day)
+    return {
+        "year": day_obj.getLunarYear(),
+        "month": day_obj.getLunarMonth(),
+        "day": day_obj.getLunarDay(),
+        "is_leap": bool(day_obj.isLunarLeap()),
+    }
+
+
+def lunar_to_solar(year: int, month: int, day: int, is_leap: bool = False) -> tuple[int, int, int]:
+    """음력 → 양력 변환. (year, month, day) tuple 반환."""
+    day_obj = sxtwl.fromLunar(year, month, day, is_leap)
+    return (
+        day_obj.getSolarYear(),
+        day_obj.getSolarMonth(),
+        day_obj.getSolarDay(),
+    )
diff --git a/saju-lab/tests/test_lunar.py b/saju-lab/tests/test_lunar.py
new file mode 100644
index 0000000..5f44d3b
--- /dev/null
+++ b/saju-lab/tests/test_lunar.py
@@ -0,0 +1,34 @@
+"""음력↔양력 변환 테스트 (sxtwl 기반)."""
+from app.calculator import lunar
+
+
+def test_solar_to_lunar_known_date():
+    # 2024년 추석 = 양력 2024-09-17 = 음력 2024-08-15
+    result = lunar.solar_to_lunar(2024, 9, 17)
+    assert result["year"] == 2024
+    assert result["month"] == 8
+    assert result["day"] == 15
+    assert result["is_leap"] is False
+
+
+def test_lunar_to_solar_known_date():
+    # 음력 2024-08-15 → 양력 2024-09-17
+    result = lunar.lunar_to_solar(2024, 8, 15, is_leap=False)
+    assert result == (2024, 9, 17)
+
+
+def test_solar_to_lunar_new_years():
+    # 양력 2024-01-01 → 음력 2023-11-20 (음력 새해는 보통 1~2월)
+    result = lunar.solar_to_lunar(2024, 1, 1)
+    assert result["year"] in (2023, 2024)
+    assert result["month"] in range(1, 13)
+
+
+def test_roundtrip():
+    # 양력 → 음력 → 양력 = 원본
+    solar_input = (1990, 5, 15)
+    lunar_mid = lunar.solar_to_lunar(*solar_input)
+    solar_back = lunar.lunar_to_solar(
+        lunar_mid["year"], lunar_mid["month"], lunar_mid["day"], lunar_mid["is_leap"]
+    )
+    assert solar_back == solar_input