diff --git a/saju-lab/app/calculator/constants.py b/saju-lab/app/calculator/constants.py
new file mode 100644
index 0000000..4a2b103
--- /dev/null
+++ b/saju-lab/app/calculator/constants.py
@@ -0,0 +1,66 @@
+"""사주 계산 상수 — saju-web/lib/saju-calculator.ts 의 상수와 1:1 매핑."""
+
+HEAVENLY_STEMS = ["甲", "乙", "丙", "丁", "戊", "己", "庚", "辛", "壬", "癸"]
+HEAVENLY_STEMS_KR = ["갑", "을", "병", "정", "무", "기", "경", "신", "임", "계"]
+
+EARTHLY_BRANCHES = ["子", "丑", "寅", "卯", "辰", "巳", "午", "未", "申", "酉", "戌", "亥"]
+EARTHLY_BRANCHES_KR = ["자", "축", "인", "묘", "진", "사", "오", "미", "신", "유", "술", "해"]
+
+FIVE_ELEMENTS = {
+    "甲": "木", "乙": "木",
+    "丙": "火", "丁": "火",
+    "戊": "土", "己": "土",
+    "庚": "金", "辛": "金",
+    "壬": "水", "癸": "水",
+    "寅": "木", "卯": "木",
+    "巳": "火", "午": "火",
+    "辰": "土", "戌": "土", "丑": "土", "未": "土",
+    "申": "金", "酉": "金",
+    "子": "水", "亥": "水",
+}
+
+FIVE_ELEMENTS_KR = {
+    "木": "목", "火": "화", "土": "토", "金": "금", "水": "수",
+}
+
+IS_YANG_STEM = {
+    "甲": True,  "乙": False,
+    "丙": True,  "丁": False,
+    "戊": True,  "己": False,
+    "庚": True,  "辛": False,
+    "壬": True,  "癸": False,
+}
+
+IS_YANG_BRANCH = {
+    "子": True,  "丑": False,
+    "寅": True,  "卯": False,
+    "辰": True,  "巳": False,
+    "午": True,  "未": False,
+    "申": True,  "酉": False,
+    "戌": True,  "亥": False,
+}
+
+# 지장간: { 지지: [본기, 중기, 여기] (없으면 생략) }
+HIDDEN_STEMS = {
+    "子": ["癸"],
+    "丑": ["己", "癸", "辛"],
+    "寅": ["甲", "丙", "戊"],
+    "卯": ["乙"],
+    "辰": ["戊", "乙", "癸"],
+    "巳": ["丙", "庚", "戊"],
+    "午": ["丁", "己"],
+    "未": ["己", "丁", "乙"],
+    "申": ["庚", "壬", "戊"],
+    "酉": ["辛"],
+    "戌": ["戊", "辛", "丁"],
+    "亥": ["壬", "甲"],
+}
+
+# 본기, 중기, 여기 가중치
+HIDDEN_STEM_WEIGHTS = [1.0, 0.5, 0.3]
+
+# 상생 사이클 (목→화→토→금→수→목)
+SHENG_CYCLE = {"木": "火", "火": "土", "土": "金", "金": "水", "水": "木"}
+
+# 상극 사이클 (목→토→수→화→금→목)
+KE_CYCLE = {"木": "土", "土": "水", "水": "火", "火": "金", "金": "木"}
diff --git a/saju-lab/tests/test_constants.py b/saju-lab/tests/test_constants.py
new file mode 100644
index 0000000..25701fd
--- /dev/null
+++ b/saju-lab/tests/test_constants.py
@@ -0,0 +1,34 @@
+from app.calculator import constants
+
+
+def test_heavenly_stems_10():
+    assert len(constants.HEAVENLY_STEMS) == 10
+    assert constants.HEAVENLY_STEMS[0] == "甲"
+    assert constants.HEAVENLY_STEMS[9] == "癸"
+
+
+def test_earthly_branches_12():
+    assert len(constants.EARTHLY_BRANCHES) == 12
+    assert constants.EARTHLY_BRANCHES[0] == "子"
+    assert constants.EARTHLY_BRANCHES[11] == "亥"
+
+
+def test_five_elements_mapping():
+    assert constants.FIVE_ELEMENTS["甲"] == "木"
+    assert constants.FIVE_ELEMENTS["丁"] == "火"
+    assert constants.FIVE_ELEMENTS["亥"] == "水"
+    assert constants.FIVE_ELEMENTS["申"] == "金"
+    assert constants.FIVE_ELEMENTS["未"] == "土"
+
+
+def test_hidden_stems():
+    assert constants.HIDDEN_STEMS["子"] == ["癸"]
+    assert constants.HIDDEN_STEMS["丑"] == ["己", "癸", "辛"]
+    assert constants.HIDDEN_STEMS["寅"] == ["甲", "丙", "戊"]
+
+
+def test_yang_yin_stems():
+    assert constants.IS_YANG_STEM["甲"] is True
+    assert constants.IS_YANG_STEM["乙"] is False
+    assert constants.IS_YANG_STEM["庚"] is True
+    assert constants.IS_YANG_STEM["辛"] is False