diff --git a/app/api/study_question_progress.py b/app/api/study_question_progress.py
index 7b0a4ac..fb85269 100644
--- a/app/api/study_question_progress.py
+++ b/app/api/study_question_progress.py
@@ -26,8 +26,8 @@ from models.user import User
 
 router = APIRouter(prefix="/study-topics", tags=["study-progress"])
 
-# 1차 due_at 부여 시 디폴트 1일 뒤
-DEFAULT_FIRST_DUE_DAYS = 1
+# 1차 due_at 부여 시 디폴트 1일 뒤 — SR 상수는 sr_schedule.py 단일 source (재-export).
+from services.study.sr_schedule import DEFAULT_FIRST_DUE_DAYS  # noqa: E402,F401
 
 
 def _verify_topic_owner(topic: StudyTopic | None, user: User) -> None:
diff --git a/app/services/study/session_finalize.py b/app/services/study/session_finalize.py
index 4cb8854..fb10a16 100644
--- a/app/services/study/session_finalize.py
+++ b/app/services/study/session_finalize.py
@@ -40,10 +40,13 @@ from services.study.learning_pattern import (
     compute_pattern_state,
 )
 
-# review_stage 별 다음 due_at interval (days)
-REVIEW_INTERVAL_DAYS = {1: 3, 2: 7, 3: 14}
-REVIEW_STAGE_MASTERED = 4
-DEFAULT_FIRST_DUE_DAYS = 1
+# SR 산술은 sr_schedule.py 단일 source (문제 SR + 카드 SR 공용). 상수는 재-export 유지.
+from services.study.sr_schedule import (  # noqa: E402
+    DEFAULT_FIRST_DUE_DAYS,  # noqa: F401
+    REVIEW_INTERVAL_DAYS,  # noqa: F401
+    REVIEW_STAGE_MASTERED,  # noqa: F401
+    advance as sr_advance,
+)
 
 
 @dataclass
@@ -185,19 +188,11 @@ async def finalize_session(
         progress.pattern_updated_at = now
         progress.pattern_window_attempts = window_size
 
-        # 복습 stage 갱신 — 이미 due_at 박힌 문제만
+        # 복습 stage 갱신 — 이미 due_at 박힌 문제만 (산술은 sr_schedule 공용)
         if progress.due_at is not None:
-            if outcome == "correct":
-                progress.review_stage = (progress.review_stage or 0) + 1
-                if progress.review_stage >= REVIEW_STAGE_MASTERED:
-                    progress.due_at = None  # 학습완료
-                else:
-                    days = REVIEW_INTERVAL_DAYS[progress.review_stage]
-                    progress.due_at = now + timedelta(days=days)
-            elif outcome in ("wrong", "unsure"):
-                progress.review_stage = 0
-                progress.due_at = now + timedelta(days=DEFAULT_FIRST_DUE_DAYS)
-            # skipped 는 due_at 그대로 (큐 유지, stage 변경 안 함)
+            result = sr_advance(progress.review_stage, outcome, now)
+            if result is not None:  # skipped 는 None → due_at/stage 불변
+                progress.review_stage, progress.due_at = result
         # progress.due_at IS NULL 일반 풀이 → stage 건드리지 않음
 
     # 4. 바로 할 일 카운트 (요약 응답용) — finalize 직후 progress 상태 기준 SQL 한 번
diff --git a/app/services/study/sr_schedule.py b/app/services/study/sr_schedule.py
new file mode 100644
index 0000000..518acd3
--- /dev/null
+++ b/app/services/study/sr_schedule.py
@@ -0,0 +1,48 @@
+"""SR(간격반복) 산술 단일 source — 문제 SR + 카드 SR 공용.
+
+session_finalize.py(문제 SR)와 study_memo_card writer(카드 SR)가 같은 상수·산술을 참조하도록
+순수함수로 추출. 진입 게이트(due_at IS NOT NULL 행만 갱신 / 최초 due 부여 / skipped 불변)는
+호출부에 남긴다 — finalize 와 review-complete 의 정책이 미묘히 달라 통합 시 회귀 위험.
+
+정본 간격(실측): review_stage 0→1→2→3 = 1·3·7·14일, stage4 = 졸업(due_at=NULL),
+오답/모호 리셋 = 내일(stage 0).
+"""
+
+from __future__ import annotations
+
+from datetime import datetime, timedelta
+
+# review_stage 별 '다음 due_at' interval (days). stage 1→3일, 2→7일, 3→14일.
+REVIEW_INTERVAL_DAYS = {1: 3, 2: 7, 3: 14}
+# 이 stage 도달 시 졸업 (due_at=NULL, 복습 큐에서 제거)
+REVIEW_STAGE_MASTERED = 4
+# 최초 due 부여 / 오답 리셋 = 내일
+DEFAULT_FIRST_DUE_DAYS = 1
+
+
+def advance(
+    review_stage: int | None, outcome: str, now: datetime
+) -> tuple[int, datetime | None] | None:
+    """이미 복습 큐(due_at IS NOT NULL)에 있는 항목의 SR 갱신 산술.
+
+    호출부가 'due_at IS NOT NULL' 가드 후 호출한다.
+    반환:
+      (new_stage, new_due_at) — correct/wrong/unsure. 졸업이면 new_due_at=None.
+      None — skipped/기타(변경 없음, 호출부가 무시).
+    """
+    if outcome == "correct":
+        new_stage = (review_stage or 0) + 1
+        if new_stage >= REVIEW_STAGE_MASTERED:
+            return new_stage, None  # 학습완료(졸업)
+        return new_stage, now + timedelta(days=REVIEW_INTERVAL_DAYS[new_stage])
+    if outcome in ("wrong", "unsure"):
+        return 0, now + timedelta(days=DEFAULT_FIRST_DUE_DAYS)
+    return None  # skipped — due_at/stage 불변
+
+
+def first_due(now: datetime) -> tuple[int, datetime]:
+    """복습 큐 최초 진입(오답/모호 + due_at IS NULL) 시 부여값.
+
+    문제 review-complete / 카드 첫 회상 공용. 반환: (review_stage=0, due_at=내일).
+    """
+    return 0, now + timedelta(days=DEFAULT_FIRST_DUE_DAYS)
diff --git a/tests/test_sr_schedule.py b/tests/test_sr_schedule.py
new file mode 100644
index 0000000..77dd1cf
--- /dev/null
+++ b/tests/test_sr_schedule.py
@@ -0,0 +1,93 @@
+"""sr_schedule 공용추출 회귀 테스트 (B1).
+
+문제 SR 동작이 추출 전과 동일함을 보장 — advance() 가 구 session_finalize 분기 로직과
+바이트 동등(전진 1·3·7·14 / 졸업 / 오답 리셋 / skipped 불변 / 상수값)인지 검증.
+"""
+
+from __future__ import annotations
+
+import sys
+from datetime import datetime, timedelta, timezone
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(ROOT / "app"))
+
+from services.study import sr_schedule as sr  # noqa: E402
+
+NOW = datetime(2026, 6, 7, 12, 0, 0, tzinfo=timezone.utc)
+
+
+def _old_logic(review_stage, outcome, now):
+    """추출 전 session_finalize.py:188-201 의 산술을 그대로 재현 (동등성 기준)."""
+    if outcome == "correct":
+        new_stage = (review_stage or 0) + 1
+        if new_stage >= 4:
+            return new_stage, None
+        return new_stage, now + timedelta(days={1: 3, 2: 7, 3: 14}[new_stage])
+    elif outcome in ("wrong", "unsure"):
+        return 0, now + timedelta(days=1)
+    return None  # skipped
+
+
+def test_constants():
+    assert sr.REVIEW_INTERVAL_DAYS == {1: 3, 2: 7, 3: 14}
+    assert sr.REVIEW_STAGE_MASTERED == 4
+    assert sr.DEFAULT_FIRST_DUE_DAYS == 1
+
+
+def test_advance_correct_progression():
+    assert sr.advance(None, "correct", NOW) == (1, NOW + timedelta(days=3))
+    assert sr.advance(0, "correct", NOW) == (1, NOW + timedelta(days=3))
+    assert sr.advance(1, "correct", NOW) == (2, NOW + timedelta(days=7))
+    assert sr.advance(2, "correct", NOW) == (3, NOW + timedelta(days=14))
+
+
+def test_advance_graduation():
+    # stage 3 → correct → stage 4 = 졸업(due_at=None)
+    assert sr.advance(3, "correct", NOW) == (4, None)
+    assert sr.advance(4, "correct", NOW) == (5, None)
+
+
+def test_advance_reset():
+    assert sr.advance(0, "wrong", NOW) == (0, NOW + timedelta(days=1))
+    assert sr.advance(2, "wrong", NOW) == (0, NOW + timedelta(days=1))
+    assert sr.advance(2, "unsure", NOW) == (0, NOW + timedelta(days=1))
+
+
+def test_advance_skipped_no_change():
+    assert sr.advance(1, "skipped", NOW) is None
+    assert sr.advance(3, "skipped", NOW) is None
+
+
+def test_first_due():
+    assert sr.first_due(NOW) == (0, NOW + timedelta(days=1))
+
+
+def test_equivalence_with_old_logic():
+    # 전 stage × 전 outcome 조합에서 추출 함수 == 구 로직.
+    for stage in (None, 0, 1, 2, 3, 4):
+        for outcome in ("correct", "wrong", "unsure", "skipped"):
+            assert sr.advance(stage, outcome, NOW) == _old_logic(stage, outcome, NOW), \
+                f"mismatch stage={stage} outcome={outcome}"
+
+
+def test_reexport_preserved():
+    # 기존 import 경로 (session_finalize / study_question_progress) 가 상수를 재-export.
+    from services.study import session_finalize as sf
+    assert sf.REVIEW_INTERVAL_DAYS == sr.REVIEW_INTERVAL_DAYS
+    assert sf.REVIEW_STAGE_MASTERED == sr.REVIEW_STAGE_MASTERED
+    assert sf.DEFAULT_FIRST_DUE_DAYS == sr.DEFAULT_FIRST_DUE_DAYS
+
+
+_TESTS = [v for k, v in dict(globals()).items() if k.startswith("test_")]
+
+if __name__ == "__main__":
+    # session_finalize import 는 무거운 의존(ai 등) 가능 — reexport 테스트만 조건부.
+    ran = 0
+    for t in _TESTS:
+        if t.__name__ == "test_reexport_preserved":
+            continue
+        t()
+        ran += 1
+    print(f"OK ({ran} pure tests; reexport는 pytest에서)")