diff --git a/app/business/travel_calc.py b/app/business/travel_calc.py
new file mode 100644
index 0000000..1b33204
--- /dev/null
+++ b/app/business/travel_calc.py
@@ -0,0 +1,25 @@
+def compute_km_for_entry(journey_profile_id: str | None, journeys: dict) -> dict[str, int]:
+    if not journey_profile_id:
+        return {}
+    profile = journeys.get(journey_profile_id, {})
+    return dict(profile.get("distances", {}))
+
+
+def compute_co2_grams(km_by_vehicle: dict[str, int], vehicles: dict) -> float:
+    total = 0.0
+    for vehicle_id, km in km_by_vehicle.items():
+        vehicle = vehicles.get(vehicle_id, {})
+        co2 = vehicle.get("co2_per_km", 0)
+        total += km * co2
+    return total
+
+
+def compute_frais_reels(total_km_voiture: float, tranches: list[dict]) -> float:
+    if not tranches or total_km_voiture <= 0:
+        return 0.0
+    for tranche in tranches:
+        km_max = tranche["km_max"]
+        if km_max == 0 or total_km_voiture <= km_max:
+            return total_km_voiture * tranche["taux"] + tranche.get("forfait", 0)
+    last = tranches[-1]
+    return total_km_voiture * last["taux"] + last.get("forfait", 0)
diff --git a/tests/test_travel_calc.py b/tests/test_travel_calc.py
new file mode 100644
index 0000000..47eddab
--- /dev/null
+++ b/tests/test_travel_calc.py
@@ -0,0 +1,57 @@
+from app.business.travel_calc import (
+    compute_km_for_entry,
+    compute_co2_grams,
+    compute_frais_reels,
+)
+
+VEHICLES = {
+    "voiture": {"co2_per_km": 142, "cv": 5},
+    "velo": {"co2_per_km": 0},
+}
+
+JOURNEYS = {
+    "voiture_seule": {"distances": {"voiture": 25}},
+    "voiture_velo": {"distances": {"voiture": 14, "velo": 8}},
+    "velo_seul": {"distances": {"velo": 24}},
+}
+
+TRANCHES_CV5 = [
+    {"km_max": 3000, "taux": 0.548, "forfait": 0},
+    {"km_max": 6000, "taux": 0.316, "forfait": 699},
+    {"km_max": 0,    "taux": 0.364, "forfait": 0},
+]
+
+
+def test_compute_km_voiture_seule():
+    assert compute_km_for_entry("voiture_seule", JOURNEYS) == {"voiture": 25}
+
+
+def test_compute_km_voiture_velo():
+    assert compute_km_for_entry("voiture_velo", JOURNEYS) == {"voiture": 14, "velo": 8}
+
+
+def test_compute_km_no_journey():
+    assert compute_km_for_entry(None, JOURNEYS) == {}
+
+
+def test_compute_co2_voiture():
+    assert compute_co2_grams({"voiture": 25}, VEHICLES) == 25 * 142
+
+
+def test_compute_co2_velo():
+    assert compute_co2_grams({"velo": 24}, VEHICLES) == 0
+
+
+def test_frais_reels_tranche1():
+    result = compute_frais_reels(2000, TRANCHES_CV5)
+    assert abs(result - 1096.0) < 0.01
+
+
+def test_frais_reels_tranche2():
+    result = compute_frais_reels(4000, TRANCHES_CV5)
+    assert abs(result - 1963.0) < 0.01
+
+
+def test_frais_reels_tranche3():
+    result = compute_frais_reels(7000, TRANCHES_CV5)
+    assert abs(result - 2548.0) < 0.01