diff --git a/raillabel/format/num.py b/raillabel/format/num.py
index 21e7a23..acd2d62 100644
--- a/raillabel/format/num.py
+++ b/raillabel/format/num.py
@@ -4,6 +4,7 @@
 from __future__ import annotations
 
 from dataclasses import dataclass
+from uuid import UUID
 
 from raillabel.json_format import JSONNum
 
@@ -29,3 +30,12 @@ def from_json(cls, json: JSONNum) -> Num:
             val=json.val,
             sensor_id=json.coordinate_system,
         )
+
+    def to_json(self, uid: UUID) -> JSONNum:
+        """Export this object into the RailLabel JSON format."""
+        return JSONNum(
+            name=self.name,
+            val=self.val,
+            coordinate_system=self.sensor_id,
+            uid=uid,
+        )
diff --git a/tests/test_raillabel/format/test_num.py b/tests/test_raillabel/format/test_num.py
index 2e0fae7..092c700 100644
--- a/tests/test_raillabel/format/test_num.py
+++ b/tests/test_raillabel/format/test_num.py
@@ -14,15 +14,20 @@
 
 
 @pytest.fixture
-def num_json() -> JSONNum:
+def num_json(num_id) -> JSONNum:
     return JSONNum(
-        uid="78f0ad89-2750-4a30-9d66-44c9da73a714",
+        uid=num_id,
         name="velocity",
         val=49.21321,
         coordinate_system="gps_imu",
     )
 
 
+@pytest.fixture
+def num_id() -> UUID:
+    return UUID("78f0ad89-2750-4a30-9d66-44c9da73a714")
+
+
 @pytest.fixture
 def num() -> Num:
     return Num(
@@ -40,5 +45,10 @@ def test_from_json(num, num_json):
     assert actual == num
 
 
+def test_to_json(num, num_json, num_id):
+    actual = num.to_json(num_id)
+    assert actual == num_json
+
+
 if __name__ == "__main__":
     pytest.main([__file__, "-v"])