diff --git a/spoolman/api/v1/models.py b/spoolman/api/v1/models.py
index 331119c..62ddf3a 100644
--- a/spoolman/api/v1/models.py
+++ b/spoolman/api/v1/models.py
@@ -91,7 +91,15 @@ class Spool(BaseModel):
     first_used: Optional[datetime] = Field(description="First logged occurence of spool usage.")
     last_used: Optional[datetime] = Field(description="Last logged occurence of spool usage.")
     filament: Filament = Field(description="The filament type of this spool.")
-    weight: float = Field(ge=0, description="Remaining weight of filament on the spool.", example=500)
+    remaining_weight: Optional[float] = Field(
+        default=None,
+        ge=0,
+        description=(
+            "Estimated remaining weight of filament on the spool. Only set if the filament type has a weight set."
+        ),
+        example=500,
+    )
+    used_weight: float = Field(ge=0, description="Consumed weight of filament from the spool.", example=500)
     location: Optional[str] = Field(max_length=64, description="Where this spool can be found.", example="Shelf A")
     lot_nr: Optional[str] = Field(
         max_length=64,
@@ -107,13 +115,19 @@ class Spool(BaseModel):
     @staticmethod
     def from_db(item: models.Spool) -> "Spool":
         """Create a new Pydantic spool object from a database spool object."""
+        filament = Filament.from_db(item.filament)
+        remaining_weight: Optional[float] = None
+        if filament.weight is not None:
+            remaining_weight = max(filament.weight - item.used_weight, 0)
+
         return Spool(
             id=item.id,
             registered=item.registered,
             first_used=item.first_used,
             last_used=item.last_used,
-            filament=Filament.from_db(item.filament),
-            weight=item.weight,
+            filament=filament,
+            used_weight=item.used_weight,
+            remaining_weight=remaining_weight,
             location=item.location,
             lot_nr=item.lot_nr,
             comment=item.comment,
diff --git a/spoolman/api/v1/spool.py b/spoolman/api/v1/spool.py
index 1c60523..db888f9 100644
--- a/spoolman/api/v1/spool.py
+++ b/spoolman/api/v1/spool.py
@@ -27,14 +27,14 @@ class SpoolParameters(BaseModel):
     first_used: Optional[datetime] = Field(description="First logged occurence of spool usage.")
     last_used: Optional[datetime] = Field(description="Last logged occurence of spool usage.")
     filament_id: int = Field(description="The ID of the filament type of this spool.")
-    weight: Optional[float] = Field(
+    remaining_weight: Optional[float] = Field(
         ge=0,
         description=(
-            "Remaining weight of filament on the spool. "
-            "Leave empty to assume a full spool based on the weight parameters of the filament type."
+            "Remaining weight of filament on the spool. Can only be used if the filament type has a weight set."
         ),
-        example=500,
+        example=800,
     )
+    used_weight: Optional[float] = Field(ge=0, description="Used weight of filament on the spool.", example=200)
     location: Optional[str] = Field(max_length=64, description="Where this spool can be found.", example="Shelf A")
     lot_nr: Optional[str] = Field(
         max_length=64,
@@ -132,7 +132,11 @@ async def get(
 @router.post(
     "/",
     name="Add spool",
-    description="Add a new spool to the database.",
+    description=(
+        "Add a new spool to the database. "
+        "Only specify either remaining_weight or used_weight. "
+        "If no weight is set, the spool will be assumed to be full."
+    ),
     response_model_exclude_none=True,
     response_model=Spool,
     responses={
@@ -143,11 +147,18 @@ async def create(  # noqa: ANN201
     db: Annotated[AsyncSession, Depends(get_db_session)],
     body: SpoolParameters,
 ):
+    if body.remaining_weight is not None and body.used_weight is not None:
+        return JSONResponse(
+            status_code=400,
+            content={"message": "Only specify either remaining_weight or used_weight."},
+        )
+
     try:
         db_item = await spool.create(
             db=db,
             filament_id=body.filament_id,
-            weight=body.weight,
+            remaining_weight=body.remaining_weight,
+            used_weight=body.used_weight,
             first_used=body.first_used,
             last_used=body.last_used,
             location=body.location,
@@ -165,26 +176,46 @@ async def create(  # noqa: ANN201
 @router.patch(
     "/{spool_id}",
     name="Update spool",
-    description="Update any attribute of a spool. Only fields specified in the request will be affected.",
+    description=(
+        "Update any attribute of a spool. "
+        "Only fields specified in the request will be affected. "
+        "remaining_weight and used_weight can't be set at the same time."
+    ),
     response_model_exclude_none=True,
+    response_model=Spool,
+    responses={
+        400: {"model": Message},
+    },
 )
-async def update(
+async def update(  # noqa: ANN201
     db: Annotated[AsyncSession, Depends(get_db_session)],
     spool_id: int,
     body: SpoolUpdateParameters,
-) -> Spool:
+):
     patch_data = body.dict(exclude_unset=True)
 
+    if body.remaining_weight is not None and body.used_weight is not None:
+        return JSONResponse(
+            status_code=400,
+            content={"message": "Only specify either remaining_weight or used_weight."},
+        )
+
     if "filament_id" in patch_data and body.filament_id is None:
         raise RequestValidationError(
             [ErrorWrapper(ValueError("filament_id cannot be unset"), ("query", "filament_id"))],
         )
 
-    db_item = await spool.update(
-        db=db,
-        spool_id=spool_id,
-        data=patch_data,
-    )
+    try:
+        db_item = await spool.update(
+            db=db,
+            spool_id=spool_id,
+            data=patch_data,
+        )
+    except ItemCreateError as exc:
+        return JSONResponse(
+            status_code=400,
+            content={"message": str(exc)},
+        )
 
     return Spool.from_db(db_item)
 
diff --git a/spoolman/database/models.py b/spoolman/database/models.py
index 1090261..6733977 100644
--- a/spoolman/database/models.py
+++ b/spoolman/database/models.py
@@ -50,7 +50,7 @@ class Spool(Base):
     last_used: Mapped[Optional[datetime]] = mapped_column()
     filament_id: Mapped[int] = mapped_column(ForeignKey("filament.id"))
     filament: Mapped["Filament"] = relationship(back_populates="spools")
-    weight: Mapped[float] = mapped_column()
+    used_weight: Mapped[float] = mapped_column()
     location: Mapped[Optional[str]] = mapped_column(String(64))
     lot_nr: Mapped[Optional[str]] = mapped_column(String(64))
     comment: Mapped[Optional[str]] = mapped_column(String(1024))
diff --git a/spoolman/database/spool.py b/spoolman/database/spool.py
index 77b28aa..6876461 100644
--- a/spoolman/database/spool.py
+++ b/spoolman/database/spool.py
@@ -16,7 +16,8 @@ async def create(
     *,
     db: AsyncSession,
     filament_id: int,
-    weight: Optional[float] = None,
+    remaining_weight: Optional[float] = None,
+    used_weight: Optional[float] = None,
     first_used: Optional[datetime] = None,
     last_used: Optional[datetime] = None,
     location: Optional[str] = None,
@@ -25,14 +26,17 @@ async def create(
 ) -> models.Spool:
     """Add a new spool to the database. Leave weight empty to assume full spool."""
     filament_item = await filament.get_by_id(db, filament_id)
-    if weight is None:
-        if filament_item.weight is None:
-            raise ItemCreateError("The weight for neither the spool nor its filament type is set.")
-        weight = filament_item.weight
+    if used_weight is None:
+        if remaining_weight is not None:
+            if filament_item.weight is None:
+                raise ItemCreateError("remaining_weight can only be used if the filament type has a weight set.")
+            used_weight = max(filament_item.weight - remaining_weight, 0)
+        else:
+            used_weight = 0
 
     db_item = models.Spool(
         filament=filament_item,
-        weight=weight,
+        used_weight=used_weight,
         first_used=first_used,
         last_used=last_used,
         location=location,
@@ -105,6 +109,10 @@ async def update(
     for k, v in data.items():
         if k == "filament_id":
             spool.filament = await filament.get_by_id(db, v)
+        elif k == "remaining_weight":
+            if spool.filament.weight is None:
+                raise ItemCreateError("remaining_weight can only be used if the filament type has a weight set.")
+            spool.used_weight = max(spool.filament.weight - v, 0)
         else:
             setattr(spool, k, v)
     await db.flush()
@@ -119,33 +127,33 @@ async def delete(db: AsyncSession, spool_id: int) -> None:
 
 # TODO: Make unit tests for race conditions on these
 async def use_weight(db: AsyncSession, spool_id: int, weight: float) -> models.Spool:
-    """Reduce the weight of a spool.
+    """Consume filament from a spool by weight.
 
     Does nothing if the spool is empty.
 
     Args:
         db (AsyncSession): Database session
         spool_id (int): Spool ID
-        weight (float): Weight loss in grams
+        weight (float): Filament weight to consume, in grams
 
     Returns:
         models.Spool: Updated spool object
     """
     spool = await get_by_id(db, spool_id, with_for_update=True)
-    spool.weight -= min(spool.weight, weight)
+    spool.used_weight += weight
     await db.flush()
     return spool
 
 
 async def use_length(db: AsyncSession, spool_id: int, length: float) -> models.Spool:
-    """Reduce the weight of a spool by using a length of filament.
+    """Consume filament from a spool by length.
 
     Does nothing if the spool is empty.
 
     Args:
         db (AsyncSession): Database session
         spool_id (int): Spool ID
-        length (float): Length of filament to reduce by
+        length (float): Length of filament to consume, in mm
 
     Returns:
         models.Spool: Updated spool object
@@ -154,11 +162,10 @@ async def use_length(db: AsyncSession, spool_id: int, length: float) -> models.S
 
     filament = spool.filament
 
-    weight = weight_from_length(
+    spool.used_weight += weight_from_length(
         length=length,
         radius=filament.diameter / 2,
         density=filament.density,
     )
-    spool.weight -= min(spool.weight, weight)
     await db.flush()
     return spool