diff --git a/parcels/particlefile.py b/parcels/particlefile.py
index 4ab406b4a..993fd448c 100644
--- a/parcels/particlefile.py
+++ b/parcels/particlefile.py
@@ -304,7 +304,7 @@ def write(self, pset, time: float | timedelta | np.timedelta64 | None, indices=N
 
             if self.create_new_zarrfile:
                 if self.chunks is None:
-                    self._chunks = (len(ids), 1)
+                    self._chunks = (len(pset), 1)
                 if pset._repeatpclass is not None and self.chunks[0] < 1e4:  # type: ignore[index]
                     warnings.warn(
                         f"ParticleFile chunks are set to {self.chunks}, but this may lead to "