diff --git a/openmc/capi/filter.py b/openmc/capi/filter.py
index 0b85e026ea..05ec85d010 100644
--- a/openmc/capi/filter.py
+++ b/openmc/capi/filter.py
@@ -402,7 +402,7 @@ class _FilterMapping(Mapping):
 
     def __iter__(self):
         for i in range(len(self)):
-            yield _get_filter(i + 1).id
+            yield _get_filter(i).id
 
     def __len__(self):
         return _dll.tally_filters_size()
diff --git a/openmc/capi/tally.py b/openmc/capi/tally.py
index b8ce0bee58..eec3a2b66b 100644
--- a/openmc/capi/tally.py
+++ b/openmc/capi/tally.py
@@ -251,13 +251,13 @@ class Tally(_FortranObjectWithID):
         filt_idx = POINTER(c_int32)()
         n = c_int()
         _dll.openmc_tally_get_filters(self._index, filt_idx, n)
-        return [_get_filter(filt_idx[i]+1) for i in range(n.value)]
+        return [_get_filter(filt_idx[i]) for i in range(n.value)]
 
     @filters.setter
     def filters(self, filters):
         # Get filter indices as int32_t[]
         n = len(filters)
-        indices = (c_int32*n)(*(f._index-1 for f in filters))
+        indices = (c_int32*n)(*(f._index for f in filters))
 
         _dll.openmc_tally_set_filters(self._index, n, indices)
 
diff --git a/src/tallies/filter.cpp b/src/tallies/filter.cpp
index 0351fd4dc7..e9aae53a7a 100644
--- a/src/tallies/filter.cpp
+++ b/src/tallies/filter.cpp
@@ -114,7 +114,7 @@ extern "C" size_t tally_filters_size()
 
 int verify_filter(int32_t index)
 {
-  if (index < 1 || index > model::tally_filters.size()) {
+  if (index < 0 || index >= model::tally_filters.size()) {
     set_errmsg("Filter index is out of bounds.");
     return OPENMC_E_OUT_OF_BOUNDS;
   }
@@ -126,8 +126,7 @@ openmc_filter_get_id(int32_t index, int32_t* id)
 {
   if (int err = verify_filter(index)) return err;
 
-  // TODO: off-by-one
-  *id = model::tally_filters[index-1]->id_;
+  *id = model::tally_filters[index]->id_;
   return 0;
 }
 
@@ -141,9 +140,8 @@ openmc_filter_set_id(int32_t index, int32_t id)
     return OPENMC_E_INVALID_ID;
   }
 
-  // TODO: off-by-one
-  model::tally_filters[index-1]->id_ = id;
-  model::filter_map[id] = index - 1;
+  model::tally_filters[index]->id_ = id;
+  model::filter_map[id] = index;
   return 0;
 }
 
@@ -152,8 +150,7 @@ openmc_filter_get_type(int32_t index, char* type)
 {
   if (int err = verify_filter(index)) return err;
 
-  // TODO: off-by-one
-  std::strcpy(type, model::tally_filters[index-1]->type().c_str());
+  std::strcpy(type, model::tally_filters[index]->type().c_str());
   return 0;
 }
 
@@ -166,8 +163,7 @@ openmc_get_filter_index(int32_t id, int32_t* index)
     return OPENMC_E_INVALID_ID;
   }
 
-  // TODO: off-by-one
-  *index = it->second + 1;
+  *index = it->second;
   return 0;
 }
 
@@ -184,9 +180,8 @@ openmc_get_filter_next_id(int32_t* id)
 extern "C" int
 openmc_new_filter(const char* type, int32_t* index)
 {
-  allocate_filter(type);
-  // TODO: off-by-one
   *index = model::tally_filters.size();
+  allocate_filter(type);
   return 0;
 }
 
diff --git a/src/tallies/filter_cell.cpp b/src/tallies/filter_cell.cpp
index 80b6678a51..4e25e8541b 100644
--- a/src/tallies/filter_cell.cpp
+++ b/src/tallies/filter_cell.cpp
@@ -76,8 +76,7 @@ openmc_cell_filter_get_bins(int32_t index, int32_t** cells, int32_t* n)
 {
   if (int err = verify_filter(index)) return err;
 
-  // TODO: off-by-one
-  const auto& filt = model::tally_filters[index-1].get();
+  const auto& filt = model::tally_filters[index].get();
   if (filt->type() != "cell") {
     set_errmsg("Tried to get cells from a non-cell filter.");
     return OPENMC_E_INVALID_TYPE;
diff --git a/src/tallies/filter_energy.cpp b/src/tallies/filter_energy.cpp
index bd1de4b462..90cd7b3bfe 100644
--- a/src/tallies/filter_energy.cpp
+++ b/src/tallies/filter_energy.cpp
@@ -123,7 +123,7 @@ openmc_energy_filter_get_bins(int32_t index, double** energies, int32_t* n)
   if (int err = verify_filter(index)) return err;
 
   // Get a pointer to the filter and downcast.
-  const auto& filt_base = model::tally_filters[index-1].get();
+  const auto& filt_base = model::tally_filters[index].get();
   auto* filt = dynamic_cast<EnergyFilter*>(filt_base);
 
   // Check the filter type.
@@ -145,7 +145,7 @@ openmc_energy_filter_set_bins(int32_t index, int32_t n, const double* energies)
   if (int err = verify_filter(index)) return err;
 
   // Get a pointer to the filter and downcast.
-  const auto& filt_base = model::tally_filters[index-1].get();
+  const auto& filt_base = model::tally_filters[index].get();
   auto* filt = dynamic_cast<EnergyFilter*>(filt_base);
 
   // Check the filter type.
diff --git a/src/tallies/filter_legendre.cpp b/src/tallies/filter_legendre.cpp
index 55f225b777..a5f583a158 100644
--- a/src/tallies/filter_legendre.cpp
+++ b/src/tallies/filter_legendre.cpp
@@ -52,7 +52,7 @@ openmc_legendre_filter_get_order(int32_t index, int* order)
   if (int err = verify_filter(index)) return err;
 
   // Get a pointer to the filter and downcast.
-  const auto& filt_base = model::tally_filters[index-1].get();
+  const auto& filt_base = model::tally_filters[index].get();
   auto* filt = dynamic_cast<LegendreFilter*>(filt_base);
 
   // Check the filter type.
@@ -73,7 +73,7 @@ openmc_legendre_filter_set_order(int32_t index, int order)
   if (int err = verify_filter(index)) return err;
 
   // Get a pointer to the filter and downcast.
-  const auto& filt_base = model::tally_filters[index-1].get();
+  const auto& filt_base = model::tally_filters[index].get();
   auto* filt = dynamic_cast<LegendreFilter*>(filt_base);
 
   // Check the filter type.
diff --git a/src/tallies/filter_material.cpp b/src/tallies/filter_material.cpp
index 0dee964def..488f962fa7 100644
--- a/src/tallies/filter_material.cpp
+++ b/src/tallies/filter_material.cpp
@@ -77,7 +77,7 @@ openmc_material_filter_get_bins(int32_t index, int32_t** bins, int32_t* n)
   if (int err = verify_filter(index)) return err;
 
   // Get a pointer to the filter and downcast.
-  const auto& filt_base = model::tally_filters[index-1].get();
+  const auto& filt_base = model::tally_filters[index].get();
   auto* filt = dynamic_cast<MaterialFilter*>(filt_base);
 
   // Check the filter type.
@@ -99,7 +99,7 @@ openmc_material_filter_set_bins(int32_t index, int32_t n, const int32_t* bins)
   if (int err = verify_filter(index)) return err;
 
   // Get a pointer to the filter and downcast.
-  const auto& filt_base = model::tally_filters[index-1].get();
+  const auto& filt_base = model::tally_filters[index].get();
   auto* filt = dynamic_cast<MaterialFilter*>(filt_base);
 
   // Check the filter type.
diff --git a/src/tallies/filter_mesh.cpp b/src/tallies/filter_mesh.cpp
index c17055a9e0..324e38cc7b 100644
--- a/src/tallies/filter_mesh.cpp
+++ b/src/tallies/filter_mesh.cpp
@@ -90,7 +90,7 @@ openmc_mesh_filter_get_mesh(int32_t index, int32_t* index_mesh)
   if (int err = verify_filter(index)) return err;
 
   // Get a pointer to the filter and downcast.
-  const auto& filt_base = model::tally_filters[index-1].get();
+  const auto& filt_base = model::tally_filters[index].get();
   auto* filt = dynamic_cast<MeshFilter*>(filt_base);
 
   // Check the filter type.
@@ -111,7 +111,7 @@ openmc_mesh_filter_set_mesh(int32_t index, int32_t index_mesh)
   if (int err = verify_filter(index)) return err;
 
   // Get a pointer to the filter and downcast.
-  const auto& filt_base = model::tally_filters[index-1].get();
+  const auto& filt_base = model::tally_filters[index].get();
   auto* filt = dynamic_cast<MeshFilter*>(filt_base);
 
   // Check the filter type.
diff --git a/src/tallies/filter_sph_harm.cpp b/src/tallies/filter_sph_harm.cpp
index c772d13ac2..d75be0f9d9 100644
--- a/src/tallies/filter_sph_harm.cpp
+++ b/src/tallies/filter_sph_harm.cpp
@@ -100,7 +100,7 @@ check_sphharm_filter(int32_t index)
   }
 
   // Get a pointer to the filter and downcast.
-  const auto& filt_base = model::tally_filters[index-1].get();
+  const auto& filt_base = model::tally_filters[index].get();
   auto* filt = dynamic_cast<SphericalHarmonicsFilter*>(filt_base);
 
   // Check the filter type.
diff --git a/src/tallies/filter_sptl_legendre.cpp b/src/tallies/filter_sptl_legendre.cpp
index 7757d1f697..b5d75f4d24 100644
--- a/src/tallies/filter_sptl_legendre.cpp
+++ b/src/tallies/filter_sptl_legendre.cpp
@@ -107,7 +107,7 @@ check_sptl_legendre_filter(int32_t index)
   }
 
   // Get a pointer to the filter and downcast.
-  const auto& filt_base = model::tally_filters[index-1].get();
+  const auto& filt_base = model::tally_filters[index].get();
   auto* filt = dynamic_cast<SpatialLegendreFilter*>(filt_base);
 
   // Check the filter type.
diff --git a/src/tallies/filter_zernike.cpp b/src/tallies/filter_zernike.cpp
index 467c64d65c..3ea4492388 100644
--- a/src/tallies/filter_zernike.cpp
+++ b/src/tallies/filter_zernike.cpp
@@ -132,7 +132,7 @@ check_zernike_filter(int32_t index)
   }
 
   // Get a pointer to the filter and downcast.
-  const auto& filt_base = model::tally_filters[index-1].get();
+  const auto& filt_base = model::tally_filters[index].get();
   auto* filt = dynamic_cast<ZernikeFilter*>(filt_base);
 
   // Check the filter type.