diff --git a/flopy4/adapters.py b/flopy4/adapters.py
index f4a36ddf..8a3cec3e 100644
--- a/flopy4/adapters.py
+++ b/flopy4/adapters.py
@@ -142,3 +142,18 @@ def get_cellid(nn: int, grid: Grid) -> tuple[int, ...]:
             return (nn,)
         case _:
             raise TypeError(f"Unsupported grid type: {type(grid)}")
+
+
+def get_nn(cellid, **kwargs):
+    ndim = len(cellid)
+    match ndim:
+        case 1:
+            return cellid[0]
+        case 2:
+            k, j = cellid
+            return k * kwargs["ncpl"] + j
+        case 3:
+            k, i, j = cellid
+            return k * kwargs["nrow"] * kwargs["ncol"] + i * kwargs["ncol"] + j
+        case _:
+            raise ValueError(f"Invalid cellid: {cellid}")
diff --git a/flopy4/mf6/codec/converter.py b/flopy4/mf6/codec/converter.py
index c2ff04f1..f3077c09 100644
--- a/flopy4/mf6/codec/converter.py
+++ b/flopy4/mf6/codec/converter.py
@@ -7,7 +7,7 @@
 from xarray import DataArray
 from xattree import get_xatspec
 
-from flopy4.adapters import get_cellid
+from flopy4.adapters import get_cellid, get_nn
 from flopy4.mf6.component import Component
 from flopy4.mf6.config import SPARSE_THRESHOLD
 from flopy4.mf6.constants import FILL_DNODATA
@@ -66,19 +66,6 @@ def final(arr):
             arr[arr == FILL_DNODATA] = field.default or FILL_DNODATA
             return arr
 
-    def _get_nn(cellid):
-        match len(cellid):
-            case 1:
-                return cellid[0]
-            case 2:
-                k, j = cellid
-                return k * dims["ncpl"] + j
-            case 3:
-                k, i, j = cellid
-                return k * dims["nrow"] * dims["ncol"] + i * dims["ncol"] + j
-            case _:
-                raise ValueError(f"Invalid cellid: {cellid}")
-
     # populate array. TODO: is there a way to do this
     # without hardcoding awareness of kper and cellid?
     if "nper" in dims:
@@ -90,13 +77,13 @@ def _get_nn(cellid):
                     set_(a, period, kper)
                 case _:
                     for cellid, v in period.items():
-                        nn = _get_nn(cellid)
+                        nn = get_nn(cellid, **dims)
                         set_(a, v, kper, nn)
             if kper == "*":
                 break
     else:
         for cellid, v in value.items():
-            nn = _get_nn(cellid)
+            nn = get_nn(cellid, **dims)
             set_(a, v, nn)
     return final(a)