diff --git a/odc/geo/_cog.py b/odc/geo/_cog.py
index 3004a34e..599ce447 100644
--- a/odc/geo/_cog.py
+++ b/odc/geo/_cog.py
@@ -17,8 +17,8 @@
 from rasterio.shutil import copy as rio_copy  # pylint: disable=no-name-in-module
 
 from .geobox import GeoBox
-from .math import align_up
-from .types import SomeShape, shape_, wh_
+from .math import align_down_pow2, align_up
+from .types import Shape2d, SomeShape, shape_, wh_
 from .warp import resampling_s2rio
 
 # pylint: disable=too-many-locals,too-many-branches,too-many-arguments,too-many-statements
@@ -60,6 +60,33 @@ def _adjust_blocksize(block: int, dim: int = 0) -> int:
     return align_up(block, 16)
 
 
+def _num_overviews(block: int, dim: int) -> int:
+    c = 0
+    while block < dim:
+        dim = dim // 2
+        c += 1
+    return c
+
+
+def _compute_cog_spec(
+    data_shape: SomeShape,
+    tile_shape: SomeShape,
+    *,
+    max_pad: Optional[int] = None,
+) -> Tuple[Shape2d, Shape2d, int]:
+    data_shape = shape_(data_shape)
+    tile_shape = shape_(shape_(tile_shape).map(_adjust_blocksize))
+    n1, n2 = (_num_overviews(b, dim) for dim, b in zip(data_shape.xy, tile_shape.xy))
+    n = max(n1, n2)
+    pad = 2**n
+    if max_pad is not None and max_pad < pad:
+        pad = 0 if max_pad == 0 else align_down_pow2(max_pad)
+
+    if pad > 0:
+        data_shape = shape_(data_shape.map(lambda d: align_up(d, pad)))
+    return (data_shape, tile_shape, n)
+
+
 def _default_cog_opts(
     *, blocksize: int = 512, shape: SomeShape = (0, 0), is_float: bool = False, **other
 ) -> Dict[str, Any]:
@@ -101,7 +128,7 @@ def _write_cog(
     ovr_blocksize: Optional[int] = None,
     use_windowed_writes: bool = False,
     intermediate_compression: Union[bool, str, Dict[str, Any]] = False,
-    **extra_rio_opts
+    **extra_rio_opts,
 ) -> Union[Path, bytes]:
     if blocksize is None:
         blocksize = 512
@@ -233,7 +260,7 @@ def write_cog(
     overview_levels: Optional[List[int]] = None,
     use_windowed_writes: bool = False,
     intermediate_compression: Union[bool, str, Dict[str, Any]] = False,
-    **extra_rio_opts
+    **extra_rio_opts,
 ) -> Union[Path, bytes]:
     """
     Save ``xarray.DataArray`` to a file in Cloud Optimized GeoTiff format.
@@ -318,7 +345,7 @@ def to_cog(
     overview_levels: Optional[List[int]] = None,
     use_windowed_writes: bool = False,
     intermediate_compression: Union[bool, str, Dict[str, Any]] = False,
-    **extra_rio_opts
+    **extra_rio_opts,
 ) -> bytes:
     """
     Compress ``xarray.DataArray`` into Cloud Optimized GeoTiff bytes in memory.
@@ -387,7 +414,7 @@ def write_cog_layers(
     ovr_blocksize: Optional[int] = None,
     intermediate_compression: Union[bool, str, Dict[str, Any]] = False,
     use_windowed_writes: bool = False,
-    **extra_rio_opts
+    **extra_rio_opts,
 ) -> Union[Path, bytes, None]:
     """
     Write COG from externally computed overviews.
diff --git a/tests/test_cog.py b/tests/test_cog.py
index 44556a8e..ce61d000 100644
--- a/tests/test_cog.py
+++ b/tests/test_cog.py
@@ -1,3 +1,8 @@
+from typing import Optional, Tuple
+
+import pytest
+
+from odc.geo._cog import _compute_cog_spec, _num_overviews
 from odc.geo.gridspec import GridSpec
 from odc.geo.xr import xr_zeros
 
@@ -37,3 +42,48 @@ def test_write_cog_ovr():
 
     img_bytes2 = img.odc.write_cog(":mem:", blocksize=32, overviews=ovrs)
     assert len(img_bytes) == len(img_bytes2)
+
+
+@pytest.mark.parametrize(
+    ["block", "dim", "n_expect"],
+    [
+        (2**5, 2**10, 5),
+        (2**5, 2**10 - 1, 5),
+        (2**5, 2**10 + 1, 5),
+        (256, 78, 0),
+        (1024, 1_000_000, -1),
+        (512, 3_040_000, -1),
+    ],
+)
+def test_num_overviews(block: int, dim: int, n_expect: int):
+    if n_expect >= 0:
+        assert _num_overviews(block, dim) == n_expect
+    else:
+        n = _num_overviews(block, dim)
+        assert dim // (2**n) <= block
+
+
+@pytest.mark.parametrize(
+    ("shape", "tshape", "max_pad"),
+    [
+        [(1024, 2048), (256, 128), None],
+    ],
+)
+def test_cog_spec(
+    shape: Tuple[int, int],
+    tshape: Tuple[int, int],
+    max_pad: Optional[int],
+):
+    _shape, _tshape, nlevels = _compute_cog_spec(shape, tshape, max_pad=max_pad)
+    assert _shape[0] >= shape[0]
+    assert _shape[1] >= shape[1]
+    assert _tshape[0] >= tshape[0]
+    assert _tshape[1] >= tshape[1]
+    assert _tshape[0] % 16 == 0
+    assert _tshape[1] % 16 == 0
+
+    assert max(_shape) // (2**nlevels) <= max(tshape)
+
+    if max_pad is not None:
+        assert _shape[0] - shape[0] <= max_pad
+        assert _shape[1] - shape[1] <= max_pad