diff --git a/subiquity/common/filesystem/sizes.py b/subiquity/common/filesystem/sizes.py
index c494e721..b63c9dbf 100644
--- a/subiquity/common/filesystem/sizes.py
+++ b/subiquity/common/filesystem/sizes.py
@@ -13,11 +13,19 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+import math
+
 import attr
 
+from curtin import swap
+
 from subiquity.models.filesystem import (
+    align_up,
+    align_down,
     MiB,
+    GiB,
     )
+from subiquity.common.types import GuidedResizeValues
 
 
 BIOS_GRUB_SIZE_BYTES = 1 * MiB
@@ -75,3 +83,66 @@ def get_efi_size(available_space):
 def get_bootfs_size(available_space):
     all_factors = (uefi_scale, bootfs_scale, rootfs_scale)
     return scale_partitions(all_factors, available_space)[1]
+
+
+# Calculation of guided resize values is primarly focues on finding a suggested
+# midpoint - what will we resize down to while leaving some room for the
+# existing partition and the new install?
+# 1) Obtain the suggested size for the install
+#    (see calculate_suggested_install_min)
+# 2) Look at the output from the resize tool to see the theoretical minimum
+#    size of the partition we might resize, and pad it a bit (2 GiB or 25%)
+# 3) Subtract the two minimum suggested sizes to obtain the space that we can
+#    decide to keep with the existing partition, or allocate to the new install
+# 4) Assume that the installs will grow proportionally to their minimum sizes,
+#    and split according to the ratio of the minimum sizes
+def calculate_guided_resize(part_min: int, part_size: int, install_min: int,
+                            part_align: int = MiB) -> GuidedResizeValues:
+    if part_min < 0:
+        return None
+
+    other_room_to_grow = max(2 * GiB, math.ceil(.25 * part_min))
+    padded_other_min = part_min + other_room_to_grow
+    other_min = min(align_up(padded_other_min, part_align), part_size)
+
+    plausible_free_space = part_size - other_min
+    if plausible_free_space < install_min:
+        return None
+
+    other_max = align_down(part_size - install_min, part_align)
+    resize_window = other_max - other_min
+    ratio = other_min / (other_min + install_min)
+    raw_recommended = math.ceil(resize_window * ratio) + other_min
+    recommended = align_up(raw_recommended, part_align)
+    return GuidedResizeValues(
+            minimum=other_min, recommended=recommended, maximum=other_max)
+
+
+# Factors for suggested minimum install size:
+# 1) Source minimum - The minimum reported as part of source selection.  This
+#    is absolute bare minimum information to get bits on the disk and doesn’t
+#    factor in filesystem overhead.   Obtained from the size value of the
+#    chosen source as found at /casper/install-sources.yaml.
+# 2) Room for boot - we employ a scaling system to help select the recommended
+#    size of a dedicated /boot and/or efi system partition (see above).  If
+#    /boot is not actually a separate partition, this space needs to be
+#    accounted for as part of the planned rootfs size.  The files that would
+#    otherwise be stored in a dedicated UEFI partition are presumed to be
+#    negligible on non-UEFI systems.  As /boot itself uses a scaling sizing
+#    system, use the bootfs_scale.maximum value for the purposes of finding
+#    minimum suggested size.  The maximum value is to be used instead of the
+#    minimum as the boot scaling system will also make adjustments, and we
+#    don’t want to short change the other calculations.
+# 3) Room to grow - while meaningful work can sometimes be possible on a full
+#    disk, it’s not the sort of thing to suggest in a guided install.
+#    Suggest for room to grow max(2GiB, 80% of source minimum).
+# 4) Swap - Ubuntu policy recommends swap, either in the form of a partition or
+#    a swapfile.  Curtin has an existing swap size recommendation at
+#    curtin.swap.suggested_swapsize().
+def calculate_suggested_install_min(source_min: int,
+                                    part_align: int = MiB) -> int:
+    room_for_boot = bootfs_scale.maximum
+    room_to_grow = max(2 * GiB, math.ceil(.8 * source_min))
+    room_for_swap = swap.suggested_swapsize()
+    total = source_min + room_for_boot + room_to_grow + room_for_swap
+    return align_up(total, part_align)
diff --git a/subiquity/common/filesystem/tests/test_sizes.py b/subiquity/common/filesystem/tests/test_sizes.py
index 191441ca..048f06cd 100644
--- a/subiquity/common/filesystem/tests/test_sizes.py
+++ b/subiquity/common/filesystem/tests/test_sizes.py
@@ -14,15 +14,19 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import unittest
+from unittest import mock
 
 from subiquity.common.filesystem.sizes import (
     bootfs_scale,
+    calculate_guided_resize,
+    calculate_suggested_install_min,
     get_efi_size,
     get_bootfs_size,
     PartitionScaleFactors,
     scale_partitions,
     uefi_scale,
     )
+from subiquity.common.types import GuidedResizeValues
 
 
 class TestPartitionSizeScaling(unittest.TestCase):
@@ -74,3 +78,68 @@ class TestPartitionSizeScaling(unittest.TestCase):
         bootfs_size = get_bootfs_size(disk_size)
         self.assertTrue(bootfs_scale.maximum > bootfs_size)
         self.assertTrue(bootfs_size > bootfs_scale.minimum)
+
+
+class TestCalculateGuidedResize(unittest.TestCase):
+    def test_ignore_nonresizable(self):
+        actual = calculate_guided_resize(
+                part_min=-1, part_size=100 << 30, install_min=10 << 30)
+        self.assertIsNone(actual)
+
+    def test_too_small(self):
+        actual = calculate_guided_resize(
+                part_min=95 << 30, part_size=100 << 30, install_min=10 << 30)
+        self.assertIsNone(actual)
+
+    def test_even_split(self):
+        # 8 GiB * 1.25 == 10 GiB
+        size = 10 << 30
+        actual = calculate_guided_resize(
+                part_min=8 << 30, part_size=100 << 30, install_min=size)
+        expected = GuidedResizeValues(
+                minimum=size, recommended=50 << 30, maximum=(100 << 30) - size)
+        self.assertEqual(expected, actual)
+
+    def test_weighted_split(self):
+        actual = calculate_guided_resize(
+                part_min=40 << 30, part_size=240 << 30, install_min=10 << 30)
+        expected = GuidedResizeValues(
+                minimum=50 << 30, recommended=200 << 30, maximum=230 << 30)
+        self.assertEqual(expected, actual)
+
+
+class TestCalculateInstallMin(unittest.TestCase):
+    @mock.patch('subiquity.common.filesystem.sizes.swap.suggested_swapsize')
+    @mock.patch('subiquity.common.filesystem.sizes.bootfs_scale')
+    def test_small_setups(self, bootfs_scale, swapsize):
+        swapsize.return_value = 1 << 30
+        bootfs_scale.maximum = 1 << 30
+        source_min = 1 << 30
+        # with a small source, we hit the default 2GiB padding
+        self.assertEqual(5 << 30, calculate_suggested_install_min(source_min))
+
+    @mock.patch('subiquity.common.filesystem.sizes.swap.suggested_swapsize')
+    @mock.patch('subiquity.common.filesystem.sizes.bootfs_scale')
+    def test_small_setups_big_swap(self, bootfs_scale, swapsize):
+        swapsize.return_value = 10 << 30
+        bootfs_scale.maximum = 1 << 30
+        source_min = 1 << 30
+        self.assertEqual(14 << 30, calculate_suggested_install_min(source_min))
+
+    @mock.patch('subiquity.common.filesystem.sizes.swap.suggested_swapsize')
+    @mock.patch('subiquity.common.filesystem.sizes.bootfs_scale')
+    def test_small_setups_big_boot(self, bootfs_scale, swapsize):
+        swapsize.return_value = 1 << 30
+        bootfs_scale.maximum = 10 << 30
+        source_min = 1 << 30
+        self.assertEqual(14 << 30, calculate_suggested_install_min(source_min))
+
+    @mock.patch('subiquity.common.filesystem.sizes.swap.suggested_swapsize')
+    @mock.patch('subiquity.common.filesystem.sizes.bootfs_scale')
+    def test_big_source(self, bootfs_scale, swapsize):
+        swapsize.return_value = 1 << 30
+        bootfs_scale.maximum = 2 << 30
+        source_min = 10 << 30
+        # a bigger source should hit 80% padding
+        expected = (10 + 8 + 1 + 2) << 30
+        self.assertEqual(expected, calculate_suggested_install_min(source_min))
diff --git a/subiquity/common/types.py b/subiquity/common/types.py
index dacab493..e2eeef21 100644
--- a/subiquity/common/types.py
+++ b/subiquity/common/types.py
@@ -327,9 +327,17 @@ class StorageResponseV2:
     disks: List[Disk]
     need_root: bool  # if True, there is not yet a partition mounted at "/"
     need_boot: bool  # if True, there is not yet a boot partition
+    install_minimum_size: int
     error_report: Optional[ErrorReportRef] = None
 
 
+@attr.s(auto_attribs=True)
+class GuidedResizeValues:
+    minimum: int
+    recommended: int
+    maximum: int
+
+
 @attr.s(auto_attribs=True)
 class AddPartitionV2:
     disk_id: str
diff --git a/subiquity/models/filesystem.py b/subiquity/models/filesystem.py
index b28230a0..db873186 100644
--- a/subiquity/models/filesystem.py
+++ b/subiquity/models/filesystem.py
@@ -36,6 +36,7 @@ from subiquity.common.types import Bootloader, OsProber
 log = logging.getLogger('subiquity.models.filesystem')
 
 MiB = 1024 * 1024
+GiB = 1024 * 1024 * 1024
 
 
 def _set_backlinks(obj):
diff --git a/subiquity/server/controllers/filesystem.py b/subiquity/server/controllers/filesystem.py
index 95f93cac..8e38ea2a 100644
--- a/subiquity/server/controllers/filesystem.py
+++ b/subiquity/server/controllers/filesystem.py
@@ -24,7 +24,6 @@ from typing import List
 
 import pyudev
 
-
 from subiquitycore.async_helpers import (
     run_in_thread,
     schedule_task,
@@ -314,12 +313,17 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
                 return p
         raise ValueError(f'Partition {number} on {disk.id} not found')
 
+    def calculate_suggested_install_min(self):
+        source_min = self.app.base_model.source.current.size
+        return sizes.calculate_suggested_install_min(source_min)
+
     async def v2_GET(self) -> StorageResponseV2:
         disks = self.model._all(type='disk')
         return StorageResponseV2(
                 disks=[labels.for_client(d) for d in disks],
                 need_root=not self.model.is_root_mounted(),
                 need_boot=self.model.needs_bootloader_partition(),
+                install_minimum_size=self.calculate_suggested_install_min(),
                 )
 
     async def v2_POST(self) -> StorageResponseV2: