diff --git a/src/con_duct/cli.py b/src/con_duct/cli.py
index 258d00aa..4ca8600a 100644
--- a/src/con_duct/cli.py
+++ b/src/con_duct/cli.py
@@ -428,6 +428,7 @@ def _create_ls_parser() -> argparse.ArgumentParser:
             "exit_code",
             "wall_clock_time",
             "peak_rss",
+            "files_size",
         ],
     )
     parser.add_argument(
diff --git a/src/con_duct/ls.py b/src/con_duct/ls.py
index 403eed6a..fd9fbba1 100644
--- a/src/con_duct/ls.py
+++ b/src/con_duct/ls.py
@@ -3,6 +3,7 @@
 import glob
 import json
 import logging
+from pathlib import Path
 import re
 from types import ModuleType
 from typing import Any, Dict, List, Optional
@@ -32,6 +33,7 @@
     "average_vsz": "{value!S}",
     "end_time": "{value:.2f!N}",
     "exit_code": "{value!E}",
+    "files_size": "{value!S}",
     "memory_total": "{value!S}",
     "peak_pcpu": "{value:.2f!N}%",
     "peak_pmem": "{value:.2f!N}%",
@@ -65,9 +67,26 @@
 LS_FIELD_CHOICES: List[str] = (
     list(VALUE_TRANSFORMATION_MAP.keys()) + NON_TRANSFORMED_FIELDS
 )
+COMPUTED_FIELDS: List[str] = ["files_size"]
 MINIMUM_SCHEMA_VERSION: str = "0.2.0"
 
 
+def compute_files_size(prefix: str) -> int:
+    """Compute total size in bytes of all files for a given session prefix.
+
+    Files that cannot be accessed (e.g. due to permissions) are silently skipped.
+    """
+    total = 0
+    for path_str in glob.glob(glob.escape(prefix) + "*"):
+        path = Path(path_str)
+        if path.is_file():
+            try:
+                total += path.stat().st_size
+            except OSError as e:
+                lgr.warning("Could not get size of %s: %s", path_str, e)
+    return total
+
+
 def load_duct_runs(
     info_files: List[str], eval_filter: Optional[str] = None
 ) -> List[Dict[str, Any]]:
@@ -87,6 +106,7 @@ def load_duct_runs(
                     )
                     continue
                 ensure_compliant_schema(this)
+                this["files_size"] = compute_files_size(this["prefix"])
                 if eval_filter is not None and not (
                     eval_results := eval(eval_filter, _flatten_dict(this), dict(re=re))
                 ):
diff --git a/test/test_ls.py b/test/test_ls.py
index 7d7dd232..392e645d 100644
--- a/test/test_ls.py
+++ b/test/test_ls.py
@@ -15,6 +15,7 @@
     MINIMUM_SCHEMA_VERSION,
     _flatten_dict,
     _restrict_row,
+    compute_files_size,
     ensure_compliant_schema,
     load_duct_runs,
     ls,
@@ -30,6 +31,7 @@ def test_load_duct_runs_sanity() -> None:
         result = load_duct_runs(["/test/path_info.json"])
     assert len(result) == 1
     assert result[0]["prefix"] == "/test/path_"
+    assert "files_size" in result[0]
 
 
 def test_load_duct_runs_skips_unsupported_schema() -> None:
@@ -164,6 +166,20 @@ def side_effect(filename: str) -> Any:
     assert "Skipping empty file" in caplog.text
 
 
+def test_compute_files_size_sums_all_files() -> None:
+    """Test that compute_files_size sums sizes of all files with the given prefix."""
+    with tempfile.TemporaryDirectory() as tmpdir:
+        prefix = os.path.join(tmpdir, "run_")
+        # Use arbitrary suffixes; compute_files_size globs all files with the prefix
+        for suffix, content in [("a", b"hello"), ("b", b"world!"), ("c", b"{}...")]:
+            with open(f"{prefix}{suffix}", "wb") as f:
+                f.write(content)
+        expected = sum(len(c) for c in [b"hello", b"world!", b"{}..."])
+        assert compute_files_size(prefix) == expected
+        # Non-existent prefix returns 0
+        assert compute_files_size(os.path.join(tmpdir, "nonexistent_")) == 0
+
+
 class TestLS(unittest.TestCase):
     def setUp(self) -> None:
         """Create a temporary directory and test files."""
@@ -218,7 +234,7 @@ def _run_ls(
             args = argparse.Namespace(
                 paths=[os.path.join(self.temp_dir.name, path) for path in paths],
                 colors=False,
-                fields=["prefix", "schema_version"],
+                fields=["prefix", "schema_version", "files_size"],
                 eval_filter=None,
                 format=fmt,
                 func=ls,
@@ -243,6 +259,8 @@ def test_ls_sanity(self) -> None:
         ]
         assert len(prefixes) == 1
         assert any("file1" in p for p in prefixes)
+        assert "Files Size:" in result
+        assert any(unit in result for unit in ["Byte", "kB", "MB", "GB"])
 
     def test_ls_with_filter(self) -> None:
         """Basic sanity test to ensure ls() runs without crashing."""
@@ -341,6 +359,7 @@ def test_ls_json_output(self) -> None:
         parsed = json.loads(result)
         assert len(parsed) == 1
         assert "prefix" in parsed[0]
+        assert "files_size" in parsed[0]
 
     def test_ls_json_pp_output(self) -> None:
         """Test pretty-printed JSON output format."""
diff --git a/test/test_schema.py b/test/test_schema.py
index 158abb91..bb34a5bd 100644
--- a/test/test_schema.py
+++ b/test/test_schema.py
@@ -3,7 +3,7 @@
 from pathlib import Path
 from utils import run_duct_command
 from con_duct._constants import SUFFIXES
-from con_duct.ls import LS_FIELD_CHOICES, _flatten_dict
+from con_duct.ls import COMPUTED_FIELDS, LS_FIELD_CHOICES, _flatten_dict
 
 
 def test_info_fields(temp_output_dir: str) -> None:
@@ -29,4 +29,5 @@ def test_info_fields(temp_output_dir: str) -> None:
     actual_info_schema = _flatten_dict(json.loads(info_file.read_text())).keys()
     os.remove(info_file)
 
-    assert set(actual_info_schema) == set(LS_FIELD_CHOICES)
+    schema_field_choices = [f for f in LS_FIELD_CHOICES if f not in COMPUTED_FIELDS]
+    assert set(actual_info_schema) == set(schema_field_choices)