diff --git a/constructor/header.sh b/constructor/header.sh
index d35bab72b..80a753f8d 100644
--- a/constructor/header.sh
+++ b/constructor/header.sh
@@ -349,6 +349,14 @@ if ! mkdir -p "$PREFIX"; then
     exit 1
 fi
 
+total_installation_size="__TOTAL_INSTALLATION_SIZE__"
+free_disk_space="$(df -Pk "$PREFIX" | tail -n 1 | awk '{print $4}')"
+free_disk_space_with_buffer="$((free_disk_space - 100 * 1024 * 1024))"  # add 100MB of buffer
+if [ "$free_disk_space_with_buffer" -lt "$total_installation_size" ]; then
+    printf "ERROR: Not enough free disk space: %s < %s\\n" "$free_disk_space" "$total_installation_size" >&2
+    exit 1
+fi
+
 # pwd does not convert two leading slashes to one
 # https://github.com/conda/constructor/issues/284
 PREFIX=$(cd "$PREFIX"; pwd | sed 's@//@/@')
diff --git a/constructor/shar.py b/constructor/shar.py
index 299f3ab9f..1ed5e79ee 100644
--- a/constructor/shar.py
+++ b/constructor/shar.py
@@ -18,6 +18,7 @@
 from .preconda import write_files as preconda_write_files
 from .utils import (
     add_condarc,
+    approx_size_kb,
     filename_dist,
     fill_template,
     get_final_channels,
@@ -90,6 +91,7 @@ def get_header(conda_exec, tarball, info):
         'pycache': '__pycache__',
         'SHORTCUTS': shortcuts_flags(info),
         'REGISTER_ENVS': str(info.get("register_envs", True)).lower(),
+        'TOTAL_INSTALLATION_SIZE': approx_size_kb(info, "total")
     }
     if has_license:
         replace['LICENSE'] = read_ascii_only(info['license_file'])