diff --git a/get_data.lib.php b/get_data.lib.php
index 5a62561..8d631b4 100644
--- a/get_data.lib.php
+++ b/get_data.lib.php
@@ -373,17 +373,30 @@ function upload_dir() {
   return $dir;
 }
 
+/*
+ * A size in megabytes from the configuration, in bytes. An empty, missing
+ * or bad value, or one that is not above zero, gives $default_mb.
+ */
+function config_megabytes($key, $default_mb) {
+  $value = trim((string)config($key, ''));
+  if (!is_numeric($value) || (float)$value <= 0)
+    $value = $default_mb;
+  return (int)((float)$value * 1024 * 1024);
+}
+
 /*
  * The largest image a story can have, in bytes, from UPLOAD_MAX_MB (1 MB
  * by default). NULL means no limit, which UPLOAD_MAX_MB=none gives.
  */
 function upload_max_bytes() {
-  $value = strtolower(trim((string)config('UPLOAD_MAX_MB', '')));
-  if ($value === 'none')
+  if (strtolower(trim((string)config('UPLOAD_MAX_MB', ''))) === 'none')
     return NULL;
-  if (!is_numeric($value) || (float)$value <= 0)
-    $value = 1;
-  return (int)((float)$value * 1024 * 1024);
+  return config_megabytes('UPLOAD_MAX_MB', 1);
+}
+
+// The most that the upload directory holds, in bytes, from UPLOAD_DIR_LIMIT_MB (100 MB by default)
+function upload_dir_limit_bytes() {
+  return config_megabytes('UPLOAD_DIR_LIMIT_MB', 100);
 }
 
 // The same in megabytes for a message, such as "1" or "2.5"
@@ -408,8 +421,7 @@ define('UPLOAD_MAX_AGE', 86400);
 function store_upload($tmp_file) {
   prune_uploads();
   remove_pending_upload();
-  $limit = (float)config('UPLOAD_DIR_LIMIT_MB', 100) * 1024 * 1024;
-  if (upload_dir_size() + (int)@filesize($tmp_file) > $limit)
+  if (upload_dir_size() + (int)@filesize($tmp_file) > upload_dir_limit_bytes())
     return FALSE;
   $name = bin2hex(random_bytes(16));
   $file = upload_dir() . '/' . $name;
diff --git a/tests/php/Unit/UploadTest.php b/tests/php/Unit/UploadTest.php
index 3b1c63f..e8196d6 100644
--- a/tests/php/Unit/UploadTest.php
+++ b/tests/php/Unit/UploadTest.php
@@ -136,6 +136,18 @@ class UploadTest extends TestCase {
     $this->assertSame(2 + 2 + 5, upload_dir_size());
   }
 
+  public function testTheUploadDirLimitComesFromTheConfiguration(): void {
+    $this->setEnv('UPLOAD_DIR_LIMIT_MB', '2.5');
+    $this->assertSame((int)(2.5 * 1024 * 1024), upload_dir_limit_bytes());
+  }
+
+  public function testAnEmptyOrBadUploadDirLimitMeansTheDefault(): void {
+    foreach (array('', ' ', 'lots', '0', '-2', 'none') as $value) {
+      $this->setEnv('UPLOAD_DIR_LIMIT_MB', $value);
+      $this->assertSame(100 * 1024 * 1024, upload_dir_limit_bytes(), "'$value'");
+    }
+  }
+
   public function testStoreUploadPrunesAndDropsTheLastUploadFirst(): void {
     $last = $this->pending();
     $files = $this->others();