diff --git a/includes/functions.php b/includes/functions.php
index d7ee59f..3ed1a4a 100644
--- a/includes/functions.php
+++ b/includes/functions.php
@@ -68,3 +68,17 @@ function listPath(
     . slug($list)
     . '.json';
 }
+
+function calculateNextInterval(array $word, bool $correct): int
+{
+    $correctCount = $word['correct'] ?? 0;
+
+    if (!$correct) {
+        return 1;
+    }
+
+    // gecontroleerde groei (SM-lite)
+    $interval = 1 + ($correctCount * 2);
+
+    return min(30, $interval);
+}
\ No newline at end of file