diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index 54b4c4a..edce48c 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -470,7 +470,8 @@
              int (*f_rng)(void *, unsigned char *, size_t), void *p_rng );
 
 /**
- * \brief           Calculate the size of a signature made with this key.
+ * \brief           Provide an upper bound for the size of a signature made
+ *                  with this key.
  *
  * \param ctx       PK context to use
  *