diff --git a/tests/src/helpers.c b/tests/src/helpers.c
index da2c047..557c13c 100644
--- a/tests/src/helpers.c
+++ b/tests/src/helpers.c
@@ -354,7 +354,7 @@
     if( *pX != NULL )
         return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
 
-    *plimbs = ( input->len + sizeof( **pX ) - 1 ) / sizeof( **pX );
+    *plimbs = CHARS_TO_LIMBS( input->len );
     if( *plimbs == 0 )
         return( 0 );
     *pX = mbedtls_calloc( *plimbs, sizeof( **pX ) );