diff --git a/src/std_fns.rs b/src/std_fns.rs
index 315da3b..b4a8e88 100644
--- a/src/std_fns.rs
+++ b/src/std_fns.rs
@@ -906,7 +906,8 @@ pub fn str_to_mega_radix(stack: &mut Stack) -> OError {
 pub fn mega_to_str_radix(stack: &mut Stack) -> OError {
     require_int_on_stack!(radix, stack, "mega-to-str-radix");
     require_on_stack!(mega, Mega, stack, "mega-to-str-radix");
-    let mut result = Vec::new();
+    // capacity because O(n)
+    let mut result = Vec::with_capacity((mega as f64).powf(1.0 / radix as f64) as usize + 2);
     let neg = mega < 0;
     let mut mega = mega;
     if neg {