diff --git a/src/lib.rs b/src/lib.rs
index c4a96c6..f1a790c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -13,15 +13,19 @@ pub fn left_pad(s: &str, pad: usize) -> String
 /// If str.len() is not less than pad, then the string is returned verbatim
 pub fn left_pad_char(s: &str, pad: usize, padchar: char) -> String
 {
-    let mut out = String::new();
+    // pad_len is the number of padchar that actually need to be put before the string.
+    let pad_len = pad.saturating_sub(s.len());
+    let required_capacity = s.as_bytes().len() + (pad_len as usize) * padchar.len_utf8();
+    let mut out = String::with_capacity(required_capacity);
 
-    let len = s.len();
-    if pad > len {
-        for _ in 0..pad-len {
-            out.push(padchar);
-        }
+    for _ in 0..pad_len {
+        out.push(padchar);
     }
     out.push_str(s);
+    // Assert a correct prediction of the required capacity.
+    debug_assert_eq!(out.as_bytes().len(), required_capacity);
+    // Assert no further allocation was done.
+    debug_assert_eq!(out.capacity(), required_capacity);
 
     out
 }
@@ -36,5 +40,6 @@ fn pad_test() {
     assert_eq!(left_pad("foo", 5), "  foo");
     assert_eq!(left_pad_char("foo", 7, 'X'), "XXXXfoo");
     assert_eq!(left_pad_char("bar", 5, '-'), "--bar");
+    assert_eq!(left_pad_char("rust", 7, '\u{2764}'), "❤❤❤rust");
 }