diff --git a/crates/js/src/formcalc.rs b/crates/js/src/formcalc.rs
index 78314637e..21a41f52e 100644
--- a/crates/js/src/formcalc.rs
+++ b/crates/js/src/formcalc.rs
@@ -2305,8 +2305,11 @@ impl Interp<'_> {
                 }
                 let num = self.to_num(&v);
                 let decimals = pic.split_once('.').map_or(0, |(_, f)| f.chars().filter(|c| matches!(c, '9' | 'z' | 'Z')).count()).min(20);
+                // `9` in the integer part forces a digit; `z` does not.
+                let min_int = pic.split('.').next().map_or(0, |i| i.chars().filter(|c| *c == '9').count()).min(20);
+                let width = min_int + if decimals > 0 { decimals + 1 } else { 0 };
                 let grouped = pic.contains(',');
-                let mut text = fixed(num.abs(), decimals);
+                let mut text = format!("{:0>width$}", fixed(num.abs(), decimals));
                 if grouped {
                     let (int, frac) = text.split_once('.').map_or((text.as_str(), ""), |(a, b)| (a, b));
                     let mut g = String::new();
diff --git a/crates/js/src/tests.rs b/crates/js/src/tests.rs
index 8fd7bf13b..804341b16 100644
--- a/crates/js/src/tests.rs
+++ b/crates/js/src/tests.rs
@@ -734,6 +734,15 @@ mod formcalc {
         assert!(go("Nope(1)", "form1[0].page1[0].total[0]", "calculate").error.unwrap().contains("unknown function"));
     }
 
+    #[test]
+    fn format_nine_pads_the_integer_part() {
+        assert_eq!(calc(r#"Format("99999", 42)"#), "00042");
+        assert_eq!(calc(r#"Format("9999", 5)"#), "0005");
+        assert_eq!(calc(r#"Format("999.99", 4.5)"#), "004.50");
+        assert_eq!(calc(r#"Format("zzz9", 42)"#), "42");
+        assert_eq!(calc(r#"Format("99", 12345)"#), "12345");
+    }
+
     #[test]
     fn date2num_rejects_days_past_the_end_of_the_month() {
         assert_eq!(calc(r#"Date2Num("2023-02-30")"#), "0");