diff --git a/src/main/angular/src/app/editor/siPrefix.ts b/src/main/angular/src/app/editor/siPrefix.ts
index e7e0b3d..aabff20 100644
--- a/src/main/angular/src/app/editor/siPrefix.ts
+++ b/src/main/angular/src/app/editor/siPrefix.ts
@@ -4,6 +4,8 @@ export function siPrefix(value: number, unit: string, minDigits: number): string
   if (value === 0) {
     return `0${unit}`;
   }
+  const negative = value < 0;
+  value = negative ? -value : value;
   const exp0 = Math.log10(value);
   const group = Math.floor(exp0 / 3);
   const index = group + 5;
@@ -14,5 +16,5 @@ export function siPrefix(value: number, unit: string, minDigits: number): string
   const hasDigits = Math.floor(Math.log10(newValue)) + 1;
   const decimals = Math.max(0, minDigits - hasDigits);
   const newValueStr2 = newValue.toFixed(decimals);
-  return `${newValueStr2}${prefix}${unit}`;
+  return `${negative ? '-' : ''}${newValueStr2}${prefix}${unit}`;
 }