diff --git a/android/app/src/main/java/tw/local/botrates/MainActivity.kt b/android/app/src/main/java/tw/local/botrates/MainActivity.kt
index 5b4aed1..431fd8f 100644
--- a/android/app/src/main/java/tw/local/botrates/MainActivity.kt
+++ b/android/app/src/main/java/tw/local/botrates/MainActivity.kt
@@ -47,8 +47,11 @@ import androidx.compose.ui.unit.sp
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
+import java.math.BigDecimal
+import java.math.RoundingMode
 import java.net.HttpURLConnection
 import java.net.URL
+import java.text.DecimalFormat
 import java.text.SimpleDateFormat
 import java.util.Date
 import java.util.Locale
@@ -82,13 +85,14 @@ private fun fetchRates(): List<Rate> {
 private fun parseAmount(s: String): Double? =
     s.filter { it != ',' && !it.isWhitespace() }.takeIf { it.isNotEmpty() }?.toDoubleOrNull()
 
-private fun formatTwd(v: Double): String {
-    val rounded = kotlin.math.round(v * 100.0) / 100.0
-    return "%,.2f".format(rounded)
-}
+private fun roundHalfUp(v: Double): BigDecimal =
+    BigDecimal.valueOf(v).setScale(2, RoundingMode.HALF_UP)
+
+private fun formatTwd(v: Double): String =
+    DecimalFormat("#,##0.00").format(roundHalfUp(v))
 
 private fun formatTwdRaw(v: Double): String =
-    "%.2f".format(kotlin.math.round(v * 100.0) / 100.0)
+    roundHalfUp(v).toPlainString()
 
 class MainActivity : ComponentActivity() {
     override fun onCreate(savedInstanceState: Bundle?) {