diff --git a/src/builtin_math.cpp b/src/builtin_math.cpp
index 9c8f4c8ae..310d2e554 100644
--- a/src/builtin_math.cpp
+++ b/src/builtin_math.cpp
@@ -228,6 +228,15 @@ static int evaluate_expression(const wchar_t *cmd, const parser_t &parser, io_st
 
     int retval = STATUS_CMD_OK;
     te_error_t error;
+    // Switch locale while computing stuff.
+    // This means that the "." is always the radix character,
+    // so numbers work the same across locales.
+    //
+    // TODO: Technically this is only needed for *output* currently,
+    // because we already use wcstod_l while computing,
+    // but we can't have math print numbers that it won't then also read.
+    char *saved_locale = strdup(setlocale(LC_NUMERIC, nullptr));
+    setlocale(LC_NUMERIC, "C");
     double v = te_interp(expression.c_str(), &error);
 
     if (error.position == 0) {
@@ -257,6 +266,8 @@ static int evaluate_expression(const wchar_t *cmd, const parser_t &parser, io_st
         streams.err.append_format(L"%*ls%ls\n", error.position - 1, L" ", L"^");
         retval = STATUS_CMD_ERROR;
     }
+    setlocale(LC_NUMERIC, saved_locale);
+    free(saved_locale);
     return retval;
 }