diff --git a/c/builtin.c b/c/builtin.c
index b65bb09b..7943675d 100644
--- a/c/builtin.c
+++ b/c/builtin.c
@@ -100,6 +100,30 @@ static void f_divide(jv input[], jv output[]) {
   }  
 }
 
+static void f_tonumber(jv input[], jv output[]) {
+  if (jv_get_kind(input[0]) == JV_KIND_NUMBER) {
+    output[0] = input[0];
+  } else if (jv_get_kind(input[0]) == JV_KIND_STRING) {
+    jv parsed = jv_parse(jv_string_value(input[0]));
+    if (!jv_is_valid(parsed)) {
+      jv_free(input[0]);
+      output[0] = parsed;
+    } else if (jv_get_kind(parsed) != JV_KIND_NUMBER) {
+      output[0] = jv_invalid_with_msg(jv_string_fmt("'%s' is not a number",
+                                                    jv_string_value(input[0])));
+      jv_free(input[0]);
+    } else {
+      jv_free(input[0]);
+      output[0] = parsed;
+    }
+  } else {
+    output[0] = jv_invalid_with_msg(jv_string_fmt("Cannot parse %s as a number",
+                                                  jv_kind_name(jv_get_kind(input[0]))));
+    jv_free(input[0]);
+    return;
+  }
+}
+
 struct cfunction function_list[] = {
   {f_true, "true", CALL_BUILTIN_1_1},
   {f_false, "false", CALL_BUILTIN_1_1},
@@ -108,5 +132,6 @@ struct cfunction function_list[] = {
   {f_minus, "_minus", CALL_BUILTIN_3_1},
   {f_multiply, "_multiply", CALL_BUILTIN_3_1},
   {f_divide, "_divide", CALL_BUILTIN_3_1},
+  {f_tonumber, "tonumber", CALL_BUILTIN_1_1},
 };
 struct symbol_table builtins = {function_list, sizeof(function_list)/sizeof(function_list[0])};
diff --git a/c/testdata b/c/testdata
index eb9f0c5f..45140242 100644
--- a/c/testdata
+++ b/c/testdata
@@ -183,6 +183,10 @@ null
 null
 10
 
+1 + tonumber + ("10" | tonumber)
+4
+15
+
 #
 # User-defined functions
 # Oh god.