diff --git a/Source/CMakeLists.txt b/Source/CMakeLists.txt
index 566099554..de91907a7 100644
--- a/Source/CMakeLists.txt
+++ b/Source/CMakeLists.txt
@@ -107,6 +107,7 @@ set(libdevilutionx_SRCS
   utils/console.cpp
   utils/display.cpp
   utils/file_util.cpp
+  utils/format_int.cpp
   utils/language.cpp
   utils/logged_fstream.cpp
   utils/paths.cpp
diff --git a/Source/qol/xpbar.cpp b/Source/qol/xpbar.cpp
index f2cece0c9..9ef1fe3f6 100644
--- a/Source/qol/xpbar.cpp
+++ b/Source/qol/xpbar.cpp
@@ -7,6 +7,8 @@
 
 #include <array>
 
+#include <fmt/core.h>
+
 #include "DiabloUI/art_draw.h"
 #include "control.h"
 #include "engine/point.hpp"
diff --git a/Source/utils/format_int.cpp b/Source/utils/format_int.cpp
new file mode 100644
index 000000000..68e98d8ad
--- /dev/null
+++ b/Source/utils/format_int.cpp
@@ -0,0 +1,48 @@
+#include "utils/format_int.hpp"
+
+#include <fmt/compile.h>
+#include <fmt/core.h>
+
+#include "utils/language.h"
+#include "utils/stdcompat/string_view.hpp"
+
+namespace devilution {
+
+std::string FormatInteger(int n)
+{
+	constexpr size_t GroupSize = 3;
+
+	char buf[40];
+	char *begin = buf;
+	const char *end = fmt::format_to(buf, FMT_COMPILE("{}"), n);
+	const size_t len = end - begin;
+
+	std::string out;
+	const size_t prefixLen = n < 0 ? 1 : 0;
+	const size_t numLen = len - prefixLen;
+	if (numLen <= GroupSize) {
+		out.append(begin, len);
+		return out;
+	}
+
+	const string_view separator = _(/* TRANSLATORS: Thousands separator */ ",");
+	out.reserve(len + separator.size() * (numLen - 1) / GroupSize);
+	if (n < 0) {
+		out += '-';
+		++begin;
+	}
+
+	size_t mlen = numLen % GroupSize;
+	if (mlen == 0)
+		mlen = GroupSize;
+	out.append(begin, mlen);
+	begin += mlen;
+	for (; begin != end; begin += GroupSize) {
+		AppendStrView(out, separator);
+		out.append(begin, GroupSize);
+	}
+
+	return out;
+}
+
+} // namespace devilution
diff --git a/Source/utils/format_int.hpp b/Source/utils/format_int.hpp
index eae4bae91..733eb5512 100644
--- a/Source/utils/format_int.hpp
+++ b/Source/utils/format_int.hpp
@@ -1,33 +1,12 @@
-#pragma once
-
-#include <string>
-
-#include <fmt/format.h>
-
-#include "utils/language.h"
-#include "utils/stdcompat/string_view.hpp"
-
-namespace devilution {
-
-/**
- * @brief Formats integer with thousands separator.
- */
-inline std::string FormatInteger(int n)
-{
-	std::string number = fmt::format("{:d}", n);
-	std::string out = "";
-
-	int length = number.length();
-	int mlength = length % 3;
-	if (mlength == 0)
-		mlength = 3;
-	out.append(number.substr(0, mlength));
-	for (int i = mlength; i < length; i += 3) {
-		AppendStrView(out, _(/* TRANSLATORS: Thousands separator */ ","));
-		out.append(number.substr(i, 3));
-	}
-
-	return out;
-}
-
-} // namespace devilution
+#pragma once
+
+#include <string>
+
+namespace devilution {
+
+/**
+ * @brief Formats integer with thousands separator.
+ */
+std::string FormatInteger(int n);
+
+} // namespace devilution
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 38189e8f4..ca1120dd4 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -25,6 +25,7 @@ set(tests
   drlg_l4_test
   effects_test
   file_util_test
+  format_int_test
   inv_test
   lighting_test
   math_test
diff --git a/test/format_int_test.cpp b/test/format_int_test.cpp
new file mode 100644
index 000000000..4514a10a3
--- /dev/null
+++ b/test/format_int_test.cpp
@@ -0,0 +1,40 @@
+
+#include <gtest/gtest.h>
+
+#include "utils/format_int.hpp"
+
+namespace devilution {
+namespace {
+
+TEST(FormatIntegerTest, OneDigit)
+{
+	EXPECT_EQ(FormatInteger(1), "1");
+	EXPECT_EQ(FormatInteger(-1), "-1");
+}
+
+TEST(FormatIntegerTest, TwoDigits)
+{
+	EXPECT_EQ(FormatInteger(12), "12");
+	EXPECT_EQ(FormatInteger(-12), "-12");
+}
+
+TEST(FormatIntegerTest, ThreeDigits)
+{
+	EXPECT_EQ(FormatInteger(123), "123");
+	EXPECT_EQ(FormatInteger(-123), "-123");
+}
+
+TEST(FormatIntegerTest, FourDigits)
+{
+	EXPECT_EQ(FormatInteger(1234), "1,234");
+	EXPECT_EQ(FormatInteger(-1234), "-1,234");
+}
+
+TEST(FormatIntegerTest, SevenDigits)
+{
+	EXPECT_EQ(FormatInteger(1234567), "1,234,567");
+	EXPECT_EQ(FormatInteger(-1234567), "-1,234,567");
+}
+
+} // namespace
+} // namespace devilution