diff --git a/ChangeLog b/ChangeLog
index 98c3f77e..d4e06437 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,7 +1,7 @@
 2022-01-23  Alex Kotov  <kotovalexarian@gmail.com>
 
 	* include/kernaux/assert.h: Require semicolon after macros
-	* include/kernaux/libc.h: Add funcs "isdigit", "isspace"
+	* include/kernaux/libc.h: Add funcs "atoi", "isdigit", "isspace"
 
 2022-01-22  Alex Kotov  <kotovalexarian@gmail.com>
 
diff --git a/README.md b/README.md
index 3433203f..df8bf4ce 100644
--- a/README.md
+++ b/README.md
@@ -89,6 +89,7 @@ stable options.
 * `--with-libc` - provides the replacement for some standard C functions. Useful
   in freestanding environment, where no libc is present. You can also separately
   include or exclude components:
+  * `--with[out]-libc-atoi`
   * `--with[out]-libc-isdigit`
   * `--with[out]-libc-isspace`
   * `--with[out]-libc-memset`
diff --git a/configure.ac b/configure.ac
index 4970ac11..4ac98338 100644
--- a/configure.ac
+++ b/configure.ac
@@ -35,6 +35,7 @@ AC_ARG_WITH(  [units],        AS_HELP_STRING([--without-units],       [without m
 
 dnl Packages (disabled by default)
 AC_ARG_WITH(  [libc],         AS_HELP_STRING([--with-libc],           [with libc replacement]))
+AC_ARG_WITH(  [libc-atoi],    AS_HELP_STRING([--with-libc-atoi],      [with atoi replacement]))
 AC_ARG_WITH(  [libc-isdigit], AS_HELP_STRING([--with-libc-isdigit],   [with isdigit replacement]))
 AC_ARG_WITH(  [libc-isspace], AS_HELP_STRING([--with-libc-isspace],   [with isspace replacement]))
 AC_ARG_WITH(  [libc-memset],  AS_HELP_STRING([--with-libc-memset],    [with memset replacement]))
@@ -61,6 +62,7 @@ AS_IF([test "$with_all" = no], do_without_all)
 
 AC_DEFUN([do_with_libc],
 [
+if test -z "$with_libc_atoi";    then with_libc_atoi=yes;    fi
 if test -z "$with_libc_isdigit"; then with_libc_isdigit=yes; fi
 if test -z "$with_libc_isspace"; then with_libc_isspace=yes; fi
 if test -z "$with_libc_memset";  then with_libc_memset=yes;  fi
@@ -104,6 +106,7 @@ AM_CONDITIONAL([WITH_PRINTF],       [test "$with_printf"       != no])
 AM_CONDITIONAL([WITH_UNITS],        [test "$with_units"        != no])
 
 dnl Packages (disabled by default)
+AM_CONDITIONAL([WITH_LIBC_ATOI],    [test "$with_libc_atoi"    = yes])
 AM_CONDITIONAL([WITH_LIBC_ISDIGIT], [test "$with_libc_isdigit" = yes])
 AM_CONDITIONAL([WITH_LIBC_ISSPACE], [test "$with_libc_isspace" = yes])
 AM_CONDITIONAL([WITH_LIBC_MEMSET],  [test "$with_libc_memset"  = yes])
@@ -138,6 +141,7 @@ AS_IF([test "$with_printf"       != no], [AC_DEFINE([WITH_PRINTF],           [1]
 AS_IF([test "$with_units",       != no], [AC_DEFINE([WITH_UNITS],            [1], [with measurement units utils])])
 
 dnl Packages (disabled by default)
+AS_IF([test "$with_libc_atoi"    = yes], [AC_DEFINE([WITH_LIBC_ATOI],        [1], [with atoi replacement])])
 AS_IF([test "$with_libc_isdigit" = yes], [AC_DEFINE([WITH_LIBC_ISDIGIT],     [1], [with isdigit replacement])])
 AS_IF([test "$with_libc_isspace" = yes], [AC_DEFINE([WITH_LIBC_ISSPACE],     [1], [with isspace replacement])])
 AS_IF([test "$with_libc_memset"  = yes], [AC_DEFINE([WITH_LIBC_MEMSET],      [1], [with memset replacement])])
diff --git a/include/kernaux/libc.h b/include/kernaux/libc.h
index 69471f59..d9c55002 100644
--- a/include/kernaux/libc.h
+++ b/include/kernaux/libc.h
@@ -11,6 +11,9 @@ extern "C" {
 int isdigit(int c);
 int isspace(int c);
 
+// <stdlib.h>
+int atoi(const char *str);
+
 // <string.h>
 void *memset(void *s, int c, size_t n);
 char *strcpy(char *dest, const char *src);
diff --git a/src/libc.c b/src/libc.c
index 3dc58894..0f28b822 100644
--- a/src/libc.c
+++ b/src/libc.c
@@ -4,6 +4,23 @@
 
 #include <kernaux/libc.h>
 
+#include <stdbool.h>
+
+#ifdef WITH_LIBC_ATOI
+int atoi(const char *str)
+{
+    while (isspace(*str)) ++str;
+    bool is_negative = false;
+    switch (*str) {
+    case '-': is_negative = true;
+    case '+': ++str;
+    }
+    int result = 0;
+    while (isdigit(*str)) result = 10 * result - (*str++ - '0');
+    return is_negative ? result : -result;
+}
+#endif // WITH_LIBC_ATOI
+
 #ifdef WITH_LIBC_ISDIGIT
 int isdigit(const int c)
 {
diff --git a/src/printf.c b/src/printf.c
index 1278a41c..e55d1d41 100644
--- a/src/printf.c
+++ b/src/printf.c
@@ -444,11 +444,9 @@ void _out_fct(char character, void* buffer, size_t idx, size_t maxlen)
 // internal ASCII string to unsigned int conversion
 unsigned int _atoi(const char** str)
 {
-    unsigned int i = 0u;
-    while (isdigit(**str)) {
-        i = i * 10u + (unsigned int)(*((*str)++) - '0');
-    }
-    return i;
+    const int result = atoi(*str);
+    while (isdigit(**str)) (*str)++;
+    return result;
 }
 
 // output the specified string in reverse, taking care of any zero-padding