diff --git a/iores.cpp b/iores.cpp
index 5ba42df..62319aa 100644
--- a/iores.cpp
+++ b/iores.cpp
@@ -34,6 +34,7 @@
 #include "ioreth.hpp"
 #include "util.hpp"
 #include "rand.hpp"
+#include "unit_int.hpp"
 
 class Options
 {
@@ -140,16 +141,16 @@ class Options
 
             switch (c) {
             case 's': /* disk access range in blocks */
-                accessRange_ = ::atol(optarg);
+                accessRange_ = fromUnitIntString(optarg);
                 break;
             case 'b': /* blocksize */
-                blockSize_ = ::atol(optarg);
+                blockSize_ = fromUnitIntString(optarg);
                 break;
             case 'p': /* period */
                 period_ = ::atol(optarg);
                 break;
             case 'c': /* count */
-                count_ = ::atol(optarg);
+                count_ = fromUnitIntString(optarg);
                 break;
             case 'w': /* write */
                 mode_ = WRITE_MODE;
diff --git a/ioth.cpp b/ioth.cpp
index 8bd3e38..9081b95 100644
--- a/ioth.cpp
+++ b/ioth.cpp
@@ -35,6 +35,7 @@
 #include "ioreth.hpp"
 #include "util.hpp"
 #include "thread_pool.hpp"
+#include "unit_int.hpp"
 
 /**
  * Parse commane-line arguments as options.
@@ -127,16 +128,16 @@ class Options
 
             switch (c) {
             case 's': /* start offset in blocks */
-                startBlockId_ = ::atol(optarg);
+                startBlockId_ = fromUnitIntString(optarg);
                 break;
             case 'b': /* blocksize */
-                blockSize_ = ::atol(optarg);
+                blockSize_ = fromUnitIntString(optarg);
                 break;
             case 'p': /* period */
                 period_ = ::atol(optarg);
                 break;
             case 'c': /* count */
-                count_ = ::atol(optarg);
+                count_ = fromUnitIntString(optarg);
                 break;
             case 'w': /* write */
                 mode_ = WRITE_MODE;
diff --git a/unit_int.hpp b/unit_int.hpp
new file mode 100644
index 0000000..e81798a
--- /dev/null
+++ b/unit_int.hpp
@@ -0,0 +1,100 @@
+#pragma once
+
+#include <string>
+#include <cstdio>
+#include <cstdlib>
+#include <exception>
+#include <cinttypes>
+
+/**
+ * Formst string with va_list.
+ */
+inline std::string formatStringV(const char *format, va_list ap)
+{
+    char *p = nullptr;
+    int ret = ::vasprintf(&p, format, ap);
+    if (ret < 0) throw std::runtime_error("vasprintf failed.");
+    try {
+        std::string s(p, ret);
+        ::free(p);
+        return s;
+    } catch (...) {
+        ::free(p);
+        throw;
+    }
+}
+
+/**
+ * Create a std::string using printf() like formatting.
+ */
+inline std::string formatString(const char * format, ...)
+{
+    std::string s;
+    std::exception_ptr ep;
+    va_list args;
+    va_start(args, format);
+    try {
+        s = formatStringV(format, args);
+    } catch (...) {
+        ep = std::current_exception();
+    }
+    va_end(args);
+    if (ep) std::rethrow_exception(ep);
+    return s;
+}
+
+/**
+ * Convert size string with unit suffix to unsigned integer.
+ */
+inline uint64_t fromUnitIntString(const std::string &valStr)
+{
+    if (valStr.empty()) {
+        throw std::runtime_error("fromUnitIntString: invalid argument.");
+    }
+    char *endp;
+    uint64_t val = ::strtoll(valStr.c_str(), &endp, 10);
+    int shift = 0;
+    switch (*endp) {
+    case 'e': case 'E': shift = 60; break;
+    case 'p': case 'P': shift = 50; break;
+    case 't': case 'T': shift = 40; break;
+    case 'g': case 'G': shift = 30; break;
+    case 'm': case 'M': shift = 20; break;
+    case 'k': case 'K': shift = 10; break;
+    case '\0': break;
+    default:
+        throw std::runtime_error("fromUnitIntString: invalid suffix charactor.");
+    }
+    if (((val << shift) >> shift) != val) {
+        throw std::runtime_error("fromUnitIntString: overflow.");
+    }
+    return val << shift;
+}
+
+/**
+ * Unit suffixes:
+ *   k: 2^10
+ *   m: 2^20
+ *   g: 2^30
+ *   t: 2^40
+ *   p: 2^50
+ *   e: 2^60
+ */
+inline std::string toUnitIntString(uint64_t val)
+{
+    uint64_t mask = (1ULL << 10) - 1;
+    const char units[] = " kmgtpezy";
+
+    size_t i = 0;
+    while (i < sizeof(units)) {
+        if ((val & ~mask) != val) { break; }
+        i++;
+        val >>= 10;
+    }
+
+    if (0 < i && i < sizeof(units)) {
+        return formatString("%" PRIu64 "%c", val, units[i]);
+    } else {
+        return formatString("%" PRIu64 "", val);
+    }
+}