diff --git a/base/include/android-base/parseint.h b/base/include/android-base/parseint.h
index 2c8570e..1b7cc5f 100644
--- a/base/include/android-base/parseint.h
+++ b/base/include/android-base/parseint.h
@@ -19,6 +19,7 @@
 
 #include <errno.h>
 #include <stdlib.h>
+#include <string.h>
 
 #include <limits>
 #include <string>
@@ -31,14 +32,20 @@
 // otherwise valid values will be rejected. Returns boolean success; 'out'
 // is untouched if parsing fails.
 template <typename T>
-bool ParseUint(const char* s, T* out,
-               T max = std::numeric_limits<T>::max()) {
+bool ParseUint(const char* s, T* out, T max = std::numeric_limits<T>::max(),
+               bool allow_suffixes = false) {
   int base = (s[0] == '0' && (s[1] == 'x' || s[1] == 'X')) ? 16 : 10;
   errno = 0;
   char* end;
   unsigned long long int result = strtoull(s, &end, base);
-  if (errno != 0 || s == end || *end != '\0') {
-    return false;
+  if (errno != 0 || end == s) return false;
+  if (*end != '\0') {
+    const char* suffixes = "bkmgtpe";
+    const char* suffix;
+    if (!allow_suffixes || (suffix = strchr(suffixes, tolower(*end))) == nullptr) return false;
+#if __clang__  // TODO: win32 still builds with GCC :-(
+    if (__builtin_mul_overflow(result, 1ULL << (10 * (suffix - suffixes)), &result)) return false;
+#endif
   }
   if (max < result) {
     return false;
@@ -49,9 +56,20 @@
 
 // TODO: string_view
 template <typename T>
-bool ParseUint(const std::string& s, T* out,
-               T max = std::numeric_limits<T>::max()) {
-  return ParseUint(s.c_str(), out, max);
+bool ParseUint(const std::string& s, T* out, T max = std::numeric_limits<T>::max(),
+               bool allow_suffixes = false) {
+  return ParseUint(s.c_str(), out, max, allow_suffixes);
+}
+
+template <typename T>
+bool ParseByteCount(const char* s, T* out, T max = std::numeric_limits<T>::max()) {
+  return ParseUint(s, out, max, true);
+}
+
+// TODO: string_view
+template <typename T>
+bool ParseByteCount(const std::string& s, T* out, T max = std::numeric_limits<T>::max()) {
+  return ParseByteCount(s.c_str(), out, max);
 }
 
 // Parses the signed decimal integer in the string 's' and sets 'out' to
diff --git a/base/parseint_test.cpp b/base/parseint_test.cpp
index 483b1d3..fb1c339 100644
--- a/base/parseint_test.cpp
+++ b/base/parseint_test.cpp
@@ -96,3 +96,44 @@
   ASSERT_FALSE(android::base::ParseInt("456x", &u));
   ASSERT_EQ(123u, u);
 }
+
+TEST(parseint, ParseByteCount) {
+  uint64_t i = 0;
+  ASSERT_TRUE(android::base::ParseByteCount("123b", &i));
+  ASSERT_EQ(123ULL, i);
+
+  ASSERT_TRUE(android::base::ParseByteCount("8k", &i));
+  ASSERT_EQ(8ULL * 1024, i);
+
+  ASSERT_TRUE(android::base::ParseByteCount("8M", &i));
+  ASSERT_EQ(8ULL * 1024 * 1024, i);
+
+  ASSERT_TRUE(android::base::ParseByteCount("6g", &i));
+  ASSERT_EQ(6ULL * 1024 * 1024 * 1024, i);
+
+  ASSERT_TRUE(android::base::ParseByteCount("1T", &i));
+  ASSERT_EQ(1ULL * 1024 * 1024 * 1024 * 1024, i);
+
+  ASSERT_TRUE(android::base::ParseByteCount("2p", &i));
+  ASSERT_EQ(2ULL * 1024 * 1024 * 1024 * 1024 * 1024, i);
+
+  ASSERT_TRUE(android::base::ParseByteCount("4e", &i));
+  ASSERT_EQ(4ULL * 1024 * 1024 * 1024 * 1024 * 1024 * 1024, i);
+}
+
+TEST(parseint, ParseByteCount_invalid_suffix) {
+  unsigned u;
+  ASSERT_FALSE(android::base::ParseByteCount("1x", &u));
+}
+
+TEST(parseint, ParseByteCount_overflow) {
+  uint64_t u64;
+  ASSERT_FALSE(android::base::ParseByteCount("4294967295E", &u64));
+
+  uint16_t u16;
+  ASSERT_TRUE(android::base::ParseByteCount("63k", &u16));
+  ASSERT_EQ(63U * 1024, u16);
+  ASSERT_TRUE(android::base::ParseByteCount("65535b", &u16));
+  ASSERT_EQ(65535U, u16);
+  ASSERT_FALSE(android::base::ParseByteCount("65k", &u16));
+}
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index 41a3d6b..780ff50 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -82,7 +82,7 @@
 // libsparse will support INT_MAX, but this results in large allocations, so
 // let's keep it at 1GB to avoid memory pressure on the host.
 static constexpr int64_t RESPARSE_LIMIT = 1 * 1024 * 1024 * 1024;
-static int64_t sparse_limit = -1;
+static uint64_t sparse_limit = 0;
 static int64_t target_sparse_limit = -1;
 
 static unsigned g_base_addr = 0x10000000;
@@ -375,7 +375,7 @@
             " -w                         Wipe userdata.\n"
             " -s SERIAL                  Specify a USB device.\n"
             " -s tcp|udp:HOST[:PORT]     Specify a network device.\n"
-            " -S SIZE[K|M|G]             Use sparse files above this limit (0 to disable).\n"
+            " -S SIZE[K|M|G]             Break into sparse files no larger than SIZE.\n"
             " --slot SLOT                Use SLOT; 'all' for both slots, 'other' for\n"
             "                            non-current slot (default: current active slot).\n"
             " --set-active[=SLOT]        Sets the active slot before rebooting.\n"
@@ -730,13 +730,10 @@
 }
 
 static int64_t get_sparse_limit(Transport* transport, int64_t size) {
-    int64_t limit;
-
-    if (sparse_limit == 0) {
-        return 0;
-    } else if (sparse_limit > 0) {
-        limit = sparse_limit;
-    } else {
+    int64_t limit = sparse_limit;
+    if (limit == 0) {
+        // Unlimited, so see what the target device's limit is.
+        // TODO: shouldn't we apply this limit even if you've used -S?
         if (target_sparse_limit == -1) {
             target_sparse_limit = get_target_sparse_limit(transport);
         }
@@ -1197,47 +1194,6 @@
     fb_queue_command(command, "");
 }
 
-static int64_t parse_num(const char *arg)
-{
-    char *endptr;
-    unsigned long long num;
-
-    num = strtoull(arg, &endptr, 0);
-    if (endptr == arg) {
-        return -1;
-    }
-
-    if (*endptr == 'k' || *endptr == 'K') {
-        if (num >= (-1ULL) / 1024) {
-            return -1;
-        }
-        num *= 1024LL;
-        endptr++;
-    } else if (*endptr == 'm' || *endptr == 'M') {
-        if (num >= (-1ULL) / (1024 * 1024)) {
-            return -1;
-        }
-        num *= 1024LL * 1024LL;
-        endptr++;
-    } else if (*endptr == 'g' || *endptr == 'G') {
-        if (num >= (-1ULL) / (1024 * 1024 * 1024)) {
-            return -1;
-        }
-        num *= 1024LL * 1024LL * 1024LL;
-        endptr++;
-    }
-
-    if (*endptr != '\0') {
-        return -1;
-    }
-
-    if (num > INT64_MAX) {
-        return -1;
-    }
-
-    return num;
-}
-
 static std::string fb_fix_numeric_var(std::string var) {
     // Some bootloaders (angler, for example), send spurious leading whitespace.
     var = android::base::Trim(var);
@@ -1471,8 +1427,9 @@
                     serial = optarg;
                     break;
                 case 'S':
-                    sparse_limit = parse_num(optarg);
-                    if (sparse_limit < 0) die("invalid sparse limit");
+                    if (!android::base::ParseByteCount(optarg, &sparse_limit)) {
+                        die("invalid sparse limit %s", optarg);
+                    }
                     break;
                 case 'v':
                     set_verbose();