diff --git a/Makefile b/Makefile
index 5424c3a1858..d8bf04dcb61 100644
--- a/Makefile
+++ b/Makefile
@@ -184,12 +184,14 @@ LIBRARIES += glog gflags protobuf boost_system boost_filesystem m hdf5_hl hdf5
 USE_LEVELDB ?= 1
 USE_LMDB ?= 1
 USE_OPENCV ?= 1
+LMDB_MAP_SIZE ?= 1099511627776
 
 ifeq ($(USE_LEVELDB), 1)
 	LIBRARIES += leveldb snappy
 endif
 ifeq ($(USE_LMDB), 1)
 	LIBRARIES += lmdb
+	COMMON_FLAGS += -D_LMDB_MAP_SIZE=$(LMDB_MAP_SIZE)
 endif
 ifeq ($(USE_OPENCV), 1)
 	LIBRARIES += opencv_core opencv_highgui opencv_imgproc 
diff --git a/Makefile.config.example b/Makefile.config.example
index 8fd49c9c1a7..8477f4e1b6c 100644
--- a/Makefile.config.example
+++ b/Makefile.config.example
@@ -16,6 +16,7 @@
 #	You should not set this flag if you will be reading LMDBs with any
 #	possibility of simultaneous read and write
 # ALLOW_LMDB_NOLOCK := 1
+# LMDB_MAP_SIZE := 4294967295 # 1TB is the default - uncomment this to avoid overflow on 32bit systems.
 
 # Uncomment if you're using OpenCV 3
 # OPENCV_VERSION := 3
diff --git a/examples/mnist/convert_mnist_data.cpp b/examples/mnist/convert_mnist_data.cpp
index 16d28093dd5..46a0515aeb4 100644
--- a/examples/mnist/convert_mnist_data.cpp
+++ b/examples/mnist/convert_mnist_data.cpp
@@ -23,6 +23,7 @@
 #include <string>
 
 #include "caffe/proto/caffe.pb.h"
+#include "caffe/util/db_lmdb.hpp"
 #include "caffe/util/format.hpp"
 
 #if defined(USE_LEVELDB) && defined(USE_LMDB)
@@ -93,7 +94,9 @@ void convert_dataset(const char* image_filename, const char* label_filename,
     CHECK_EQ(mkdir(db_path, 0744), 0)
         << "mkdir " << db_path << "failed";
     CHECK_EQ(mdb_env_create(&mdb_env), MDB_SUCCESS) << "mdb_env_create failed";
-    CHECK_EQ(mdb_env_set_mapsize(mdb_env, 1099511627776), MDB_SUCCESS)  // 1TB
+    // Check that the db size is the user specified one, or the max of size_t.
+    CHECK_EQ(mdb_env_set_mapsize(mdb_env, BIGNUM_OR_MAX(_LMDB_MAP_SIZE)), \
+                                MDB_SUCCESS)
         << "mdb_env_set_mapsize failed";
     CHECK_EQ(mdb_env_open(mdb_env, db_path, 0, 0664), MDB_SUCCESS)
         << "mdb_env_open failed";
diff --git a/include/caffe/util/db_lmdb.hpp b/include/caffe/util/db_lmdb.hpp
index 4e1568ace50..1b0a686387f 100644
--- a/include/caffe/util/db_lmdb.hpp
+++ b/include/caffe/util/db_lmdb.hpp
@@ -2,12 +2,19 @@
 #ifndef CAFFE_UTIL_DB_LMDB_HPP
 #define CAFFE_UTIL_DB_LMDB_HPP
 
+#include <limits>
 #include <string>
 
 #include "lmdb.h"
 
 #include "caffe/util/db.hpp"
 
+#define BIGNUM_OR_MAX(A) ((A) > (std::numeric_limits<size_t>::max()) ?\
+                         (std::numeric_limits<size_t>::max()) : (A))
+#ifndef _LMDB_MAP_SIZE
+#define _LMDB_MAP_SIZE 1099511627776
+#endif
+
 namespace caffe { namespace db {
 
 inline void MDB_CHECK(int mdb_status) {
diff --git a/src/caffe/util/db_lmdb.cpp b/src/caffe/util/db_lmdb.cpp
index 0bc82b53e2b..817a05eecc2 100644
--- a/src/caffe/util/db_lmdb.cpp
+++ b/src/caffe/util/db_lmdb.cpp
@@ -7,7 +7,8 @@
 
 namespace caffe { namespace db {
 
-const size_t LMDB_MAP_SIZE = 1099511627776;  // 1 TB
+// User specified size or max value of size_t.
+const size_t LMDB_MAP_SIZE = BIGNUM_OR_MAX(_LMDB_MAP_SIZE);
 
 void LMDB::Open(const string& source, Mode mode) {
   MDB_CHECK(mdb_env_create(&mdb_env_));