diff --git a/README.md b/README.md
index c0c5217..c9886a9 100644
--- a/README.md
+++ b/README.md
@@ -23,7 +23,7 @@ Run the script `terrain_rivers.py` via command line. You can optionally append t
 ```
 ./terrain_rivers.py 1000
 ```
-For a default 401x401 map, it should take between 1 and 2 minutes. It will generate 5 files directly in the mod folder, containing the map data.
+For a default 400x400 map, it should take between 1 and 2 minutes. It will generate 5 files directly in the mod folder, containing the map data.
 
 ## Map generation
 Just create a Minetest world with `singlenode` mapgen, enable this mod and start the world. The data files are immediately copied in the world folder so you can re-generate them afterwards, it won't affect the old worlds.
diff --git a/terrain_rivers.py b/terrain_rivers.py
index d7c7137..a526d7b 100755
--- a/terrain_rivers.py
+++ b/terrain_rivers.py
@@ -15,14 +15,14 @@ argc = len(sys.argv)
 if argc > 1:
 	mapsize = int(sys.argv[1])
 else:
-	mapsize = 401
+	mapsize = 400
 
-scale = (mapsize-1) / 2
-n = np.zeros((mapsize, mapsize))
+scale = mapsize / 2
+n = np.zeros((mapsize+1, mapsize+1))
 
 # Set noise parameters
 params = {
-    "octaves" : int(np.ceil(np.log2(mapsize-1)))+1,
+    "octaves" : int(np.ceil(np.log2(mapsize)))+1,
     "persistence" : 0.5,
     "lacunarity" : 2.,
 }
@@ -32,8 +32,8 @@ xbase = np.random.randint(65536)
 ybase = np.random.randint(65536)
 
 # Generate the noise
-for x in range(mapsize):
-    for y in range(mapsize):
+for x in range(mapsize+1):
+    for y in range(mapsize+1):
         n[x,y] = noise.snoise2(x/scale + xbase, y/scale + ybase, **params)
 
 nn = n*mapsize/5 + mapsize/20
@@ -87,7 +87,7 @@ save(model.dirs, 'dirs', dtype='u1')
 save(model.rivers, 'rivers', dtype='>u4')
 
 with open('size', 'w') as sfile:
-    sfile.write('{:d}\n{:d}'.format(mapsize, mapsize))
+    sfile.write('{:d}\n{:d}'.format(mapsize+1, mapsize+1))
 
 # Display the map if matplotlib is found
 try: