diff --git a/CHANGELOG.md b/CHANGELOG.md
index c582978..fb4e539 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,8 @@ It's about time I started a changelog! This will serve from now on as the main c
  - `//count`: Make numbers human-readable
      - Tip: Use a monospace font for the chat window, and the columns will be aligned!
  - Add `//hollow` for hollowing out areas (a step towards parity with Minecraft WorldEdit)
+ - `//subdivide`: Improve performance of initial chunk counting algorithm - it should get started on the job _much_ quicker now (especially on large regions)
+ - Bugfix: Fix obscure crash in calls to `human_size` ("unknown" will now be returned if passed junk)
 
 
 ## v1.10 (16th January 2021)
diff --git a/test_count_chunks.lua b/test_count_chunks.lua
new file mode 100644
index 0000000..d8a7cab
--- /dev/null
+++ b/test_count_chunks.lua
@@ -0,0 +1,76 @@
+--[[
+test_count_chunks
+
+Test script for the new count_chunks implementation for //subdivide.
+The new implementation does the maths properly and is functionally identical to
+the previous algorithm - we've tested it with 1000 totally random pos1, pos2,
+and chunk_size values.
+]]--
+
+local function count_chunks(pos1, pos2, chunk_size)
+	local count = 0
+	for z = pos2.z, pos1.z, -chunk_size.z do
+		for y = pos2.y, pos1.y, -chunk_size.y do
+			for x = pos2.x, pos1.x, -chunk_size.x do
+				count = count + 1
+			end
+		end
+	end
+	return count
+end
+
+local function count_chunks_new(pos1, pos2, chunk_size)
+	-- Assume pos1 & pos2 are sorted
+	local dimensions = {
+		x = pos2.x - pos1.x + 1,
+		y = pos2.y - pos1.y + 1,
+		z = pos2.z - pos1.z + 1,
+	}
+	-- print("[new] dimensions", dimensions.x, dimensions.y, dimensions.z)
+	return math.ceil(dimensions.x / chunk_size.x)
+		* math.ceil(dimensions.y / chunk_size.y)
+		* math.ceil(dimensions.z / chunk_size.z)
+end
+
+
+local pos1	= { x = 16000, y = 0, z = 16000 }
+local pos2	= { x = 16100, y = 100, z = 16100 }
+local count	= { x = 7, y = 9, z = 10 }
+
+print("Reference: ", count_chunks(pos1, pos2, count))
+print("New: ", count_chunks_new(pos1, pos2, count))
+
+local ok = 0
+local failed = false
+for i=1,1000 do
+	pos1.x = 16000 + math.floor(math.random() * 99)
+	pos1.y = 16000 + math.floor(math.random() * 99)
+	pos1.z = 16000 + math.floor(math.random() * 99)
+
+	pos2.x = 16100 + math.floor(math.random() * 2000)
+	pos2.y = 16100 + math.floor(math.random() * 2000)
+	pos2.z = 16100 + math.floor(math.random() * 2000)
+	
+	count.x = math.floor(math.random() * 25) + 1
+	count.y = math.floor(math.random() * 25) + 1
+	count.z = math.floor(math.random() * 25) + 1
+	
+	io.write(ok..": ( "..pos1.x..", "..pos1.y..", "..pos1.z.." ) - ( "..pos2.x..", "..pos2.y..", "..pos2.z.." ), size ( ", count.x, ", ", count.y, ", ", count.z, " ) | ")
+	
+	local val_ref = count_chunks(pos1, pos2, count)
+	local val_new = count_chunks_new(pos1, pos2, count)
+	
+	if val_ref == val_new then
+		print("OK")
+		ok = ok + 1
+	else
+		print("FAIL! ( "..val_ref.." / "..val_new.." )")
+		failed = true
+		break
+	end
+	
+end
+
+io.write(ok.." ok")
+if failed then io.write(", 1 failed") end
+print()
diff --git a/worldeditadditions/lib/subdivide.lua b/worldeditadditions/lib/subdivide.lua
index 8a0dfd0..c57a563 100644
--- a/worldeditadditions/lib/subdivide.lua
+++ b/worldeditadditions/lib/subdivide.lua
@@ -6,19 +6,22 @@
 local wea = worldeditadditions
 
 -- Counts the number of chunks in the given area.
--- TODO: Do the maths properly here instead of using a loop - the loop is *very* inefficient - especially for large areas
+-- Maths is now done properly. Values from this new implementation were tested
+-- with 1000 random pos1, pos2, and chunk_size combinations and found to be identical.
 local function count_chunks(pos1, pos2, chunk_size)
-	local count = 0
-	for z = pos2.z, pos1.z, -chunk_size.z do
-		for y = pos2.y, pos1.y, -chunk_size.y do
-			for x = pos2.x, pos1.x, -chunk_size.x do
-				count = count + 1
-			end
-		end
-	end
-	return count
+	-- Assume pos1 & pos2 are sorted
+	local dimensions = {
+		x = pos2.x - pos1.x + 1,
+		y = pos2.y - pos1.y + 1,
+		z = pos2.z - pos1.z + 1,
+	}
+	-- print("[new] dimensions", dimensions.x, dimensions.y, dimensions.z)
+	return math.ceil(dimensions.x / chunk_size.x)
+		* math.ceil(dimensions.y / chunk_size.y)
+		* math.ceil(dimensions.z / chunk_size.z)
 end
 
+
 local function merge_stats(a, b)
 	for key,value in pairs(a) do
 		if not b[key] then b[key] = 0 end