diff --git a/code/setup.dm b/code/setup.dm
index 39a9b4b06fd..0fe085d5a6b 100644
--- a/code/setup.dm
+++ b/code/setup.dm
@@ -21,7 +21,7 @@
 
 #define BREATH_VOLUME 0.5	//liters in a normal breath
 #define BREATH_MOLES (ONE_ATMOSPHERE * BREATH_VOLUME /(T20C*R_IDEAL_GAS_EQUATION))
-#define BREATH_PERCENTAGE BREATH_MOLES/MOLES_CELLSTANDARD
+#define BREATH_PERCENTAGE BREATH_VOLUME/CELL_VOLUME
 	//Amount of air to take a from a tile
 #define HUMAN_NEEDED_OXYGEN	MOLES_CELLSTANDARD*BREATH_PERCENTAGE*0.16
 	//Amount of air needed before pass out/suffocation commences