diff --git a/code/modules/organs/external/_external.dm b/code/modules/organs/external/_external.dm
index 8a6ae7ed48d..fc46018a4f7 100644
--- a/code/modules/organs/external/_external.dm
+++ b/code/modules/organs/external/_external.dm
@@ -77,6 +77,8 @@
 	var/atom/movable/applied_pressure
 	var/atom/movable/splinted
 
+	var/internal_organs_size = 0       // Currently size cost of internal organs in this body part
+
 	// HUD element variable, see organ_icon.dm get_damage_hud_image()
 	var/image/hud_damage_image
 
@@ -298,6 +300,10 @@
 		O = O.parent
 	return 0
 
+/obj/item/organ/external/proc/update_internal_organs_cost()
+	internal_organs_size = 0
+	for(var/obj/item/organ/internal/org in internal_organs)
+		internal_organs_size += org.get_storage_cost()
 
 /obj/item/organ/external/proc/dislocate()
 	if(dislocated == -1)
diff --git a/code/modules/organs/internal/_internal.dm b/code/modules/organs/internal/_internal.dm
index 5c0b6fddbd2..10cc7974ced 100644
--- a/code/modules/organs/internal/_internal.dm
+++ b/code/modules/organs/internal/_internal.dm
@@ -23,6 +23,8 @@
 				. = INITIALIZE_HINT_QDEL
 				CRASH("[src] spawned in [holder] without a parent organ: [parent_organ].")
 			E.internal_organs |= src
+			E.cavity_max_w_class = max(E.cavity_max_w_class, w_class)
+			E.update_internal_organs_cost()
 
 /obj/item/organ/internal/Destroy()
 	if(owner)
@@ -53,7 +55,9 @@
 		owner.internal_organs_by_name[organ_tag] = null
 		owner.internal_organs_by_name -= organ_tag
 		owner.internal_organs_by_name -= null
+		var/obj/item/organ/external/E = owner.get_organ(parent_organ)
 		owner.internal_organs -= src
+		E.update_internal_organs_cost()
 
 		if(detach)
 			var/obj/item/organ/external/affected = owner.get_organ(parent_organ)
diff --git a/code/modules/surgery/implant.dm b/code/modules/surgery/implant.dm
index d66aebbaf29..94bfff70223 100644
--- a/code/modules/surgery/implant.dm
+++ b/code/modules/surgery/implant.dm
@@ -106,7 +106,7 @@
 /decl/surgery_step/cavity/place_item/pre_surgery_step(mob/living/user, mob/living/carbon/human/target, target_zone, obj/item/tool)
 	var/obj/item/organ/external/affected = target.get_organ(target_zone)
 	if(affected && affected.cavity)
-		var/max_volume = BASE_STORAGE_CAPACITY(affected.cavity_max_w_class)
+		var/max_volume = BASE_STORAGE_CAPACITY(affected.cavity_max_w_class) + affected.internal_organs_size
 		if(tool.w_class > affected.cavity_max_w_class)
 			to_chat(user, SPAN_WARNING("\The [tool] is too big for [affected.cavity_name] cavity."))
 			return FALSE
@@ -115,6 +115,8 @@
 			if(istype(I,/obj/item/implant))
 				continue
 			total_volume += I.get_storage_cost()
+			for(var/obj/item/organ/internal/org in affected.internal_organs)
+				max_volume -= org.get_storage_cost()
 		if(total_volume > max_volume)
 			to_chat(user, SPAN_WARNING("There isn't enough space left in [affected.cavity_name] cavity for [tool]."))
 			return FALSE