diff --git a/eth_interface.md b/eth_interface.md
index 1c4f66d3..d71d21fe 100644
--- a/eth_interface.md
+++ b/eth_interface.md
@@ -19,11 +19,13 @@ We also define the following WebAssembly data types:
 
 ## useGas
 
-Subtracts an amount to the gas counter
+Reduces the gas left counter by an amount.
+
+It should also charge for memory cost by multiplying `memory_pages * memory_cost * amount`, where `memory_cost` is defined by the gas schedule.
 
 **Parameters**
 
--   `amount` **i64** the amount to subtract to the gas counter
+-   `amount` **i64** the amount to reduce the gas left counter with
 
 **Returns**