"Bitumen" is the binder in asphalt concrete; this tool uses the standard 145 lb/ft³ hot-mix density.
"Bitumen" is another name for asphalt — the same binder-and-aggregate hot mix. This page uses the identical 145 lb/ft³ engine as the main asphalt calculator, so your tonnage matches whichever term your supplier uses.
Why order extra? The headline number is the exact material your project needs. When you actually order, add about 5–10% on top — the “with 5% waste” figure in the result is a safe starting point. That margin covers what you lose to compaction, cuts around edges and corners, and ordinary spillage during handling. It is cheaper to have a little left over than to stop mid-job and wait on a second delivery.