Available to promise (ATP)
The stock you can genuinely commit to a new customer — quantity on hand less what is already reserved for other orders.
Stock on hand and stock available are not the same number. Available to promise is what is left after subtracting the units already spoken for by confirmed orders that have not yet gone out.
Where the gap bites
You have 40 pieces in the godown. Thirty are reserved against an order being packed for tomorrow. A walk-in customer asks for 20. The screen says 40 and you say yes. Someone is going to be disappointed, and it will probably be the customer who ordered first.
What to subtract
Confirmed sales orders not yet dispatched. Goods set aside for a repair or a replacement. Stock physically present but blocked — damaged, quarantined, or past a near-expiry cut-off. Anything you would not hand over today should not be in the number you promise from.
What can be added
Incoming stock with a firm arrival date can be promised against, if you are honest with the customer about the date. That is a different promise from "we have it now", and mixing the two is how delivery commitments get broken.
How KillStock handles it
Reservations and backorders are tracked so committed stock is not sold twice. On the Growth plan and above, reorder suggestions net reserved stock and backorders rather than working from the raw on-hand figure.
A common mistake
Quoting availability from a stock report that shows quantity on hand only. The report is not wrong; it is answering a different question.