Each time sales gives a customer a delivery date it is actually making a commitment. In many organizations that date is set from the salesperson's guess, the standard lead time, or customer pressure, not from actual stock and capacity. The result is repeated delays, low customer trust and constant pressure on production and the warehouse for a 'miracle'.
ATP (available to promise) is the quantity that can be promised to a new customer without conflicting with earlier commitments. In its simplest form: current stock plus planned production or receipts, minus previously confirmed orders. Cumulative ATP over time shows how much is committable at each date.
CTP (capable to promise) goes one step further. When stock is not enough, CTP checks whether, given production capacity and material supply, the goods can be made or procured by a certain date. This matters for make-to-order products or low-stock items, because the delivery date depends on free capacity, available materials and preparation time.
The key in practice is a single source for viewing stock and commitments. If sales uses one table, the warehouse another and production a third plan, the same quantity can be promised twice. Even without expensive software, a shared sheet with columns for stock, planned receipts, confirmed orders and promisable quantity is a big step.
The allocation rule matters too. When stock is limited, who gets priority? A key customer's order, an emergency order, or first come first served? These rules are best written before a crisis, so that at the moment of shortage the decision follows policy rather than pressure and negotiation. Part of the stock can be reserved for priority customers or channels.
A common mistake is giving an optimistic delivery date to win an order. It raises sales in the short term but destroys customer trust in the long term. A reliable date that is a little later is usually better than an earlier one that gets broken. Another mistake is forgetting supplier lead-time variability when counting future receipts; do not treat a receipt that has not yet arrived as too certain.
The right metric is OTIF against the promised date: what percentage of orders were delivered complete on the date committed to the customer? If this is low, the problem is often in the promising, not only in execution. Separating lateness against the promised date from lateness against the customer's requested date shows two different problems.
A practical starting point: for your 20 best-selling items, build a simple weekly ATP table and require sales to look at it before promising. After a month, compare the percentage of broken commitments with the previous month.