What Is ATP (Available to Promise)?
Also known as: available to promise, ATP quantity
Definition
ATP (available to promise) is the portion of inventory and scheduled production not yet committed to existing customer orders, calculated by date so order entry can quote a delivery a customer can rely on.
ATP (Available to Promise) Explained
ATP is computed as on-hand inventory plus scheduled receipts, minus quantities already committed to customer orders, bucketed by date. The crucial detail is the time dimension. A discrete ATP calculation looks only at the supply arriving in each bucket, so a promise cannot be made from stock that has not landed yet. Cumulative ATP carries forward unconsumed availability from earlier buckets, giving a more generous but still defensible answer. Most ERPs implement cumulative ATP with or without look-ahead, where look-ahead prevents overpromising against a later commitment.
In order entry, ATP is the difference between a real date and a guess. When a customer service representative enters a line for 200 units on the 15th, an ATP check answers whether 200 uncommitted units will exist by the 15th and, if not, the first date on which they will. Without ATP the representative either quotes from memory or promises the standard lead time regardless of actual position, both of which produce the pattern where the plant is busy and customers are still unhappy.
ATP only considers material supply that is already planned. It does not ask whether the plant has capacity to make more, and it does not create new supply. That boundary is exactly where capable to promise takes over. It is also why ATP figures degrade when the master production schedule is not maintained: the scheduled receipts feeding the calculation come from planned and firm production, so a stale schedule produces confident but wrong promises.
Allocation policy determines who gets scarce ATP. Common approaches include first come first served, customer priority tiers, and channel or region allocation where a defined quantity is reserved for a key account or a distribution partner. In aerospace and defense, contractual priority ratings under the Defense Priorities and Allocations System can legally override commercial allocation rules, which is a configuration point many commercial-first ERPs handle only with customization.
Why It Matters
- ATP converts order entry from an optimistic guess into a defensible commitment, which is the foundation of on-time delivery performance.
- Correct ATP prevents double-promising the same units to two customers, the root cause of most last-minute allocation fights.
- Because ATP reads scheduled receipts, it exposes master production schedule neglect immediately and visibly.
- Allocation rules embedded in ATP determine which customers absorb shortages, a commercial decision often left to system defaults by accident.
In Practice
Worked example: on-hand is 100, a work order for 150 completes on the 20th, and existing orders commit 80 units in the current bucket. Discrete ATP today is 20, and 150 more becomes available on the 20th. Promising 120 units for the 10th is overpromising even though total supply exceeds demand, because the timing does not work.
Frequently Asked Questions
What is the difference between discrete and cumulative ATP?
Discrete ATP evaluates each time bucket in isolation, so unused availability does not roll forward. Cumulative ATP carries unconsumed quantity into later buckets, producing larger promisable numbers. Cumulative with look-ahead is the safest common variant, because it subtracts future commitments before releasing availability and avoids promising units that a later order already needs.
Does ATP consider production capacity?
No. ATP only looks at inventory and supply that is already scheduled or planned. It will not tell you whether the plant could make more within the requested window. That question requires capable to promise, which runs a constraint check against material availability and work center capacity before returning a date.
Related Terms
CTP (Capable to Promise)
CTP (capable to promise) is an order promising method that goes beyond available inventory by simulating whether required material and work center capacity can be obtained in time, returning a delivery date the plant can actually achieve.
MPS (Master Production Schedule)
An MPS (master production schedule) is the time-phased statement of which end items a plant will produce, in what quantities, and in which periods. It is the anchor demand input MRP explodes into component requirements.
Lead Time
Lead time is the elapsed time between initiating an order and having the material available for use. In ERP it is the offset MRP applies when back-scheduling a planned order from its due date to its release date.
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Free tools
- Available to Promise (ATP) CalculatorCalculate discrete ATP from on hand, scheduled MPS quantities and committed customer orders, and find the earliest period that can cover a new request.
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- SAP SD ATP Availability Check CalculatorModel the available-to-promise logic: what quantity can be confirmed now and what is backordered, with a sweep of the requested quantity.
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