What is Available-to-Promise (ATP)?

Available-to-Promise (ATP) is an inventory management concept that calculates the quantity of a product that can be reliably committed to new customer orders at any given moment. Unlike a simple on-hand count, ATP accounts for inventory that is already spoken for—allocated to existing orders, reserved for pre-orders, held for quality inspection, or earmarked for warehouse transfers. The ATP calculation provides the true, sellable inventory number that determines whether a business can accept and fulfill a new order without risking an oversell or backorder situation.

Why It Matters

Overselling is one of the most damaging operational failures in e-commerce. When a customer orders a product that appears available but is actually committed to prior orders, the result is a canceled order, a disappointed customer, a potential chargeback, and damage to seller ratings on marketplaces. ATP prevents this by providing an accurate picture of what can actually be sold, rather than what is physically sitting in the warehouse.

For multichannel sellers, ATP becomes even more critical. Inventory shared across Amazon, Shopify, eBay, and wholesale channels must be carefully managed so that selling on one channel doesn’t create oversell situations on others. Without ATP-aware inventory management, businesses either oversell (causing cancellations) or artificially restrict channel quantities (leaving potential revenue on the table). ATP solves this by dynamically calculating availability across all channels simultaneously.

How It Works

ATP is calculated through a real-time inventory equation:

• Basic Formula: ATP = On-Hand Quantity – Allocated Quantity – Reserved Quantity – Held Quantity. For example, if a warehouse has 500 units on hand, 200 are allocated to open orders, 50 are reserved for pre-orders, and 30 are in quality hold, the ATP is 220 units.
• Forward-Looking ATP: Advanced ATP calculations also factor in incoming supply—purchase orders in transit, production orders scheduled for completion, and warehouse transfers en route. This forward-looking view allows businesses to promise inventory that hasn’t arrived yet but is reliably expected, enabling pre-orders and backorder fulfillment planning.
• Channel-Level ATP: For multichannel sellers, ATP can be calculated globally (total available across all channels) or at the channel level (specific quantities allocated to each marketplace). Channel-level ATP allows businesses to strategically distribute inventory—for example, reserving higher margins for direct channels or prioritizing marketplace listing quantities based on sales velocity.
• Real-Time Updates: ATP must recalculate continuously as orders come in, shipments go out, and inventory movements occur. A delay of even minutes can lead to overselling during high-volume sales events. Modern OMS platforms recalculate ATP in real time and push updated availability to all connected channels within seconds.
• Safety Buffers: Some businesses apply safety buffers to ATP—withholding a small percentage of available inventory to absorb count inaccuracies, damage, or unexpected demand spikes. A 5–10% buffer on ATP reduces oversell risk at the cost of slightly lower listed availability.

How Nventory Helps

Nventory calculates ATP in real time across all connected warehouses and sales channels. Every order, return, transfer, and adjustment instantly updates the ATP figure, and updated availability is pushed to all marketplaces and storefronts within seconds. Channel-specific inventory allocation rules let you control how much ATP is distributed to each channel, and configurable safety buffers protect against overselling during peak demand. The result is maximum sellable inventory with minimal oversell risk.