# Public NAT Gateway Billing

## Prerequisites

* Public NAT gateway resources are all [**pay-as-you-go**](/welcome/pricing/general-pricing-rules/pay-as-you-go.md) resources charged by hour, with a minimum of one hour price. Please first see [**General Pricing Rules**](/welcome/pricing/general-pricing-rules.md) and [**General Lifecycle Rules**](/welcome/pricing/general-lifecycle-rules.md) to know about basic pricing ways of pay-as-you-go resources.
* Public NAT gateway currently includes the following pricing items: instance fees and CU fees. As NAT gateway needs to bind elastic IPs to access Internet, you'll be charged for the elastic IPs that are associated with the NAT gateway. See [**Elastic IPv4**](/welcome/pricing/elastic-compute-billing.md#elastic-ipv4) for more details.

{% hint style="info" %} <mark style="color:blue;">**Note**</mark>

<mark style="color:blue;">The actual price varies in different locations and zones, therefore the price shown on the order page shall prevail.</mark>
{% endhint %}

## Billing Items

### Public NAT Gateway Instance

The billing method of the public NAT gateway is [**Pay-as-you-go**](/welcome/pricing/general-pricing-rules/pay-as-you-go.md). You pay for your resources afterward by hour.

### CU

A CU (Capacity Unit) is the smallest unit used to measure resources consumed by a public NAT gateway. The number of CUs is determined by the highest value between concurrent connections per minute and new connections per second (CPS), and further affected by the CU coefficient.

**Metrics**

* **Concurrent Connections**\
  Concurrent Connections refer to the number of connections that are active and being handled simultaneously by a public NAT gateway at a given time (per minutes).
* **CPS**\
  Connections Per Second (CPS) refers to the number of new connections that a public NAT gateway can handle in one second.

**CU Coefficient**

To calculate the number of CUs, divide the highest value between concurrent connections per minute and CPS by the corresponding coefficient.

<table><thead><tr><th width="281.70703125">Metric</th><th>CU Coefficient</th></tr></thead><tbody><tr><td>Concurrent Connections</td><td>10,000</td></tr><tr><td>CPS</td><td>1,000</td></tr></tbody></table>

#### **CU Fee Formula**

> * CU fee = CU unit price (USD/CU) × Number of CUs per hour × Duration (hours)
>
> * Number of CUs per hour = max {Number of CUs for concurrent connections, Number of CUs for CPS}
>
> > Number of CUs for concurrent connections = Highest concurrent connections value/CU coefficient
> >
> > Number of CUs for CPS = Highest CPS value/CU coefficient

#### CU Billing Example

You create a pay-as-you-go public NAT gateway. You release it after one hour. The following table describes the highest concurrent connections value, the highest CPS value, and the total amount of data transfer during this period.

<table><thead><tr><th width="236.663330078125">Metric</th><th>Number of CUs</th></tr></thead><tbody><tr><td>Concurrent Connections</td><td><p>The highest value within the hour is 500,000.</p><p>Number of CUs = 50,000/10,000 = 5.0</p></td></tr><tr><td>CPS</td><td><p>The highest value within the hour is 2,000.</p><p>Number of CUs = 2,000/1000 = 2.0</p></td></tr></tbody></table>

The Number of CUs per hour\
\= max {Number of CUs for concurrent connections, Number of CUs for CPS}\
\= Number of CUs for concurrent connections\
\= 5.0

Assume the CU unit price is $0.500/h, then your CU fee will be:

CU fee\
\= CU unit price (USD/CU) × Number of CUs per hour × Duration (hours)\
\= $0.500/CU/h × 5.0 CUs × 1 h\
\= $2.500


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