---
title: The 10% Rule
description: Learn about the 10% rule in airflow design, explaining how tubing length impacts outlet airflow and ways to optimize system performance
---
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# The 10% Rule
## The 10% rule states that for every 5ft added above 10ft of supply tubing, we see a 10% reduction in airflow at the outlet.
#### When building a [fastquote](https://helpcenter.unicosystem.com/knowledge/what-is-the-fastquote?hsLang=en), it's important to make sure you caclulate the right CFM in your system design.
### Introduction
The 10% rule states that for every 5ft added above 10ft of supply tubing, we see a 10% reduction in airflow at the outlet. The longer the supply tubing is, the lower the airflow at the outlet. The 10% rule is crucial to delivering well-designed systems when working with the Unico system.
### Fully Rated Outlet
If a supply tube is 10ft long, the airflow into the tubing matches what is delivered at the outlet. This is known as a "fully rated outlet" because up to 10ft, there is no loss of airflow.
### 10% Rule in Action: Example
If 35CFM enters 20ft of supply tubing, only 28CFM is delivered at the outlet. This outlet is delivering ~80% (28CFM) of the airflow of a fully rated outlet (35CFM).
### Design Example
If a room requires 350CFM to be conditioned, and fully rated outlets are 35CFM, 10 outlets are necessary to condition the space.
When the 10% rule is applied, those 10 outlets deliver less than 35CFM, resulting in an under-conditioned room:
#### **1. Apply The 10% Rule**
| **Target Outlet CFM** | **Supply Tubing** | **10% Outlet Rule CFM** |
| --- | --- | --- |
| 35CFM | 20ft | = 28CFM |
#### **2. Calculate the total CFM delivered by outlets into the room**
| **10% Outlet Rule CFM** | **Number of Outlets** | CFM delivered to room |
| --- | --- | --- |
| 28CFM | 20ft | = 280CFM |
#### **3. Check the CFM difference between what the room requires, and what is actually delivered**
| **Room CFM Requirement** | **CFM Delivered to Room** | Required vs Delivered Difference |
| --- | --- | --- |
| 350CFM | 280CFM | = 70CFM or 80% |
### **Solving the design challenge**
To balance a design to deliver the required airflow after the 10% rule has been applied, there are three main approaches:
1. Increase the plenum length to reduce the amount of supply tubing required
2. Use 2.5" supply tube to allow more airflow over the same distance
3. Add more outlets to the area to deliver more airflow
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