# How Do You Calculate CFM For Exhaust Fan?

Cubic feet per minute (CFM) is a measurement used to perform the conversion of exhaust air flow measurement. Exhaust fans come in a variety of sizes and styles depending on their design use. **When determining the proper CFM for an exhaust fan, there are three factors to consider**:

**a)** the size of the kitchen area.

**b)** the BTU (British thermal units) and the location of its range.

**c)** the amount of ductwork between the hood and the outside air.

Kitchens require 15 changes per hour (ACH) in order to meet the Building Code Institute’s main ventilation requirements. Follow the instructions for correctly calculating the CFM for kitchen hoods in your home.

Contents

- 1 What is the CFM of an exhaust fan?
- 2 Instructions for calculating the CFM for exhaust fans
- 3 A much simpler example for calculating the CFM of a kitchen hood
- 4 How many CFM is good for a range hood?
- 5 How do you size a commercial exhaust fan?
- 6 Does the hood have to be the same size as the range?
- 7 What is the electrical consumption of an extractor hood?
- 8 What is the difference between a ventilation fan and an exhaust fan?

## What is the CFM of an exhaust fan?

Flow or Volume: This is the capacity of the compressor to press the air over a unit of time and is measured in CFM (Cubic Feet Minute) which is the same as PCM (Cubic Feet per Minute) or l/min (liters per minute).

1 CFM is the acronym for 1 ft³/min = 1 ft³/min. Similarly, to convert cubic feet per minute (ft³/min) to cubic meters per hour (m³/h), multiply by: 1.699.

## Instructions for calculating the CFM for exhaust fans

**1.** Start by measuring the dimensions of the room. It is essential to have an exact length, width and height of the area that the cooker hood will use. Determine the volume of air inside the kitchen by multiplying the three numbers together.

Use a room that is 8 feet wide by 10 feet long by 8 feet high, which equals 640 cubic feet of air. The fan has to move at least 15 times an hour, so multiply the amount of air (640 cubic feet) by 15 to get a total of 9600 cubic feet of air.

The CFM is measured in minutes, so now divide that number by 60. Divide 9600 by 60 to get 160CFM as the starting point for the CFM hood rating.

**2.** Determine the number of BTU’s (British thermal units — the standard measurement for range size) produced by your range and add 100CFM per 10,000 BTU’s. In our example, it uses a range of 20,000 BTU’s located directly under the kitchen hood.

Therefore, you will have to add 200 CFM to the original chapel CFM number (160+200CFM) for a total of 360CFM to be accepted.

**3.** Measure the length of the pipe so that the exhaust hood will overcome the stale air. Use the values in the resource table below and determine the exact length of the duct.

In our example, you have a single controlled duct that is 4 feet of 6-inch pipe directly to a 90-degree elbow, within 10 feet of straight pipe and a cap.

Calculated this way: 7 feet of straight pipe + 20 feet for the elbow + 40 meters for the cap = 67CFM. Add the result to the total CFM (360CFM + 67CFM) to get a calculated CFM rating for the 472CFM bell.

In order to be compatible, you will need to install a fan in the kitchen hood that has been rated at least 430CFM or higher.

## A much simpler example for calculating the CFM of a kitchen hood

Here is another much simpler example to calculate the CFM of a kitchen hood:

**CFM is equal to the area of the hood in (square feet) multiplied by 90.**

CFM= hood area x 90

**For example:**

If you have a hood that measures 4′ x 7′, the calculation to know the CFM would be as follows:

4′ x 7′ = 28 P2

28 x 90= **2,520 CFM**

## How many CFM is good for a range hood?

Air movement or the strength of the hoods is usually calculated in cubic feet per minute (CFM) on the basis of the air movement or power of the hoods. household or commercial kitchens with extensive cooking require a hood that has at least 350 CFM.

This amount of energy is sufficient for removing unwanted excess steam and strong odors from the kitchen. A recommended rate of at least 1 CFM of ventilation per 100 BTU is for high-performance cookers or gas stoves.

For instance, a high-efficiency burner with a 35,000 BTU outlet, it requires a kitchen hood that provides at least 350 CFM to properly and effectively purify the air.

Purchasing a cooker hood that has a higher CFM does have its disadvantages, however, as it produces a louder sound.

Buying an extractor hood with a lower CFM may not suck in all unpleasant odors, smoke, or steam from the kitchen, resulting in poor indoor air quality.

## How do you size a commercial exhaust fan?

The manufacturer of the hood itself determines the airflow volume for the hood depending on the type of equipment located under the hood and depending on its position in the room (island hood) or against a wall.

The Airflow is obtained by the Manufacturer from the standards set by the National Fire Code (NFC). According to the NFC, the volume of airflow may vary according to whether the cooking is done at low, medium, high, or very high heat.

In essence, 10 feet of open flame, wok cooks will need significantly more exhaust than standard equipment from a stove, griddle, fryer, etc.

## Does the hood have to be the same size as the range?

On a residential exhaust hood, the size can be equal to the size of the range, although ideally, the size should protrude at least three inches on each side of the range.

The recommended height is 30 inches above the top of the range, on higher power ranges such as commercial use the overhang of the hood of at least three inches on each side should always be taken into consideration.

## What is the electrical consumption of an extractor hood?

An extractor hood with a normal consumption by market standards can consume up to 200 watts per hour when running at full power, while at its minimum speed it is around 70Wh.

## What is the difference between a ventilation fan and an exhaust fan?

The essential difference is the application of an extractor fan and a fresh air fan. The exhaust fan is used to remove fumes and polluted air from the interior of the building, and the fresh air fan is mainly used for cooling and ventilating purposes in the interior of the building.

**Image Credits: Flickr.com**