Chimney Draft Calculator: How Does It Work?

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An optimal chimney draft serves two main purposes: it allows efficient heating in your house and prevents harmful gasses from polluting it. But to determine whether your chimney draft is optimal, you need to calculate it using a chimney draft calculator.

A draft calculator helps calculate the draft and determine whether your chimney has backdraft problems to ensure proper functioning. It also determines whether the chimney is sized appropriately for the appliance. In short, determining proper chimney height is essential for its safety and functionality.

How Do You Calculate The Natural Draft In a Chimney?

You can calculate the chimney draft to determine fireplace cold airflow issues using the air draft formula given below:

Technicians measuring the temperature inside a chimney.

Air draft = ((Inside temperature – Outside temperature) / (Chimney height x 0.0127)) – (Wind speed x 0.015).

But to use this formula, you’ll need to measure the values required in the formula. Here’s how:

Step 1: Measure the outside and inside temperatures of the chimney

  1. Use a thermometer (costs $5-10) to measure the inside temperature. Remember to purchase one that you can use for high temperatures.
  2. Turn your fireplace on and let it heat up for 10 minutes.
  3. Measure the temperature of the flue gasses inside the chimney by inserting the thermometer probe into the flue opening.
  4. Don’t take the thermometer out immediately. Let it sit there for a minute or so.
  5. Measuring the outside temperature depends on your chimney. If it’s on the wall, measure from the ground. If it’s on the roof, climb up and take the reading from there.
  6. Record the temperature and write down your readings.


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Step 2: Determine the wind speed

An anemometer.

You can purchase a handheld anemometer (costs $20-30) to calculate the wind speed. It’s a portable device that is useful in these situations. Follow the instructions on the package to determine the wind speed near your chimney.

Step 3: Measure the height of your chimney

A man on a ladder leaning on a chimney.

There are two ways to determine the height of your chimney. You can measure the height manually or use a chimney length calculator.

Follow the steps below to measure the height of your chimney manually:

  1. Position the ladder to measure the height of your chimney from the ground up.
  2. Extend the measuring tape to the top and ensure that the measuring tape is long enough for your chimney.
  3. Record the measurement in feet or meters, according to your preference.
  4. Repeat the process to take multiple measurements for accuracy.

Follow the steps below if you want to use a chimney length calculator:

  1. Find a reliable chimney length calculator (you can easily find one online).
  2. Properly record the measurements you need to insert in the calculator. Take multiple measurements for accuracy.
  3. Some chimneys have bends and turns that reduce the accuracy of the calculator and change the dynamics altogether. Ensure that you consider the bends and turns while measuring the height of the chimney.
  4. Use 2-3 sources to get a ballpark figure that you can work around.

Traditional methods of measuring chimney height depend on guesswork. This can lead to an inaccurate chimney length. That is why you should always use a chimney length calculator. You can use traditional methods to compare your results.

The internet has various types of chimney length calculators. The most common ones are:

  1. Online Chimney Sizing Calculator: This calculator asks for data (such as outside and inside temperature, wind speed, the height of the chimney, etc.) and gives the optimal chimney length.
  2. Chimney Sizing: It calculates the chimney draft and required chimney area, and also presents the formulas it uses.
  3. Chimney Draft Calculator: This determines the draft needed by your heating system to operate smoothly. It asks for the diameter of the flue, the type of appliance used, and more.

Step 4: Use the chimney draft calculator equation to determine the natural draft

Insert the values in the following equation and calculate the natural draft of your chimney:

Air draft = ((Inside temperature – Outside temperature) / (Chimney height x 0.0127)) – (Wind speed x 0.015)

For example, let’s assume the outside temperature is 30 degrees Fahrenheit, the inside temperature is 70 degrees Fahrenheit, the height of the chimney is 15 feet, and the wind speed is 5 miles per hour. The air draft for this example would be:

Air draft = ((70°F – 30°F) / (15 x 0.0127)) – (5 x 0.015) = 0.51 inches


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How Much Draft Should a Chimney Have?

The optimal chimney draft depends upon several factors, such as heating appliance type and weather conditions. That is why it’s best to consult a professional to ensure you have an optimal chimney draft.

A chimney with an optimal draft will provide sufficient heating, keeping you warm and toasty, and keeping your house safe from harmful gasses.

If the draft is too low, the chimney will have a buildup of creosote (a black, tar-like substance) over time. This can lead to house fires. Moreover, your heating system will have to work harder to prevent the temperature from falling, leading to higher electricity bills.

If the chimney has an excess draft, the heating efficiency of your fire will significantly drop. Too much draft also raises the chances of house fires since flames are constantly drawn up into the chimney. Not to mention that this will increase fuel consumption, thereby raising your heating bills.

Does a Longer Chimney Increase Draft?

A very tall commercial chimney.

Yes, a longer chimney increases the draft. That is because at higher altitudes, the pressure difference between the top of the chimney and its base is much higher. This pressure difference results in a stronger draft. 

As the draft increases, the combustion gasses leave the chimney’s system at a faster rate. Your chimney’s overall performance will increase, plus the chimney will perform well even in colder temperatures (since the draft is strong).

Therefore, having a chimney with optimal height is important to maintain its efficiency, safety, and functionality.


The draft of a chimney is essential because it optimizes heating and ensures safety. The optimal way to calculate it is by using the chimney draft calculator formula we covered in this article. If you do not have the time or resources to calculate the draft or simply want to rely on an expert opinion, it is better to consult a professional chimney sweep.

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Thomas Green

Thomas has worked in the Chimney & Fireplace field for over 12 years. He is an expert in his trade and loves to help People with their needs. Thomas Write helpful articles so that homeowners can make the most informed decisions about their fireplace and chimney.