How to Choose the Correct Size for Your AC Unit

How to Choose the Correct Size for Your AC Unit

Size matters.

At least, it definitely matters when you’re choosing a new air filter for your AC or HVAC unit. Sure, other factors go into the choice – the MERV rating, for instance – but if you have the wrong size for your filter you end up with one of two situations:

  1. You get a filter that’s so small that it can’t even stand up in the space reserved for it.
  2. Your filter is so massive that you have to stuff it into its slot (hint – that won’t work).

Either way, you get a filter that can’t do its job. Let’s ensure that doesn’t happen by using these methods to get you the right filter size.

Method 1 – Check Your System Manual

The simplest methods are often the best, and this is as simple as it gets – check the owner’s manual that came with your AC or HVAC unit.

This is good practice even when you don’t need a filter. It will teach you about specific functions and maintenance issues you need to know about. But when it comes to your filter, the manual should tell you precisely what size you need.

Read it.

Remember it.

And get a filter that matches the size requirements (as well as any other specifications) that the manual points out.

Method 2 – Check Your Filter’s Label

So, you’ve lost your owner’s manual for your HVAC system. It happens. And to make things even worse, you can’t find the manual online so you can’t find out what size air filter you need.

Or can you?

Removing your current air filter can help you discover the size you need if you look for one thing – the filter’s label. You’ll usually find this on one side of the filter, but there’s a caveat – you’re looking for the actual size rather than the standard size.



Usually, the bold-printed size written on the filter’s frame is the “standard” size. That’s not what you need. You’re looking for a non-bold size on the frame’s side to get its “actual” size.

Method 3 – Break Out the Measuring Tape

Horror of horrors! You can’t find any sizing information on the side of your old air filter. Or, you can, but it’s the “standard” size and you need to find the actual size.

Thankfully, there’s an easy solution – break out the tape measure and a flathead screwdriver.

The need for the tape measure is obvious, as you’ll use it to measure the length, width, and height of your filter media. As for the screwdriver, you may need it to pry open the plastic frame surrounding the filter. Some manufacturers make this easier (they embed their filter media in a tray that you can open by hand), but you’ll need a screwdriver or similar tool if they don’t.

Why are you doing this?

Remember – you’re looking for the filter media’s “actual” size rather than its “standard” size. The frame is part of the standard measurement, with the filter inside often being a touch smaller.

Method 4 – Measure the Filter’s Slot

What if you don’t have an existing filter? Or, you threw the old filter media away before thinking about the fact that you needed to measure it to get its size.

Don’t worry – the filter’s frame is still useful.

Look inside and you should see a slot in which you place your filter media. That slot is cut to match the “actual” size of the filter because it’s where your filter media goes. Measure it, round up the nearest inch, and you have your filter size.

Size It Up

Four methods, ranging from the blindingly obvious to the slightly more technical, can give you the measurements you need to know which filter size to get. After that, your job is simple – find a filter media that’s up to the task. MERV ratings help here, with a filter that has a rating closer to 20 being a better performer than a filter with a rating on the lower end of the scale.

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