Normally Nominal

Posted by Erik Sternberger at

We're going to talk about a lesser known part of buying a filter today: Nominal Sizing.

What does that mean? It means your 20 x 22 x 1 MERV 11 filter is actually a 19.5 x 21.5 x 0.8 MERV 11 filter if you were to measure it. Does this mean you're getting less filter for your money or someone is cutting corners?* Nope. That is actually the real size your furnace needs!

MERV 11 Furnace Filter
Filters aren't the only products sold this way. Have you ever measured a 2 x 4? Well, if you have, you party like we do and know they are actually closer to 11x 312s. Man, that just isn't as fun to say.

If you're old enough to remember 312 inch floppy discs you'll learn they are actually 3.54 inches. If you still have one around to measure, we hope things are going well with your museum. Speaking of computers, for every hard drive in your devices, you actually "lose" 70.3MB for each gig listed. So, an 80GB hard drive is actually 74.5GB.

*If for some reason someone does try to sell you a furnace filter without actual corners, you should probably raise a concern about that. We at include all corners.

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