Which filter lets your fans move the most air?

Airflow your box would get

Is a 4-inch filter really easier to breathe through than a 2-inch?

Same brand, same MERV, both tested at 500 feet per minute. Lower is easier. A line sloping down means the 4-inch is better; flat or up means it isn't.

Raw manufacturer numbers

Clean-filter resistance as published, in inches of water. Only filters tested at the same air speed share a chart, because a number at 300 FPM can't be compared with one at 500 FPM.

What "electrostatic" means on a filter

Filters catch fine particles two ways. Mechanical media traps them in a dense web of fibers. Electrostatic media (also called electret) adds a static charge to the fibers that pulls particles in — so the filter can reach MERV 13 while staying easy to breathe through, which is exactly what you want behind PC fans. Most inexpensive residential MERV 13 filters, including the Nordic Pure and Filtrete ones here, work this way.

The catch: that static charge fades with time, humidity and dust, so a charged filter's real-world efficiency can slip below its rated MERV as it ages — it still filters mechanically, just less. Commercial mechanical-media filters (the Koch Green13, AAF and Glasfloss commercial lines here) lean less on charge and hold their rating more steadily, but they cost more and usually fight your fans harder. For a DIY box the charged synthetic is the sweet spot — just swap it on schedule instead of running it for years.