Apple - What external monitor to buy that has same clarity as the MacBook?

720, 1080, 4K or 5K are all measures that imply the total pixel count of the display.

Retina is a marketing term that describes a pixel density that is high enough that an average user in normal conditions cannot see the individual pixels.

The short answer is you can go 4K up to about 25" but by the time you get to 27", the pixel density of a 4K monitor starts to get too low to be considered retina. 25"-27"+ you need to start looking at 5K monitors to get retina pixel density.

I use a 24" LG Ultra HD (24UD58-B) and I smile each time I sit at the computer because it looks so good and the price was right.

Hope that helps!

To get the same clarity as a 13.3 inch MacBook with 2560‑by‑1600 native resolution, you would need 227 PPI (pixel per inch).

PPI is simply calculated using:

where p = pixels and i = inches.

For example, consider a 24-inch monitor with resolution 4096x2160 (4K). This monitor has about 193 PPI, which is still not as dense as a retina display, however the difference would be negligible to the naked eye.

To be pedantic, you would need a resolution of 5120x2880 (5K) on 24-inch (~245 PPI) or 27-inch (~218 PPI) monitors to be better or near retina clarity