What external monitor to buy that has same clarity as the MacBook?
Solution 1:
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!
Solution 2:
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
Solution 3:
You have a 2015 MacBook Pro 13"
From the specs listed here we can see that the built-in display is a 2560 by 1600 pixels with 227 pixels per inch. It is listed as a Retina display.
Your MacBook has a high pixel density, but is not itself 4k or 5k.
Your computer can support one display via the Thunderbolt 2 port. It can support one external display via the HDMI port.
Here are the limitations:
It can support a single display up to 3840x2160 via Thunderbolt 2 and a single 1080p display at up to 60 Hz, 3840x2160 at 30 Hz, or 4096x2160 at 24 Hz via HDMI.
Given that information, you could support a 4k monitor. You cannot support a 5k monitor.
Personally, I would not dwell too heavily on all of this technical information.
First, choose the size monitor you are most interested in. Go to a store and decide on a 25", 27", 30", etc. Whichever feels right. Then once you've nailed that down, purchase a QHD (2560 x 1440) or 4K monitor. I have personally used a QHD (2560 x 1440) monitor with a 2015 and 2017 15" Retina MacBook Pro and it works great with both. Visually, the external monitor is very similar to the actual built-in display of the MacBook.
Here's a list of 4K monitors that are similar to the actual 4K Mac monitor.
Good luck!