If you’ve tried to use HDMI 2.1 on a Mac with a high refresh rate monitor or TV with HDR, you might have found some limitations. In particular, the popular LG OLED TVs with 120 Hz or higher refresh rates can’t use the popular 1440p scaled resolution while keeping HDR on. So what ARE the best settings you CAN use at each resolution and refresh rate combination while still maintaining HDR? Let’s find out!
Mac Resolution Scaling
First, a short primer on why this is an issue in the first place. Mac OS uses a method where it doubles the pixels in each direction when you choose to use a scaled resolution in order to increase sharpness and give you the HiDPI effect they call a Retina display.
That means, for example, if you have a 4k monitor with a native resolution of 3840×2160, one of the default resolution options is actually scaled 1080p, which gives you a much sharper image, but halves the effective pixel density for desktop real estate on screen since each pixel is using up 4 actual pixels.
If you want to increase the screen real estate, you can increase the resolution to, for example 2560×1440, a historically popular resolution for those with 27 to 32 inch monitors. However, when you do this on a Mac, it will actually render the screen image at 4 times the pixel density of 5120×2880 and then scale that down for display on your 2160p monitor.
The problem is that while HDMI 2.1’s bandwidth is enough to support 2160p resolution at 120 Hz refresh rate with HDR on, it ISN’T enough to support what the Mac is sending through, which is 2880p, at 120Hz and with HDR.
Resolution Tests
I used a custom resolution utility to set and test the limits of the highest resolution you can set over HDMI 2.1 at various refresh rate levels on a 42” LG C4 OLED TV. All testing was done on my M2 Max Mac Studio, but you can expect any Mac with HDMI 2.1 to function similarly. The TV itself supports up to 144Hz refresh rate, so that’s as far as I tested. I’ll also focus on 16:9 aspect ratio resolutions since that is the most common for monitors and TVs. Note that setting it to variable refresh rate requires the same bandwidth as the highest refresh rate.
Let’s start with the highest refresh rate of 144Hz. The highest resolution you can go while still having HDR enabled is 2208×1242. If you go up one more step to 2224×1251, you lose the ability to turn on HDR. If you don’t need HDR, you can go all the way up to 2800×1575 at 144Hz before you need to lower the refresh rate to go even higher.
If we bump the refresh rate down to 120Hz, the highest resolution you can set with HDR on is 2432×1368. Going up one more step to 2448×1377 loses the ability to use HDR. While this is pretty good, it is still below my preferred resolution of 2560×1440, so the interface elements are just a little bigger than I would like. Without HDR, you can go up to 3072×1728.
One tip for the LG C4 TV is that the 144Hz mode can be toggled on or off in Game Optimizer settings, so if you want to use a higher resolution than 2208×1242 with variable refresh rate limited to 120Hz and HDR on, you can do that by turning OFF 144Hz mode first on the TV.
Going down to 100Hz next, which is still a respectable refresh rate, it is finally possible to set the resolution to 2560×1440 with HDR on. In fact, you can go all the way up to 2656×1494. Going up one more step to 2672×1503 forces HDR to go off. If you can live without HDR at 100 Hz, you can get all the way up to 3360×1890 resolution.
And finally, at the normal 60Hz refresh rate, you can go up to 3440×1935 and have HDR on. One more step up to 3456×1944 forces HDR off. You can even go all the way up to one step less than the 4k display’s native refresh rate to have scaled 3824×2151.
If you try to set it to the native refresh rate, scaling turns off, so the HDR option comes back, and you can also go up to 144Hz with no problem since it’s not trying to send through 4 times as many pixels.
If we look at the pixel count by multiplying the resolution width times the height, and then multiply by the refresh rate, we can see that the magic number seems to be around 400 million. Anything combination less than 400 million supports HDR, while any combination over 400 million does not.
Refresh Rate and Resolution Formula
If you want to quickly calculate whether a custom resolution and refresh rate will work with HDR on Mac OS, you can try using this method. For example, we can deduce that an ultrawide resolution of scaled 3440×1440 can support a refresh rate of up to 80 Hz before exceeding the 400 million limit. Similarly, using a native 2160p resolution without scaling should allow up to 192 Hz refresh rate. If you can help verify some of these theoretical numbers, please let me know in the comments and give the video a like if you’re impressed that I can do basic arithmetic.
We can also see that the pixel count times refresh rate with HDR off seems to have a breakpoint of around 640 million. Just less than that and the refresh rate and resolution combination works fine. Just past that and its no good. With that knowledge, we can extrapolate that at 60Hz, you should theoretically be able to go up to scaled 4352×2448 resolution. Of course, you’d need a monitor that is higher resolution than that to actually use it, so I can’t verify that on my 4k TV.
I won’t go any lower than 60Hz since the resolutions are already getting ridiculous with tiny UI elements, and lowering the refresh rate even further just feels bad to use.
Highest Resolution Per Refresh Rate
So to summarize, here is a table with all the highest 16:9 aspect ratio scaled resolutions per refresh rate you can use and still have HDR on, as well as as the highest possible 16:9 scaled resolutions at each refresh rate with HDR off.
What’s your preferred resolution and refresh rate?
Personally, I find myself either going with 120Hz refresh rate at 2432×1368 or stepping down to 100Hz refresh rate so I can set it at 2560×1440. Anything larger than that and the interface elements and text get too small for my comfort.
I don’t know if Apple will ever change how they do resolution scaling to make it possible to get higher refresh rate and resolution combinations with HDR on, or if we’ll just have to wait until HDMI 2.2 support comes to displays and Macs before we no longer have to compromise.