The simple version first
One nit equals one candela per square meter. A candela is roughly the brightness of a single candle at one meter distance. So a 400 nit monitor puts out roughly the light of 400 candles per square meter of screen area.
The term "nit" is informal but universally used in consumer display specs. The formal unit is cd/m2.
Reference points
To put numbers in context:
How many nits do you actually need
For a dark or dim room: 200-300 nits is plenty. Using a 1000 nit monitor at full brightness in a dark room is uncomfortable and strains your eyes.
For a moderately bright room: 300-500 nits covers most daytime use.
For a very bright office or near a window: 500-1000 nits gives comfortable visibility without squinting.
For outdoor use: 1000+ nits becomes necessary when competing with sunlight.
For HDR content: peak brightness matters more than sustained brightness. HDR highlights look best with 600+ nit peaks, but a 400 nit monitor can still show HDR if the display software does good tone mapping.
Why HDR brightness specs can mislead
Monitor manufacturers specify two types of brightness:
Typical (sustained) brightness: The maximum brightness the monitor can hold indefinitely. This is what affects everyday use.
Peak (burst) brightness: The maximum brightness the monitor can achieve briefly in a small area. Often measured over 1-10% of the screen area for less than a second.
A monitor marketed as "1000 nits HDR" might have a typical brightness of 400 nits and only achieve 1000 nits for a brief flash over a small highlight area. This is real, but the 1000 nit number overstates what you will see in practice.
VESA DisplayHDR certifications define minimum requirements for different tiers:
Using brightness correctly day to day
Your monitor brightness should be set to match your room. A bright monitor in a dark room causes eye strain just as reading a book in a dim room does. Your eyes adapt to the background ambient light and the screen should match.
A simple test: look at a white area of the screen and then at a white piece of paper in the room. If the screen looks much brighter than the paper, turn brightness down. If much dimmer, turn it up.
Our Brightness Test tool shows a range of gray steps, near-black and near-white patterns. At your calibrated brightness, you should be able to distinguish all 16 gray steps. If the darkest steps merge into black, the brightness is too low.