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About this tool
Convert nits to PQ and HLG code values, see the headroom above reference white and why untone-mapped HDR looks flat on SDR.
This explainer converts absolute brightness in nits into PQ (SMPTE ST 2084) and HLG (ITU-R BT.2100) code values, then shows how much headroom an HDR grade has above diffuse white and what happens to that headroom on an SDR screen. It tone maps with extended Reinhard against the HDR peak and contrasts that with the untone-mapped result a naive player produces, which is the usual reason an HDR grade looks flat and dark on SDR. Written for editors, colourists and developers who keep hearing about nits without a clear picture of what the numbers mean.
Open HDR Vs SDR Brightness Explainer on AltFTool — it loads instantly in your browser.
Add your input to the workspace.
Adjust the options until the result looks right.
Copy or download the output and put it to work.
PQ and HLG use their published constants, so the code values match a scope.
Brightness differences are expressed the way a photographer already thinks about them.
Side-by-side tone-mapped and untone-mapped values quantify the flat-looking playback.
A nit is one candela per square metre, a measure of light actually leaving the screen. SDR is mastered to 100 nits reference white, HDR reference white sits at 203 nits under ITU-R BT.2408, and 1000 nits is a common HDR mastering peak — roughly 2.3 stops of headroom above that reference white.
Usually the player is sending an HDR signal to a display or pipeline that treats it as SDR, so the PQ code value gets fed to a gamma curve instead of being tone mapped. Feeding a 203-nit PQ code to a 200-nit gamma 2.4 display produces about 54 nits — roughly a stop darker than it should be, which reads as flat and desaturated.
PQ is absolute: a code value maps to a fixed luminance in nits regardless of the display, which is why it needs mastering metadata. HLG is relative and scene-referred, with the signal below 1/12 of scene white following a square root and above it a logarithm, so it degrades more gracefully on an SDR display.
It depends entirely on the peak and the diffuse white you compare against, not on the label. A 1000-nit peak over a 203-nit diffuse white is about 2.3 stops of specular headroom; a 4000-nit peak over the same white is about 4.3 stops.