The short answer
A DMX512 universe has 512 channels (slots). With 8-bit control, a pixel uses one slot for each independently controlled colour channel. Divide 512 by the slots per pixel and round down to get the number of whole pixels per universe.
| Pixel type | Channels per pixel | Whole pixels per universe | Unused channels |
|---|---|---|---|
| Single white | 1 | 512 | 0 |
| Tunable white (warm + cool) | 2 | 256 | 0 |
| RGB | 3 | 170 | 2 |
| RGBW | 4 | 128 | 0 |
| RGB + warm + cool (RGBTW) | 5 | 102 | 2 |
Pixels that use 16-bit colour need two channels per colour, which halves these numbers: an RGB pixel at 16-bit uses six channels, so 85 pixels fit in a universe.
Aligned vs spill packing
Most pixel controllers and patch conventions use aligned packing: each universe holds only whole pixels, and the next pixel starts on the next universe. An RGB universe therefore stops at pixel 170 and channel 510, and channels 511 and 512 stay empty.
Some systems allow spill packing, where the 512 bytes are filled contiguously and a pixel may start in one universe and finish in the next. Spill saves a few universes on very large runs, but a pixel split across two packets is harder to debug and not every receiver accepts it. When in doubt, use aligned packing; it is what the controller documentation and most mapping software assume.
The calculation
universes per run = ceil( pixels in the run ÷ floor( 512 ÷ channels per pixel ) )Calculate per run when your controller starts each output at a new universe. In that layout, the partly filled last universe of each run still counts.
Worked example
- A cove of 10 m LED tape at 60 pixels per metre is 600 pixels.
- As RGB (3 channels): 600 ÷ 170 = 3.53, so 4 universes.
- As RGBW (4 channels): 600 ÷ 128 = 4.69, so 5 universes.
- Twelve such runs as RGBW need 60 universes, not the 57 a whole-project division would suggest.
Frame rate and headroom
A full 512-slot universe on a physical DMX512 line takes about 22.7 ms to transmit, which is where the familiar maximum of roughly 44 frames per second comes from. Art-Net and sACN carry universes over Ethernet and are not limited by the serial line, but the pixel controller's outputs and the LED driver ICs still set the practical refresh rate. Plan for the slowest link.
Leave spare capacity on every controller for late additions and for runs that turn out longer on site. Check the controller's per-output pixel limit as well as its universe licence: both can be the bottleneck.
How Lightshaped handles it
Lightshaped patches pixel runs with aligned packing, gives every run its own universe range, never lets two runs overlap and plans controller capacity from the resulting universes. The patch is exported as CSV and MVR, and it stays a design patch until the physical installation has been commissioned.
Questions people ask
- Why 170 pixels per universe and not 512 divided by 3?
- 512 ÷ 3 is 170.67. Aligned packing only places whole pixels in a universe, so it holds 170 RGB pixels and leaves two channels unused.
- How many pixels fit in one universe for RGBW?
- 128. An RGBW pixel uses four channels and 512 ÷ 4 is exactly 128.
- Do Art-Net and sACN change the number of pixels per universe?
- No. Both carry standard 512-channel DMX512 universes over IP; the pixels-per-universe maths is the same.