Short answer:
If you mean the color red in physics, it corresponds to light with wavelengths of roughly 620–750 nanometers in the visible spectrum, making it the longest-wavelength visible color for humans.

What “how long is red” usually means

The phrase “how long is red” sounds odd at first, because colors do not have length in everyday terms.
In science, though, we talk about the wavelength of light, which does have a length, measured in nanometers.

  • Visible light is a band of electromagnetic radiation that human eyes can see.
  • Different colors correspond to different wavelengths.
  • Red sits at the long-wavelength end of the visible spectrum, next to orange and opposite violet.

So “how long is red?” in a scientific sense = “what wavelength range does red light have?”

The wavelength range of red

Scientists usually define red light for human vision as:

  • About 620–750 nanometers (nm) in wavelength.
  • Below about 620 nm, you move into orange and yellow; above about 750 nm, light becomes infrared , which humans cannot see but can feel as heat.

Some sources give slightly different edges (like 625–740 nm), but they all agree that red covers the longest visible wavelengths our eyes can perceive.

Why “long” matters for red

Thinking in terms of wavelength:

  • Red’s “length” is greater than that of green or blue light.
  • That is why we say red is at the long-wavelength end of the visible spectrum.
  • Just beyond red is infrared , which is literally “below red” (from Latin infra) and is invisible to us.

An everyday way to imagine it:
If you drew a rainbow as a wave, the waves for red would be the “stretched out” ones , while violet would be the “tightly packed” ones.

Mini FAQ: Other angles on “how long is red”

  • Is red always the same length?
    Not exactly—different shades people call “red” might be nearer 620 nm (more orange‑red) or nearer 750 nm (deep red), but they all sit in that long‑wavelength band.
  • Is red the longest color humans can see?
    Yes, for most people, red is the longest visible wavelength ; anything longer is infrared and invisible to the human eye.
  • Does language change what counts as red?
    Across cultures and history, people have grouped colors differently, but physically, the light we call red still occupies that same long‑wavelength region.

TL;DR:
For light and color, “how long is red?” translates to “what’s red’s wavelength range?” and the answer is about 620–750 nm, the longest visible wavelengths humans can see.

Information gathered from public forums or data available on the internet and portrayed here.