Good question-- See the link. Roughly the visible region is 300
nm to 650. so that's 350 nm total. The electromagnetic spectrum
goes from 10 Megameters to 1 picometer. So the percentage of the
spectrum occupied by visible light is tiny, just 3.5 X 10-9 divided
by 10^19 ---- So that's **3.5x10-26 %.** So many zeros....!
Yikes. Gamma rays (picometers) (10-12)

Hard X-rays

Soft X-Rays

Extreme ultraviolet

Near ultraviolet

Visible light

Near infrared

Moderate infrared

Far infrared

Extremely high frequency (Microwaves)

Super high frequency (Microwaves)

Ultra high frequency (Radio)

Very high frequency (Radio)

High frequency (Radio)

Medium frequency (Radio)

Low frequency (Radio)

Very low frequency (Radio)

Voice frequency (Radio)

Extremely low frequency (Radio) 10 Megameters (10 X 106)
Actually the electromagnetic spectrum goes off as far as you want
in both directions - there are electromagnetic waves with
wavelengths as small as you want, and others as large as you want.
So really the answer is 0%. If you meant to ask what percentage of
the electromagnetic energy from the sun is visible (for instance)
then the answer is nonzero, but I don't know what it is. If you
meant to ask a question of the kind that has been answered above
(what percentage of the spectrum from 10 megameters to 1 picometer
is visible) then **the question is not well defined:** the
answer depends not just on the bounds but on what scale you use. I
think the answer above is mistaken. If you go by wavelength, my
answer (to that question) would be: 3.5x10-7 divided by 107, which
is 3.5x10-14 or **3.5x10****-12****%**. If you use
frequency instead of wavelength: The bounds on wavelength given
above correspond to 30 Hz to 3x1020Hz, with visible light between
4.6x1014Hz and 1015Hz. Then, the answer would be 5.4x1014 divided
by 3x1020, which is 1.8x10-6, or **0.00018%**. If you want to
use a logarithmic scale, which is more sensible in some ways, then
the answer is much higher. I don't know exactly because I don't
have a calculator with logarithms here, but it's **around
1.5%**.