Light Attenuation Depth Calculator
How much surface light reaches a given depth in water, from the attenuation coefficient — with the euphotic depth where 1% remains, the depth for any chosen share, and a Secchi-disc estimate.
Light falls off exponentially with depth: each meter removes the same fraction, set by the water's attenuation coefficient.
How the light attenuation depth calculator works
Light falls off exponentially with depth: each meter removes the same fraction, set by the water's attenuation coefficient. Clear ocean water has a coefficient near 0.04 per meter and a euphotic zone — the layer with enough light for net photosynthesis, conventionally down to 1% of surface light — over 100 meters deep. A turbid estuary at 1 per meter has a euphotic zone of under five.
The coefficient is what a Secchi disc measures indirectly: the depth at which it disappears is roughly 1.7 divided by the coefficient.
Formula: I(z) = I₀ e^(−K_d z); z_1% = ln(100) / K_d ≈ 4.6 / K_d; Secchi ≈ 1.7 / K_d
Worked examples
| Inputs | Light remaining at the depth | Note |
|---|---|---|
| Coastal water at 10 m | 22.313% | 22% — well lit |
| Open ocean | 1.8316% | a euphotic zone over 100 m |
| A turbid estuary | 2.7324% | dark below a few meters |
FAQFrequently asked questions
What is the attenuation coefficient?
The fraction of light lost per meter of water, from absorption by the water itself and scattering and absorption by particles and dissolved matter. It ranges from 0.02 in the clearest ocean to several per meter in muddy rivers.
What is the euphotic zone?
The sunlit layer where photosynthesis exceeds respiration, conventionally down to 1% of surface light. Almost all marine primary production happens in it.
What does a Secchi disc measure?
Water clarity — the depth at which a white disc disappears. It relates to the attenuation coefficient roughly as 1.7 over K, though the constant varies from 1.4 to 2 with the water.
Does color matter?
Yes — water absorbs red strongly and blue weakly, so the coefficient depends on wavelength. This page uses a single coefficient for photosynthetically active light, which is the usual practice.
Why does the euphotic depth vary so much?
Because it is inversely proportional to the coefficient. Ten times the turbidity is a tenth of the lit depth — from 115 meters in the open ocean to a few in an estuary.
Where these figures come from
- NCBI Bookshelf — Molecular Biology of the Cell — growth kinetics and molecular conventions
- National Human Genome Research Institute — the US genomics research institute
Last checked: September 2026. Formulas are the standard textbook forms; assumptions are stated on each page because they are where these models break.