Douglas Sea State Reference

Reading the sea by wave height

The Douglas sea scale assigns a single digit, 0 through 9, to the state of the sea surface based on significant wave height. It gives mariners and forecasters a compact, standardised way to describe conditions from a glassy calm to a phenomenal storm sea. This reference lists every degree with its wave-height band and a plain description, plus a lookup that classifies any height you enter.

The complete Douglas sea scale

Each degree corresponds to a band of significant wave height in metres:

0  Calm (glassy)     0 m
1  Calm (rippled)    0 – 0.10 m
2  Smooth            0.10 – 0.50 m
3  Slight            0.50 – 1.25 m
4  Moderate          1.25 – 2.50 m
5  Rough             2.50 – 4 m
6  Very rough        4 – 6 m
7  High              6 – 9 m
8  Very high         9 – 14 m
9  Phenomenal        over 14 m

The lookup takes a height and returns the band that contains it, using upper-inclusive boundaries so that, for example, exactly 2.5 m reads as degree 4 (moderate) rather than degree 5.

What significant wave height means in practice

Significant wave height is not the maximum wave height you would encounter — it is the average of the highest one-third of waves observed over a measurement period. This statistical definition was chosen because it closely matches what a trained observer would report from the deck of a ship: the human eye naturally discounts the smallest waves and pays attention to the notable ones.

In practice this means individual waves will sometimes be notably larger than the significant height. In open ocean storm conditions it is not unusual for the largest wave in any 20-minute period to be 1.5 to 1.8 times the significant height. The theoretical maximum (a rogue or freak wave) can reach twice the significant height or more, though such events are rare and not predictable from the scale.

This matters for voyage planning: a degree 6 (very rough) sea with 4–6 m significant wave height could produce individual waves over 8–10 m, which have very different implications for vessel safety than the scale description alone suggests.

Relationship to Beaufort and weather forecasts

Marine weather forecasts typically combine both scales: a Beaufort force number for wind and a Douglas sea state for the wave condition. They move together but are not directly interchangeable. Wind creates waves, but the sea state also depends on:

• Fetch — the distance of open water over which wind has blown; longer fetch builds higher seas at the same wind speed
• Duration — wind must blow long enough to build a fully developed sea
• Swell — wave energy from distant weather systems that travels independently of local wind

This is why a calm sea state (degree 1–2) can persist for hours after a wind drops, or why a degree 5–6 sea can be present even when local wind has eased, if a significant swell is running.

Using the scale

The scale is used in NAVTEX forecasts, maritime weather bulletins, and coastal weather service reports. If a forecast says “sea state 4” you know the significant wave height is 1.25–2.50 m. Offshore operations, ferry services, and ship routing systems set operational limits using Douglas scale thresholds — for example, small craft advisories often apply from degree 4 (moderate) upward.

Tips and notes

• The scale measures the sea, not the wind — pair it with Beaufort for context.
• Significant wave height is the mean of the highest third of waves, not the max.
• Individual rogue waves can be roughly twice the significant height.
• A separate Douglas swell scale grades swell length and height independently.
• Sea state lags wind; a rising wind can sit ahead of the matching degree.