Sea State Calculator
Paste a marine forecast, or enter the wave height and period, to see how long and steep the waves are and how they will feel in your boat. Enter a wind speed for its Beaufort force and the sea it builds.
Decode a marine forecast
OptionalPaste a line or a whole zone forecast from the National Weather Service, such as
Waves and your boat
Wind and the Beaufort scale
| Force | Knots | Description | Waves, ft (max) |
|---|
Beaufort table: World Meteorological Organization, as published by the National Weather Service. Wave heights are probable (and maximum) heights in the open sea, well away from land.
How the numbers are worked out
Wavelength and speed come from the deep-water wave formulas. The distance between crests is g × T² ÷ 2π, which works out to about 5.12 × T² in feet (1.56 × T² in meters). A 6-second wave is about 184 feet long and travels at about 18 knots. In water shallower than half a wavelength, the waves get shorter and steeper than these figures, which is why inlets and shoals are rougher than open water.
Steepness is the wave height divided by the wavelength. Deep-water waves break when they reach about 1 in 7. The bands used here, from gentle through very steep, are set to agree with the boaters' rule of thumb in wave period explained: when the period in seconds is no more than the height in feet, expect a steep, uncomfortable sea.
The boat guide uses the old rule of 10 feet of boat for every foot of wave, explained in how bad are 3 to 5 foot seas. The kayak guide follows our kayaking guide: under a foot for beginners, 2 to 3 feet only for experienced paddlers in sea kayaks.
Forecast heights are averages
When the National Weather Service forecasts "seas 4 ft", it means the significant wave height, the average of the highest third of the waves. Individual waves can be twice that, so a 4-foot day brings the odd wave near 8 feet. The calculator uses the height you enter, so if you are close to your limit, try the upper end of the forecast range.
Wind builds the sea
The Beaufort table gives the waves a wind builds in the open ocean after blowing for a long time over a long distance. Near the coast, seas are usually smaller but steeper. A common coastal rule of thumb is 2 to 3 foot seas at 10 to 15 knots, 3 to 5 feet at 15 to 20 knots and 4 to 6 feet at 20 to 25 knots. If the wind is rising, the sea will build behind it.
Questions
How bad are 3 to 5 foot seas?
In a 20-foot boat, 3 to 5 foot seas mean a long, rough and wet ride. By the 10-to-1 rule they are comfortable in boats of about 30 to 50 feet. A long period of 10 seconds or more makes them much gentler, and a short period of 5 seconds or less makes them much worse. Read the full guide.
What does "3 ft seas at 6 seconds" mean?
The significant wave height is 3 feet and a wave passes every 6 seconds. In deep water the crests are about 184 feet apart and travel at about 18 knots. The period is twice the height in feet, which most boats that handle 3 feet will find manageable. In a boat under about 20 feet it will still feel choppy.
How much wind is too much for a small boat?
It depends on the boat, but winds of 15 to 20 knots usually mean 3 to 5 foot seas near the coast, which is a rough ride for boats under about 25 feet. Small craft advisories start somewhere between about 20 and 25 knots on most coasts. For kayaks, above about 10 knots is hard work for beginners.
What is the Beaufort scale?
A 0 to 12 scale of wind force that links wind speed to what the sea looks like, devised by Francis Beaufort of the Royal Navy in 1805. Force 4, a moderate breeze of 11 to 16 knots, brings frequent whitecaps. Force 8, a gale, is 34 to 40 knots.
Can this tell me whether it is safe to go out?
No. It turns forecast numbers into a rough picture of the sea. Inlets, wind against tide, shallow water and crossing seas can make real conditions much worse than the numbers suggest. Check the official NWS marine forecast, ask your captain if you have one, and when in doubt, stay in.