The method
Combine the squares, keep the details
The Bureau of Meteorology's combined sea and swell method uses the square root of the sum of the squared component heights. It includes wind waves, primary swell and secondary swell:
Use non-overlapping partitions. A buoy's total significant wave height already includes its wave components; entering that total beside its swell and wind-wave parts would count some waves twice. Check your forecast's labels before transferring values.
Each bar uses Hcomponent² / ΣH². NDBC relates significant height to surface-displacement variance by Hs = 4√m0. Squared height is therefore proportional to variance. The shares are not percentages of individual waves, breaking height or wave power.
The calculation assumes separate component variances can be added. Period and direction describe each component but do not enter this height calculation. Displaying two decimal places makes the arithmetic easy to compare; it does not increase the precision of the source forecast.
Teaching example A · Fabricated inputs
A larger swell with smaller companions
The starting example contains a 1.2 m primary swell at 14 s from W, a 0.8 m secondary swell at 9 s from SW, and 0.6 m wind waves at 5 s from NW. These are invented values for learning, rather than observations from a surf spot.
- Square the heights: 1.2² = 1.44; 0.8² = 0.64; 0.6² = 0.36.
- Add those values: 1.44 + 0.64 + 0.36 = 2.44 m².
- Take the square root: √2.44 ≈ 1.56 m, or 5.12 ft.
- Compare contributions: 1.44 / 2.44 ≈ 59.0%; 0.64 / 2.44 ≈ 26.2%; 0.36 / 2.44 ≈ 14.8%.
Adding the heights directly would give 2.6 m. That sum describes neither this combined significant height nor a prediction of the next wave. At half the height, the 0.6 m component supplies one quarter of the 1.2 m component's variance.
Teaching example B · Fabricated inputs
The same total can conceal a different mix
Compare the two invented sea states below. Both round to 1.41 m combined significant height. The first contains two equal components; the second contains only one. A matching total does not preserve the component periods or directions.
| Sea state | Components | Calculation |
|---|---|---|
| Two systems | 1.00 m at 14 s from W 1.00 m at 6 s from N Wind waves: 0 m | √(1² + 1²) = 1.414… m 50% / 50% variance |
| One system | 1.414… m at 10 s from W Secondary swell: 0 m Wind waves: 0 m | √((√2)²) = 1.414… m 100% variance in one component |
In the two-system case, averaging 14 s and 6 s produces 10 s, which belongs to neither entered component. Averaging W (270°) and N (0°) arithmetically produces 135° (SE), which points away from both incoming systems. Keeping the original rows prevents those misleading summaries.
Dominant period and average period answer different questions
NDBC defines peak period from the frequency band with the greatest spectral density. Its average periods use spectral moments, with conventions varying by provider. The three entered periods cannot supply either measure for the combined sea state.
A component's total variance and its peak spectral density are different properties. A broad component can contain more variance while a narrower component has the taller peak. These rows lack the spectral shape needed to find the combined peak period.
The National Weather Service's wave-detail forecasts retain component height, period and incoming direction alongside significant height. Use that fuller description when comparing forecasts. Our guide to swell period and surf forecasts explains how to read those fields together.
Use the result for a specific question
This tool answers “how do these component heights contribute to a combined significant height?” You can change one height while holding the others fixed, compare its variance share, or download the exact input mix with both metre and foot results.
It does not model a beach's breaking wave height. Depth, seabed shape, shoreline exposure, tide and local wind are outside the calculation. Direction remains an incoming bearing rather than a measure of how much swell reaches a particular beach. The result also does not describe the largest individual wave or rate conditions for a surfer.
Before using real inputs, confirm that all rows refer to the same forecast time and location, that the heights use the same significant-height convention, and that no component appears twice. Leave absent components at zero. Read the original forecast and local information for the conditions at your destination.