Surfer’s guide to Wave Height

Wave height typically refers to the vertical distance between the crest (top) and the trough (bottom) of a wave. For surfers, this gets more complex. Swell forecasting models measure open ocean swell heights, which are not always directly equivalent to the waves breaking at your local break.

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When predicting surf conditions, the main focus is how big the waves will be. To do this effectively, you need to understand how to read a swell forecast. While we usually want to know the wave size in feet or metres, measuring a wave directly is not as simple as it sounds, and different scales or reference points are used around the world.

The main factors that determine how big the waves will be at your local break, compared to the open ocean swell reading, are the length and shape of the reef, the depth of water offshore and the direction of the swell. For example, the very long reef at Uluwatu is open to all swell directions, while the sheltered waves of Jimbaran Bay will always be much smaller. The river mouth at Balian and the right-handers at Keramas both have a channel of very deep water offshore formed by their respective rivers, which funnels and amplifies the swell. Geger Right, the longest beach in Nusa Dua also sits over very deep water offshore and picks up more swell than anywhere else on that stretch of coast.

How Surf Forecasts Measure Wave Size: Significant Wave Height

In the open ocean, wave size is determined from data collected by buoys. The average wave height is calculated along with average wave period, swell direction, water temperature, wind and other measurements. Surf forecasting services like Surfline only include the height of the biggest third of waves in their calculation — the smallest two-thirds are ignored as they would give a misleading average. These smaller waves are either too small to surf on an average day or are breaking on the inside on bigger days. As we cover in detail in our Science of Waves guide, this is called significant wave height and is the global standard used by all surf forecasting services.

For a breaking wave, you can see it but you need a measuring reference to estimate its size, and it is further complicated by the fact that the wave will break at different heights in different sections. Usually a wave starts big and gets smaller — but at breaks like Uluwatu’s Racetrack the wave can actually start small and grow bigger as it peels along the reef.

Wave Size Scales: Hawaiian, Australian and Body Reference

Due to the complicated nature of measuring wave height, several scales have developed over time.

The Hawaiian Scale: Measuring the Wave from the Back

This was in widespread use throughout the 20th century. The wave was measured from the back, giving a scale of 1 foot, 2 foot, 3 foot, 4 foot. Broadly, 1 foot is around head high and 2 foot is a significant wave. 3 foot and you may want a bigger board; 4 foot is serious size. It works as a quick way to communicate — if a surfer in Hawaii says the waves are 2 foot, you know roughly what to expect.

Since the discovery of Teahupo’o in Tahiti, the Hawaiian scale has lost credibility. This wave has no meaningful back — the trough of the wave is actually below sea level — so it cannot be measured from behind. There was also a cultural element of understating wave size for bravado, making the scale unreliable. A surfer saying the waves are 2 foot might be standing in front of a genuine 3–4 metre wall of water.

The Australian Scale: The Standard We Use in Bali

Similar to the Hawaiian scale as a communication tool, but measured differently. In broad terms: 1 foot is tiny and needs a longboard; 2 foot is small but rideable on a shortboard with waves around shoulder high; 3 foot is around head high and fun size for most surfers; 4 foot is head and a half high; 5 foot is double overhead; 6 foot is triple overhead. Above triple overhead, wave face size in metres or feet is typically used — it is quite rare outside of Hawaii and a handful of big wave spots to see surf above triple overhead, so the easy reference numbers stop working at that point.

In Bali we follow the Australian scale, and most days the surf is 2 foot (small), 3 foot (medium) or 4 foot (medium to large).

The Body Reference Scale: From Knee High to Double Overhead

Measuring compared to the height of the surfer is effective if the surfer is around average height. The scale runs: flat, knee high, waist high, shoulder high, head high, overhead, head and a half, double overhead, triple overhead, and beyond that simply massive. This is the most intuitive scale for most surfers and the most universally understood.

How Swell Travels from the Storm to Your Surf Break

Wind creates swell, which produces breaking waves when the ocean meets a shallow reef or beach. The size and quality of that swell depends on the fetch the wind blows over — the bigger the area of ocean and the longer the wind blows, the better the swell. As swell travels away from its storm origin, the waves organise into clean, parallel sets and their quality improves with longer travel distance. Wave height does reduce as the swell spreads outward from the storm — roughly 30% for every doubling of distance from the source — but the organisation and period of the swell improve significantly over those same distances.

Reading Wave Height Like a Surfer: The Practical Takeaway

An ocean wave behaves according to the same laws of physics that govern sound waves and radio waves. The difference with ocean waves is that shallow water changes their nature and forces the stored energy to be released when the water depth becomes shallow enough — producing the breaking wave a surfer rides.

As surfers, what matters most is planning ahead: a swell forecast can only ever give an average, since no model can predict the exact size of each individual wave in a set. This is exactly why, when a truly precise measurement is needed — judging scores at a big-wave contest, for example — competitors compare the wave against the surfboard riding it rather than relying on any of the scales above.

Now that you have a better understanding of wave height and swell forecasts, you are ready to go deeper. Read our guides on understanding wave energy, how to read swell period, the science of wind and how tides affect surf conditions.