Understanding swell height vs wave height


When you’re just starting to learn to surf, reading and understanding the surf report can seem like a massive hassle. You’re met with rows of arrows and numbers that can seem like a foreign language when you first see them.

We want to try and help surfers to understand how waves work and how to use the surf report to find the best possible surf conditions for you.

In this guide, we’ll be looking at the difference between swell height vs wave height and how you can use this data to predict what the waves at your local beach will look like on any given day.

If you’re new to surfing and want to learn how to read a surf report you should start with our helpful guide on how to find the best surf conditions as a beginner for a quick crash course on surf forecasting.

What is the swell height?

The swell height is calculated by buoys, as waves pass under a buoy they make the buoy rise and this rise in level is calculated and relayed back as data to forecasters.

They achieve this by taking measurements at regular intervals with the most solid data being Hsig – or significant wave height. Hsig measures the mean of the largest ⅓ of any waves measured during the sampling period.

This average is then used as a swell height to help surfers and mariners work out what the conditions in the ocean will be like for the day.

So what happens when there are two swells running from different directions?

Primary swell

Offshore buoys have the ability to understand swell size and direction which allows them to easily assess different swells in the water. When this happens data will be grouped for each swell direction and displayed as primary or secondary.

The largest swell in the water will be displayed as the primary swell which in theory should have the most impact on water users.

Secondary swell

The secondary swell will be the smaller of the two swells running in the water and in theory, should have less effect on the ocean’s conditions.

It’s important to note that secondary swells can affect the quality of waves provide by the primary swell. Typically a secondary swell will break up incoming ground swell and create wedgier conditions with considerably shorter rides.

What is the wave height?

Wave height is the actual size of waves that are breaking on the beach that you surf. Wave height is typically calculated by measuring from the trough to the peak of the wave.

Just to add to the confusion of measuring wave heights this system is not consistent across all countries and Hawaiians in particular use a very different method.

When measuring wave height in Hawaai they use the distance between the surface of the water and the peak of the wave when viewed from behind the wave. This means that the actual breaking wave may be considerably larger, sometimes 2-3x larger.

In practice, this means that if a Hawaain asks you to come surf some 4ft left-handers you may be in for a slight shock when you reach the beach and see the size of the actual breaking waves.

How swell period affects wave size

The Swell period is simply the time in between each swell. You can visualise this simply by standing at the beach, when a wave hits you start counting, when the next wave hits you, stop. The time between the first wave and the second wave is the swell period.

As a rule of thumb the larger the wave period the larger the surfable waves it provides. Let’s take a look at a real-life example:

Example 1 – In this example, the primary swell is 5ft and the swell period is 13 seconds.
Example 2 – In this example, the primary swell is 5ft and the swell period is 8 seconds.

Both of these surf reports display the same swell height but very different swell periods. Assuming both swells are hitting their surf spots directly, which one is going to create bigger surfable waves?

In case you’re not sure example 1 will provide considerably larger breaking waves than example 2.

This si something to be very aware of when you’re learning to surf, large period swells mean long gaps between set waves and what looks like a relatively small day of surf can be interspersed with large set waves with intervals of as long as 15-20 minutes.

How swell direction affects wave size

Now you’ve got a clear understanding of how the swell height and period change the size of actual waves let’s look at the final piece of the puzzle, swell direction.

Using the two examples above you can see that one swell is hitting the beach directly while the other swell is running along the length of the beach.

Head-on swells breaking directly onto the beach, reef or point break like in example 1 will always produce larger more powerful waves than swells from any other direction.

The further the angle of the swell direction moves away from the break like in example 2, the smaller the waves will get.

What’s a good wave size for beginners?

If you’re just starting out learning to surf then any waves with enough power to allow push you along are perfect, this is great for small children because you can often find small playful waves at spots that aren’t known for their surfable waves.

As you start to progress you’ll want to move onto slightly larger waves in the 2ft to 4ft range. As an adult surfer, you’ll likely need at least 2ft waves to help keep you afloat while your balance improves.

Want to learn more about how the weather conditions affect your surf and how you can use it to your advantage? Head over to our guide on onshore vs offshore winds to find more tips on reading your local surf report and finding better waves.