How to read a Swell Forecast for Uluwatu

Before heading to the beach, understanding the surf forecast will greatly enhance your surfing experience. In this guide, you’ll learn how to quickly interpret this information and make smart decisions about when to surf. Afterwards, we will dive deeper into what the detailed data presented means for planning your surf session.

While there are surf forecasts for nearly all the popular surf spots such as Bingin, Padang Padang, Balangan, Dreamland for consistency we refer only to the Uluwatu surf forecast. When we have determined the expected surf size for Uluwatu we can then estimate from this how big the other spots will be. For example, if Uluwatu is 3 foot, then we can expect Baby Padang to be 2 foot while Padang Lefts will not be breaking as too small. Balangan picks up almost as much swell as Uluwatu but this depends on the swell direction – a South Swell will mostly miss Balangan and Bingin as they face North-West but if there is a lot of west then these breaks will be almost the same size.

Canggu breaks such as Berawa, Batu Balong, Old Man’s, Echo Beach, Sandbar and The Rivermouth are less affected by swell direction as they face South West and thus open to all swells. They will generally be a littlle bit smaller than Uluwatu but unlike Uluwatu which can hold a swell of any size, Canggu breaks are not surfable on the biggest swells.

When comparing the size of Uluwatu and the South Coast breaks, swell direction is a key component as the Uluwatu is West facing while Nyang Nyang, Nungallan, Green Bowls, Pandawa, and Gunung Payung are all south facing. South coast breaks will be bigger than Uluwatu on a south swell but about the same size on a West Swell.

Uluwatu is usually bigger than Keramas and Nusa Dua breaks with the exception of Geger right and Chicken Wings which have very deep water offshore. Our Bali Surf Spots guide has detailed information on every surf break.

Introduction to terms

Swell height is the open ocean size of the wave, and while related to the size of the waves breaking at the beach, it is not the same. As every wave is unique and different size and formation, the wave model averages the size of the biggest waves to approximate the wave height. The model ignores the 2/3 smallest waves as not helpful to predicting the swell size. For surfers, these would be the smallest waves breaking closest to the shore.

Swell period is the time between waves. Agan it is a calculated average. The longer the wave period, the longer the gap between the waves and the more powerful.

Swell direction is where the wave are coming from, in relation to your beach/reef.

Energy is a calculation of the wave energy in KJ. Higher the energy the more powerful the waves. Energy can be calculated from the average wave height and period along with velocity and other variables.

Padang Padang Surf Camp use wave energy as a benchmark for estimating the wave size/power, as it is easier to communicate than the raw data of swell period and wave height, which needs to go into a complex formula. To form the overall view we do also look at wave height and period for both primary and secondary swells. We have 5 swell size categories and 5 levels of surfing ability. We match the surf spot with the guest ability and forecast wave energy using our own proprietary surf spot app to determine the best time for the tide.

Wind speed and Direction affects wave shape and local conditions. If offshore the waves could be perfect, if onshore or sideshore the wave quality will be degraded in proportion to the strength of the wind and to a lesser extent duration.

Tide affects wave shape and quality sometimes in a way similar to winds as could be the difference between surfable and non surfable. We use the Imray tides planner App which works worldwide and gives you all the tide forecast information any surfer could want – speed of tide movement, height of tide by minute.

Fetch is the distance over which wind blows across the ocean surface, influencing the size and energy of the generated waves. A longer fetch allows for the development of larger, more powerful swells, while a shorter fetch produces smaller, less energetic waves.

Bathymetry is the depth of the water and affects the time it takes for the waves to arrive from the Ocean, and also how the waves will break on the coast. Bathymetry determines where the fetch is more or less effective, as shallow areas can limit or redirect wave growth.

Surf Height Prediction (for novices reading forecasts)

If you are new to surfing, it’s best to keep things as simple as possible. Fortunately, Surfline provides a straightforward prediction of surf height, which indicates the size of the waves you can expect.

This makes it easy to identify if conditions are suitable for your skill level—beginners typically look for waves less than 4 feet without the guidance of an instructor.

Stop Light Rating (for novices to reading forecasts)

Surfline’s stoplight color coding quickly summarizes surf conditions:

Using this intuitive system, you can make informed decisions at a glance, helping you choose the right days and times to surf. As you start comparing the forecast to actual conditions, you’ll quickly build a stronger understanding of how various elements impact wave quality.

Comprehensive Reading of a Surf Forecast

Now, let’s explore the more detailed aspects of reading surf forecasts and what the specific data mean for your surfing session. Note that fetch and bathymetry is not shown in the data produced by the model, but is used by the models to calculate wave height and wave period. For average surfers, fetch is more important with regards to local winds.

Terminology

First, it’s essential to clarify some terminology. Many people mistakenly use “surf report” interchangeably with “swell forecast.” A surf report is based on historical or current data and focuses on conditions as they are, while a surf forecast predicts future conditions. Recent surf reports can influence forecasts if they provide up-to-date information on wave size and period. This data is integrated into forecasting models along with predicted wind patterns to project swell conditions for up to 12 days ahead. To simplify things, let’s focus on “swell forecast,” which predicts future swell sizes at specific locations.

Key Numbers in a Surf Forecast

There are three main metrics to look for in determining the expected wave size:

1. Wave Height (expressed in feet or meters)
2. Average Wave Period (measured in seconds)
3. Swell Direction (indicated in degrees on a 360-degree compass)

While wave height is often seen as the most critical factor, wave period plays a significantly larger role in determining the power and surfability of the waves. Additionally, swell direction greatly influences the conditions, determining whether all the swell is coming to your beach, some of it, or none at all.

Surf Forecasting Sources

Numerous surf forecasting websites provide valuable information. Our own platform offers daily surf forecasts and local condition reports. Surfline is well-known for its accurate forecasts, but in some regions (like the Indian Ocean), accuracy is hampered by a lack of buoy data. Countries like the USA, Australia, and Western Europe benefit from numerous offshore buoys that collect real-time swell data, whereas areas like Indonesia have fewer buoys, making models less reliable. Weather forecasts can also be less dependable in tropical regions due to their inherent volatility.

Understanding the Forecast Columns

The first column typically shows the expected wave height in feet. While measurements can be displayed in meters, it’s best to stick with one system for consistency. The model predicts surf size in three-hour intervals, and these predictions are rough calculations influenced by primary and secondary swell data.

The Rating system provides a rough indication of surf conditions. Caution is advised here; for instance, an “Excellent” rating can sometimes indicate surf that’s too large for most intermediate surfers. Therefore, it’s often more helpful to focus on the primary and secondary swell data, along with wind speed and direction. For surf spots like Uluwatu, tide information is crucial; a tide that is too low or too high can render surf conditions unrideable. Always consider tide charts in conjunction with swell and wind forecasts to determine the best surfing times. If you’re new to reading these forecasts, relying on surf height and rating initially works, until you gain more experience by comparing forecasts with real-time conditions.

Interpreting Swell Data

The primary swell information is generally the most critical for gauging wave size and power at your local break. Secondary swells can also affect conditions, especially when they rise while the primary swell declines.

Wave energy is directly tied to wave height and period, following the formula:

Wave Height * Wave Period * Wave Period = Wave Energy

Example 1. Dry Season Uluwatu

We will start with a very easy example. The first column shows Surf in ft which is expected size of the breaking waves. It starts off at 1-2 foot, grows to 2-3, 3-4, 3-4+ and 4-5 at end of day.

The Rating Poor To Fair is determined from the swell size which is very small and an excellent offshore wind. As the swell picks up the rating is improved to Fair and then Fair to Good. Caution needs to be taken when reading this column. For example, Fair to Good rating is given for a big powerful swell which is too big for most intermediate surfers and downright dangerous for beginners. Poor To Fair is not a helpful rating if the surf is too small to surf.

The Primary swell is very small and weak and therefore the surf in the morning will be very small. The secondary swell is also very weak and small but there is a long distance 20s swell that builds the day. It starts are the 3rd most important swell, at 9am it is the secondary swell and from noon onwards it is the primary swell. The higher the swell period, the faster the wave travels which is why you first get a 21s period swell dropping to 20s, 19s, 18s and 17s. This is the normal pattern for a very powerful ground swell. The surf will be tiny and weak early morning, building to a powerful swell by sunset.

Early morning looks a bit complicated to read with 3 different swells but they are all very small and so surf size will be likely in the waist to shoulder high range. Due to the nature of long range swells and Uluwatu’s location, there will always be head high waves even on the smallest swells, but likely to be very infrequent.

The consistency number in the morning is high. This means there will be many waves and that is due to the fact there are 3 swells combining. The surf could be a little bit messy and confusing with 3 waves coming from 3 different swell directions but when it’s very small, best to have many waves than very few. Later in the day as the long range primary swell dominates the forecast, the frequency reduces. By the nature of high period swells, above around 15s, the waves tend to be very organised in sets and can expect long delays between the biggest sets. At the very start of the new swell in the morning with 20s period, can expect very infrequent waves that are more powerful and moving faster than the waves from the shorter period swells.

The wind is directly offshore all day and a bit lighter early morning which is a normal pattern in dry season. The wave energy starts at a very low 66KJ and goes 11* the power to 680KJ when the primary swell reaches 4.4 foot @ 17s.

1012mb is high pressure which is consistent with stable weather. Sunny all day. Yabba daba do let’s go surfing. Only question is what time of the day. Late as possible for biggest waves. Will be small all morning. It will pick up in afternoon – building throughout the day. Tide chart is next tool you need to plan your session. Other options are to go to other spots on Bukit in afternoon as everywhere will be firing by sunset. Dreamland if low tide is one option. Local knowledge.

Example 2. Early Season Uluwatu

The surf feet shows 3-4+ in the morning and slowly decreasing in size throughout the day. The rating is FAIR for all period except 1. It is hard to see why 6AM has a lower rating than the others and this can be discounted.

There is a small primary swell which goes from 13s to 11s throughout the day. 11s is a very weak groundswell whereas 13s is around normal. It’s bit of a confusing picture with 2 secondary swells. The 1ft at 7s swell will not give rideable waves and can be discounted when considering the size of the waves, except that it is likely to reduce the wave quality – short period waves can be described as chop. At 6pm a small 18s long range swell shows. This could build the following day or could be too small to make any difference. The secondary swell of 2.4ft at 12s is quite significant. Both swells will be producing waves in the waist to shoulder high range with head high sets. With mixing of 3 swells it could be a bit messy.

The wind is light westerly early morning dropping to almost no wind around dawn. It is likely there will be some chop from the overnight light onshore wind, and while it should clean up during the day, no wind is not as good as an offshore wind for cleaning up the surf. Without getting too technical, an offshore wind cancels out the chop from short period waves produced by wind swells which is the case we have in this forecast from 1ft @ 7s which is caused by a local wind.

Wave energy gets lower throughout the day as the period of the primary swell drops in the fading swell. Consistency of around 50 is quite high due to 2 swells both producing waves. The wave size will be perfect for intermediate surfers, but probably less than perfect conditions with a bit of chop on the waves. Well worth a go for most people.

Example 3. Rainy Season Uluwatu

Any wave period less than around 10s is considered wind swell. which is very different to ground swell when it comes to surfing rideable waves. With regards to the data in the above example chart you can discount all of the 6s and 7s swell as it will not produce waves that are big enough to ride. The effect of very short period swell like this is to degrade the overall quality of the surf as short period waves are mostly just chop and not rideable. The 8s and 9s periods will produce some swell, and of better quality than the 6s and 7s, but negligable compared to 12s and 13s period swell. Let’s make some example calculations using the above formula.

6.2 foot * 6s * 6s = 223.20

5 foot * 13s * 13s = 845.00 which is almost 4 * the energy of the secondary swell from a smaller wave height

The overall effect of the secondary swell is not to increase the swell size but to degrade the quality. The primary swell is groundswell and would be breaking around head high at Uluwatu, bigger on the sets.

Wave energy can give a rough guide of what the trend throughout the day is and should be consistent with the size (wave height) and power (wave period) of the primary + secondary swell. Wave energy is quite stable around 500 throughout the day which would be consistent with head to overhead high waves. This is known through experience of comparing surfline forecast with actual conditions at Uluwatu.

Being able to intepret a swell forecast only has any value if it has some purpose. Mostly the key information to determine is if the surf spot is going to be good or not that day, and in dry season that would normally equate to whether or not the swell would is big enough and not too big. In rainy season when this forecast was taken from you also need to take into account the wind conditions. 20Mph onshore all day means very poor quality waves and either not rideable or just about rideable. In view that the forecast wind direction is broadly stable and constant all day, it is likely to be accurate but be aware that in the tropics, short term weather and wind forecasts are much less reliable than in temperate climates due to the energy of the sun giving more dynamic weather patters.

It’s a little bit hard to decipher the trend in the wave size and the actual size of the surf as you got primary and secondary swell interchanging but the overall picture is of a ground swell that is quite weak and small by Uluwatu standards, dropping throughout the day mixed in with a messy wind-swell. At first light 6am 4.8 foot @ 13 seconds will give overhead waves. At 12 noon 3.3 foot @ 13 is significantly smaller while at 6pm it shows 4.4 foot @ 12s. I would discount the 12 noon reading and determine the trend being from 4.8 @ 13 sunrise to 4.4 @ 12 sunset. This mean smaller less powerful waves as the day continues which is consistent with a diminishing swell.

You need to first determine what your objective from understanding the data is before you can make any use of the information. While the wave size during the day would suit the average surfer – overhead in the morning, dropping to head high in evening, this is totally negated by the 20Mph winds which are blowing constant all day. If there was variability in the wind speed or direction during the day I would summise that the forecast does not have accuracy probability and need to check conditions on the day hour by hour. In this case with solid steady winds between 20Mph-23Mph all day (small numbers are guest speeds which are by nature a little bit higher) then it’s 95%+ probability that Uluwatu is not surfable all day due to strong onshore winds making the waves unrideable.

I can understand if you have reached this far and feel disappointed. What the point then if can’t go surf. 2 points:

A: You don’t waste your time and energy vising Uluwatu
B: You use the information gleaned above to find a better spot than Uluwatu

With those winds the South coast will also be blown out. Pandawa handles a west wind better and may be surfable but will be quite big and very challending L4 to L5 surfers only. For everyone from L1 to L4 then Nusa Dua would be the best choice and most of the breaks will be around head high (similar size to Uluwatu). All the spots should be clean but there will be variabilities on the effect of the wind depending on the distance the break is to the shore and other factors such as headlands.

There you have it for almost everyone reading this. If you see a swell forecast of Bali looking like this head to Nusa Dua.

Conclusion

It takes a lot of time and effort into understanding how it works in your local surfing area, and this guide has been written to help you get started on that journey. For areas you are not familiar with, it is very useful to be able to read a forecast but to be able to ensure you are at the right spot at the right time, with so many variables such as wave energy, wind, wave height, wave period, tides, surf spot information, it’s not quick or simple. That’s why we recommend for beginner and intermediate surfers who are not familiar with an area, start out for at least the first few days with a reputable local surfing operator who will know the local conditions.

Interpreting a swell forecast has value when you understand its implications: determining whether a your local surf spot will be good or not. In the dry season, it often revolves around swell size, while in the rainy season, wind conditions are just as crucial. Strong onshore winds can render many spots unrideable. Always analyze all these factors together to make informed decisions.

To be able to determine the swell size at your local spot you would need to consider swell direction and other factors that affect the spot. This guide can help you understand the data but to get a reasonably accurate prediction of the swell size, you need to bench marks many days of comparison between surf forecast and actual.

Looking for unique, in-depth insights on the technical and scientific side of surfing? Our original, professionally made Video Tutorials—Beginner, Intermediate, and Advanced—are created just for you to improve your skills like no other.

Written by Andrew – Been visiting Bali since 1995 and lived here since 2003 surfing every day
Updated: Dec 12, 2025