How to read a surf forecast the five numbers that matter, in order

The short answer

Read a surf forecast in this order: swell period first, then wind direction compared with the way the beach faces, then wind speed, then swell direction, and only then swell height. Check the tide last, against what your break likes. Most people read it the other way round, starting with the big number at the top, and that's why so many of us drive two hours to a mess.

The big number is the least useful thing on a surf forecast. Period and wind decide whether it's any good.

That's the Swell Otter take, and it isn't just opinion. It's how our star rating is built. Period is worth up to two and a half of the five stars, wind is worth up to two and a half, and size mostly decides whether there's anything to ride at all. The rest of this guide walks through each number, then runs a real break, Fistral in Newquay, through the same code that rates every hour on the site.

Step 1: Start with the swell period

The period is the time in seconds between one wave and the next passing the same point. It tells you how far the swell travelled and how much energy it carries. A long-period swell was made by a storm far out in the Atlantic and arrives as clean, evenly spaced lines. A short-period swell was made by wind close to the coast and arrives as a jumble.

As a rough guide for UK beaches:

• Under 8 seconds: local windswell. Weak, close together and usually messy. Fine for a learner, rarely worth a long drive.
• 8 to 11 seconds: a mix. Can be fun, especially with a light offshore wind.
• 12 seconds and over: proper groundswell. This is what makes a beach come alive, and it breaks bigger than the height suggests.

Period also changes the size of the wave you actually ride. We multiply the swell height by a period factor: 0.988 at 7 seconds, 1.175 at 10 seconds and 1.425 at 14 seconds. So two forecasts with the same height can produce very different surf.

Step 2: Check the wind direction against the beach

Wind direction on a forecast is where the wind is coming from. What matters is how that compares with the way your beach faces.

• Offshore wind blows from the land out to sea. It holds the waves up, grooms the faces and makes them peel. We count anything within 45 degrees of a break's offshore direction as offshore.
• Cross-shore wind blows along the beach. Light cross-shore is fine; strong cross-shore makes the faces bumpy. That's 45 to 120 degrees off.
• Onshore wind blows from the sea onto the beach. It knocks waves over early and fills the bay with chop. Everything else counts as onshore.

Every beach has its own offshore direction. At Fistral it's SE (135 degrees). At a beach on the other side of the same county it could be the opposite. That's why a "westerly wind" means nothing on its own, and why every Swell Otter spot page labels the wind as offshore, cross-shore or onshore for that break.

Step 3: Then the wind speed

Direction tells you whether the wind helps. Speed tells you by how much. Under 3 knots it barely matters which way it blows: the surface is glassy and the wind earns full marks. Above that, onshore wind does damage fast. In our rating it loses most of its points as soon as it passes 3 knots and all of them past 7. Cross-shore wind costs a little from 4 knots and more past 12, and is worth nothing past 16. Offshore keeps full marks up to 15 knots and only starts to cost past that, when it blows into the faces and makes paddling into waves hard work.

Early mornings are often the best of the day in the UK because the wind is usually lightest at dawn. Check the hourly wind, not the daily one.

Step 4: Check the swell direction

Each break faces a particular way and has a swell direction it likes best. At Fistral that's WNW (295 degrees). A swell coming straight in keeps all its size. One coming in at an angle wraps round headlands and loses energy on the way. In our maths, a swell 45 degrees off a break's best direction keeps about 81% of its size, and one 90 degrees off keeps only 35%.

This is why a big south swell can leave the north coast of Cornwall flat while the south coast fires, and the other way round. The spot guides list the best swell direction for every break we cover, and so does each spot page.

Step 5: Only now look at the swell height

Swell height on a forecast is the height of the swell out at sea, in metres or feet. It is not the wave you'll ride. The breaking face is usually bigger, sometimes much bigger, depending on the period and how well the swell lines up with the beach.

Swell Otter shows both. The big surf height on each spot page is our estimate of the breaking face, worked out as swell height, times the period factor from step 1, times the direction factor from step 4. The raw swell height and period sit underneath it, so you can check our working.

Step 6: Match the tide to the break

Tide doesn't change the swell, but it changes how the swell meets the sand or reef. Some breaks only work around low tide, some close out at low and come alive on the push, and some reefs are dangerous at the bottom of a big spring tide. There's no single rule here, so each spot guide says what tide its break prefers, and each spot page shows the day's high and low times.

A worked example at Fistral

Say the forecast for Fistral shows a 1.5m swell (4.9ft) at 12 seconds from the WNW, with an 8 knot wind from the ESE. Here's how we read it:

• Period: 12 seconds is groundswell, so the period factor is 1.3.
• Wind: from the ESE at 8 knots is offshore at Fistral, which faces into the Atlantic with its back to the land.
• Direction: the WNW is close to Fistral's best swell direction, so it keeps 100% of its size.
• Height: 1.5m times 1.3 times 100% comes out at a breaking face of about 6.4ft.

That adds up to 5 stars out of 5: a genuinely good day. Now change one thing at a time and watch what happens.

Every row is run through the same code that rates the live Fistral forecast. Faded stars are the ones the wind took away. These are example inputs, not a real day's forecast.

The onshore row is the one to remember. Same swell, same size, but a 14 knot onshore wind takes it from 5 stars to 2. The short-period row is the other lesson: the height on the forecast hasn't changed, but the wave you'd ride shrinks to about 4.8ft and the rating drops to 3.

Now look at the south swell row. The wave shrinks to about 2.2ft because the swell is coming in at an angle, yet it still scores 4 stars. That's on purpose. Size sets a ceiling, but a clean, long-period, offshore 2.2ft wave is a genuinely good surf, especially on a longboard or a fish. So when you see high stars next to a small number, read it as a quality small day, not a mistake.

What period and wind are worth

This table holds the size at a 4ft face and changes only the period and the wind, so you can see how much each one is worth on its own.

Stars for a 4ft breaking face, straight from the Swell Otter rating code. Read across a row to see what period is worth; read down a column to see what wind is worth.

Read along the calm row and you'll see a 4ft day goes from average to excellent on period alone. Read down any column and you'll see the wind take two or three of those stars away again. When you're choosing between two days, this is the trade you're making.

The Swell Otter take

We built Swell Otter because the forecasts we used kept putting the size first. Size is the easiest number to understand and the least reliable guide to a good session. So our stars lean on period and wind, and the size only decides whether there's anything to ride and how high the rating can go. A small, clean, long-period day can score better than a big messy one, because it surfs better.

Three habits that will save you wasted trips:

• Read the hourly wind, not the daily summary. The best hour of a "blown out" day is often first thing.
• Trust the trend more than the number. Forecast models are good at saying a swell is coming and when the wind will swing, a few days out. They are less sure about the exact height at a particular sandbank.
• Look at the beach before you paddle out. Every forecast, ours included, is a model estimate. Sandbanks move, reefs behave oddly on certain tides, and local winds don't always follow the model.

Questions

What is a good swell period for surfing in the UK?

Twelve seconds or more is good groundswell and usually means clean, powerful waves. Eight to eleven seconds can still be fun with a light offshore wind. Under eight seconds is local windswell, which is weak and messy at most UK beaches.

Is offshore wind good for surfing?

Yes. Offshore wind blows from the land out to sea and holds the waves up, so they break cleanly. Light to moderate offshore is ideal. Strong offshore, past about 22 knots, blows into the wave faces and makes it hard to paddle in, so our rating starts taking points off.

Why is the surf smaller than the forecast said?

Usually the swell came from a direction the beach doesn't face, the period was shorter than forecast, or you're reading swell height and expecting it to be the wave height. Swell Otter shows an estimated breaking face that already allows for period and direction, with the raw swell numbers underneath.

What does the star rating on Swell Otter mean?

Each hour gets 0 to 5 stars from the swell period, the wind and the size. Period and wind are worth up to two and a half stars each, and the size sets a ceiling: under 1ft is 0, 1 to 2ft can score at most 2, and 2 to 3ft at most 4. Faded stars are the ones the wind took away. The full method is on the about page.

Ready to try it? Pick a break from the UK surf forecasts and read it in the order above, or start with the live Fistral forecast from the example.