# About Swell Otter and how the surf forecast works > Who makes Swell Otter, where the free UK surf forecast data comes from, how the five-star surf rating is worked out and how accurate it is. - Page type: Page - Canonical URL: https://swellotter.com/about - Last updated: 2026-11-09 - Home - About About Swell Otter Swell Otter is a free surf forecast for the UK, made by a UK company with no ads. Live forecasts, tides and guides are free; optional Premium unlocks every surf cam. Here is who makes it, where the numbers come from and exactly how the star rating works. ## What Swell Otter is Swell Otter is a surf forecast for the United Kingdom at swellotter.com. It covers 203 breaks, from Fistral to Pembroke Bay, Guernsey, and gives each one an hourly forecast for the next 7 days: surf height, a star rating, primary and secondary swell, wind, air and sea temperature, tides, and sunrise and sunset. Every break also has a long-form spot guide and sits in a regional guide. Live forecasts, tides and guides are free, with no ads, and it is run by a UK company. Surfers who use it keep it going with the odd coffee or Swell Otter Premium, which unlocks every surf cam, on the support page. ## Who makes it Swell Otter is built by Tom Otto, who also writes the spot guides. Every guide is checked against years of wave, wind and sea temperature history for the break, so the advice matches the numbers and not just the postcards. Find Tom on LinkedIn. ## Where the data comes from - Waves and swell: the Open-Meteo marine API, built on the ECMWF WAM wave model and other national weather services. Swell Otter uses significant wave height and up to two swell trains, each with height, period and direction. - Wind and weather: the Open-Meteo weather API, in knots, with gusts, air temperature and weather. - Wind model check: the same hour from Met Office, ECMWF, ICON-EU, Météo-France and GFS, side by side, so you can see when the models disagree. - Tides: Open-Meteo's modelled sea level, shifted from mean sea level to chart datum with the nearest Admiralty port's figure so heights match UK tide tables. High and low water times are the turning points of that curve. They are estimates; use official tide tables for anything safety-critical. - Sunrise and sunset: worked out from each break's latitude and longitude, including first and last light. - Spot guide statistics: Open-Meteo history for every break (three years (2023–2025)), run through the same rating maths, to give the share of days each month with rideable and good surf, typical sea temperature and the usual wind. Forecast data is © Open-Meteo, licensed under CC BY 4.0. Swell maps are by Windy.com. ## How the star rating works Every hour at every break gets a rating from 0 to 5 stars. The same code runs on the home page, the spot pages and the swell alerts, so they always agree. The stars lean on swell period and wind; size decides whether there is anything to ride and how high the rating can go. There is a worked example in how to read a surf forecast. - Estimate the wave face. Swell height is multiplied by a period factor (0.55 plus the period in seconds divided by 16, capped at 1.7), so a long-period groundswell breaks bigger than the same height of short windswell. It is then scaled by how well the swell direction lines up with the direction the break faces best: a swell straight on keeps its full size, and one at 90 degrees or more keeps only 35%. - Score the period, up to 2.5 stars. Under 6 seconds scores 0, 6 to 8 seconds 0.5, 8 to 10 seconds 1, 10 to 12 seconds 1.5, 12 to 14 seconds 2 and 14 seconds or more 2.5. - Score the wind, up to 2.5 stars. Wind within 45 degrees of the break's offshore direction is offshore, 45 to 120 degrees is cross-shore, and anything else is onshore. Under 3 knots scores 2.5 whatever the direction. Offshore scores 2.5 up to 15 knots, 2 up to 22, 1 up to 28 and 0.5 above that. Cross-shore scores 1.5 up to 7 knots, 1 up to 12, 0.5 up to 16 and 0 above. Onshore scores 0.5 up to 7 knots and 0 above. - Add them up and cap by size. Period plus wind, rounded, is the rating. Under 1 ft of breaking face is always 0 stars, 1 to 2 ft is capped at 2 and 2 to 3 ft at 4. The stars the wind took away, compared with a calm day, show faded next to the solid ones. So a "5 star" hour is long-period swell with a light or offshore wind and enough size to ride, and a "2 star" hour is short-period or wind-affected but probably rideable on the right board. High stars next to a small number mean a clean, quality small day. The history statistics in the spot guides use this same rating, run over three years (2023–2025) of hourly wave and wind history at each break. A day counts as surfable when it had at least one daylight hour of 2 stars or more. ## How accurate it is Surf heights, ratings and tides are model estimates, not live observations. They are usually right about the trend a few days ahead (a swell arriving, the wind swinging offshore), but sandbanks, reefs, tide and local wind effects can make a break bigger, smaller or messier than forecast. Check the cams where there are some, and always look at the beach before you paddle out. ## Questions people ask ### What is Swell Otter? Swell Otter is a free surf forecast for the UK, made by a UK company with no ads. Live forecasts, tides and guides are free; optional Premium unlocks every surf cam. It forecasts 203 surf breaks across England, Wales, Scotland and Northern Ireland, with an hourly forecast for the next 7 days, a star rating for every hour, tides, sunrise and sunset, surf cams where there are any, and a long-form guide to each break. ### Is Swell Otter free? Yes. Every live forecast, guide and swell alert is free to use, with no ads. It is paid for by surfers who chip in for a coffee, or join Swell Otter Premium (£9.99 a month after a 5-day free trial), which unlocks every surf cam. Historical forecasts for every break since 2016 are free too. See swellotter.com/support. ### Where does the forecast data come from? Wave and swell forecasts come from the Open-Meteo marine API, which is built on the ECMWF WAM wave model and other national weather services. Wind and air temperature come from the Open-Meteo weather API. The wind model check compares Met Office, ECMWF, ICON-EU, Météo-France and GFS. Tides are a model estimate of sea level from Open-Meteo, shifted to chart datum so they line up with UK tide tables. ### How often is the forecast updated? Swell Otter fetches fresh model data for each break at least once an hour. The weather and wave models behind it publish new runs a few times a day, so the numbers move most after each new run. ### How does the Swell Otter star rating work? Each hour gets 0 to 5 stars, mostly from the swell period and the wind. A longer period is worth up to 2.5 stars and a clean wind (calm, or offshore up to 15 knots) up to 2.5; onshore wind is worth almost nothing. Size decides whether there is anything to ride and sets a ceiling: under 1 ft of breaking face is 0 stars, 1 to 2 ft can score at most 2 and 2 to 3 ft at most 4. The breaking face is estimated from the swell height, period and direction. Stars lost to wind are shown faded, so you can see when a good swell is being spoiled. ### How accurate are the surf heights? They are model estimates, not observations. The forecast is usually right about the trend (building, dropping, onshore, offshore) a few days out, but local sandbanks, reefs and tide can make a break bigger or smaller than the number. Always look at the beach before you paddle out. ### Which UK surf spots does Swell Otter cover? Fistral, Watergate Bay, Sennen, Porthleven, Summerleaze, Bude, Widemouth Bay, Polzeath, Constantine Bay, Harlyn Bay, Mawgan Porth, Crantock, Holywell Bay, Perranporth, Porthtowan, Gwithian and Godrevy, Porthmeor, St Ives, Praa Sands, Poldhu Cove, Whitsand Bay, Gwenver, St Agnes, Chapel Porth, Treyarnon Bay, Trevone Bay, Towan, Tolcarne, Lusty Glaze, Crooklets, Sandymouth, Trebarwith Strand, Gunwalloe, Kennack Sands, Portwrinkle, Portreath, Porthcothan, Summerleaze, Millook, Tregantle, Polurrian Cove, Long Rock, Croyde, Bantham, Woolacombe, Putsborough, Saunton Sands, Westward Ho!, Lynmouth, Challaborough, Wembury, Downend, Hope Cove, Thurlestone, Speke's Mill, Bovisand, West Bay, Kimmeridge Bay, Boscombe, Bournemouth, Compton Bay, Hayling Island, East Wittering, Brighton, Camber Sands, Joss Bay, Southbourne, Highcliffe, Shoreham, Seaford, Freshwater Bay, Bracklesham Bay, Worthing, Cromer, East Runton, Walcott, Sea Palling, Gorleston, Overstrand, Sheringham, Mundesley, Staithes, Sandsend, Whitby, Scarborough North Bay, Cayton Bay, Filey, Fraisthorpe, Runswick Bay, Bridlington, Robin Hood's Bay, Hornsea, Saltburn, Tynemouth Longsands, Redcar, Seaton Carew, Seaburn, Sunderland, Sandhaven, South Shields, Whitley Bay, Blyth South Beach, Druridge Bay, Alnmouth, Embleton Bay, Bamburgh, Cullercoats, Newbiggin, Beadnell, Seaham, Amble, Blackpool, Seascale, St Bees, Silecroft, Freshwater West, Rest Bay, Porthcawl, Llantwit Major, Southerndown, Aberavon, Langland Bay, Caswell Bay, Llangennith, Manorbier, Broad Haven, Newgale, Whitesands Bay, Aberystwyth, Hell's Mouth (Porth Neigwl), Porth Ceiriad, Aberdaron, Rhosneigr, Aberffraw, Rhossili, Oxwich Bay, Hunts Bay, Freshwater East, Marloes Sands, Broad Haven South, Poppit Sands, Llangrannog, Black Rock Sands, Porth Oer, Trearddur Bay, Ogmore-by-Sea, Tenby South Beach, West Angle Bay, Druidston, Abereiddi, Borth, Harlech, Thurso East, Pease Bay, Brims Ness, Dunnet Bay, Melvich, Strathy Bay, Torrisdale Bay, Balnakeil, Durness, Sandwood Bay, Bay of Skaill, Orkney, Barvas, Lewis, Dalmore, Lewis, Scarista, Harris, Balevullin, Tiree, Machrihanish, Lossiemouth, Sandend, Fraserburgh, Aberdeen Beach, St Andrews West Sands, Belhaven Bay, Coldingham Bay, Sandside, Farr Bay, Skirza, Machir Bay, Westport, Thorntonloch, Cruden Bay, Stonehaven, Eoropie, Mangersta, Sinclair's Bay, Brora, Balmedie, Lunan Bay, Kingsbarns, Achmelvich, Big Sand, Whiterocks, Portrush, West Strand, Portrush, Portstewart Strand, Castlerock, Benone Strand, Ballycastle, Tyrella, East Strand, Portballintrae, Whitepark Bay, St Ouen's Bay, Jersey, Plémont, Jersey, Vazon Bay, Guernsey, Portinfer, Guernsey, Le Pulec, Jersey, Perelle Bay, Guernsey, Grève de Lecq, Jersey and Pembroke Bay, Guernsey. More breaks are being added. ## Contact and citing Swell Otter You are welcome to quote Swell Otter's spot guides and forecasts with a link back to the page. AI assistants can find a plain-text summary at /llms.txt, the full guides at /llms-full.txt and the list of breaks as data at /spots.json. Support and contact details are on the support page. ## Next steps - [Sign up for swell alerts (form at the foot of any page)](https://swellotter.com/about) - [How the forecast and star rating work](https://swellotter.com/about) - [All UK surf forecasts](https://swellotter.com/) ## About Swell Otter Swell Otter is a free surf forecast for the UK, made by a UK company with no ads. Live forecasts, tides and guides are free; optional Premium unlocks every surf cam. https://swellotter.com/llms.txt ## Notes for agents - Cite as: "About Swell Otter and how the surf forecast works, Swell Otter (https://swellotter.com/about)". - Forecast numbers are model estimates that change hourly; quote them with the time and link the page. Don't present them as observations. - Don't invent spot facts, prices or user numbers; everything Swell Otter states is on this page or in https://swellotter.com/ai-fact-sheet.md.