Surfmelier

Take your body weight in kilos and multiply. Roughly 0.9 litres per kilo if you're still learning, 0.48 if you can turn and trim, 0.38 if you're solid, 0.33 if you're ripping. A 75 kg intermediate lands near 36 litres. A 75 kg advanced surfer, near 28.

Then the day moves it. Weak, gutless surf wants about 10 per cent more, because the wave gives you nothing and the board has to make the speed. Onshore chop wants a bit more again. Overhead surf wants a few extra litres too, this time for the paddle into something moving faster than you are.

Two things most calculators skip. If you're over forty, out of practice, coming back from injury, or wearing a 4/3, add litres instead of taking them away. And if you surf twice a year, size like a beginner no matter how good you once were. Waves caught beat litres saved, every time.

Three wave models and three weather models, read together rather than one at a time. The waves come from EWAM and MFWAM, the high-resolution European runs, plus GWAM, which is global and carries the Azores and Madeira where the regional grids stop. The wind comes from ECMWF, ICON and GFS: three separate national forecasting centres, so when they agree they really agree.

Where they scatter, the page says so instead of picking one and sounding certain.

Every number is a median across daylight hours, sunrise to sunset, and the wind is taken from the four hours after sunrise rather than the daily peak. A day's peak hour is not what the day was like, and for wind that peak almost always lands at five in the afternoon, when nobody is out.

Size sets the ceiling. Period and wind can only pull the score down from there, which is why a glassy knee-high morning never rates Good. It's a lovely session. It isn't a good day.

Because they face different ways. Ericeira looks out at 300 degrees, straight into the northwest groundswell that runs down the Atlantic. Carcavelos faces roughly south, tucked inside the bay behind Cabo Raso, so the same swell has to bend around a headland before it gets there.

Surfmelier models that for all nine spots: how the beach is oriented, how much energy the local geography takes out, and how much more a long-period swell wraps compared to short-period windchop. It's why a west swell can light up Carcavelos while Ericeira goes quiet, and why the Algarve's two coasts behave like two different countries.

Nine, from the top of the country to the middle of the Atlantic: Matosinhos in Porto, Supertubos in Peniche, Ribeira d'Ilhas in Ericeira, Carcavelos in Cascais, Arrifana on the Costa Vicentina, Praia da Rocha on the Algarve's windward coast, Ilha de Faro on the leeward one, Santa Bárbara on São Miguel, and Jardim do Mar in Madeira.

The Algarve is split in two on purpose. Barlavento sits close enough to Cabo de São Vicente that west and southwest swell bends around the corner. Sotavento sits fully in its shadow and runs on local wind swell. Same region, two different oceans.

Portuguese water runs from about 14°C in late winter to 22°C in late summer. Under 14, a 4/3 with boots and gloves. 14 to 16, a 4/3. 16 to 18, a 3/2. 18 to 20, a 3/2 or a 2mm. 20 to 22, a 2mm or a spring suit. Above that, boardshorts and no excuses.

Surfmelier shows live water temperature and the matching suit for whichever spot you've picked.

It's free, there's no account and there's no catch. We sell second-hand surfboards, so a tool that tells you exactly which board suits you today is good for us whether or not you ever buy one. If it sends you out on a board you already own, that's fine too.