Young exhausted athlete splashing and pouring fresh water on his head to refresh during a running trail

It's a good idea to calculate your sweat rate so you know how much to put back in.

Summer is almost here, which means beer, barbies, beaches and other stuff brought to you by the letter "B" that I can't think of right now.

But if you happen to be a runner, it also means sweat. And lots of it. While it's not something generally discussed in polite circles, sweating is an unavoidable fact of life for all runners, especially as the weather heats up. And with that increasing sweat loss comes a greater focus on hydration (or, in plain English, drinking fluid).

You won't be doing your performance any favours if you allow yourself to dehydrate. Losing as little as 2 per cent of your body weight in fluid can make you feel much more tired than normal, raise your body temperature and heart rate and confuse your thinking.

The amount of fluid you lose is a very personal thing, so it's a good idea to calculate your sweat rate so you know how much to put back in.

Sports dietician Tara Diversi knows a thing or two about hydration – she is a distance runner and swimmer, who has just swum the English Channel.

"Particularly at this time of year, when it's not so hot that you are going to really fatigue or damage yourself, we get people to do a sweat rate test," she says.

Weigh yourself then head out for a run of about an hour and don't drink anything during the run. Weigh yourself afterwards and that is roughly how much fluid you with lose each hour – 1 kilogram weight loss is roughly equivalent to 1000 millilitres of sweat.

Obviously, at the height of summer, you will lose more, but this is still a useful rule of thumb.

For shorter runs of about 10 kilometres, Diversi says drinking on the run, while useful, is not as important as hydrating correctly afterwards.

"It helps you back up for your next session," she says. "You need your muscles to recover and to rehydrate if you want to run every day or even if you want to run in two days' time."

And if you don't drink enough after your run, the danger is that you will progressively accumulate dehydration.

"You might be 1 per cent dehydrated and say 'I feel fine' and every day that adds up and one day you go over the edge and say, 'Wow, I'm not feeling like I've recovered, it must be something I did in that run', but it's actually two weeks' of not rehydrating properly."

And as to what you should be drinking after a run, if you are an occasional runner or only cover short distances, then water is fine. Otherwise, a sports drink (choose your brand based on personal preference - they are all broadly similar) is a better choice as they contain both salt and carbohydrates.

At the other extreme, hyponatremia is a dangerous and potentially fatal condition generally caused by drinking too much water. And, while it is unlikely to be an issue during your regular 5-10 kilometre workout, it can be a problem in longer sessions like a marathon.

Diversi's advice to avoid the risk of hyponatremia is to drink "from thirst" while running and ensure you hydrate properly afterwards.

What are your hydration strategies? Do you have a plan or do you just wing it?