Physical health · a life skill
Advanced Hydration Planning
weigh, drink, weigh again
On long or hot sessions, a fixed bottle plan stops fitting, because people sweat at very different rates. The scale tells you yours: the weight you lose, plus what you drank, is roughly the sweat. Log a few sessions and you can plan drinks that keep you under 2% down without ever finishing heavier than you started.
- Challenging
- Life-Changing impact
- Learn it: 30 days
- Two weighings a session
Sam's numbers are an example, not a target. Taking 1 kg lost as about 1 litre of sweat, Sam's hot runs come to about 1.3 litres an hour; yours may be far from that.
The method
Weigh, drink, weigh, divide.
-
Weigh before, the same way each time
Same scale, little or no clothing, after the toilet. Write it down to a tenth of a kilo.
-
Count every drink
Use a bottle you've measured, and note how much is gone at the end, not how much you meant to drink.
-
Weigh after, and work out the change
Towel off first. Before minus after, divided by before: under 2% down is the aim. Any gain means you drank more than you sweated.
-
Plan the next one from your rate
Weight lost plus drinks is roughly the sweat. Over a few sessions in each kind of weather, that gives a rate per hour to plan the next session's drinks from.
Your own sweat rate per kind of weather, and drinks that land you between 2% down and no gain.
Example: Sam weighed 70.1 kg before a 90-minute run on a hot afternoon, drank 0.5 litre, and weighed 68.5 kg after: 1.6 kg down, 2.3%. Sweat was about 2.1 litres, 1.4 litres an hour. On the next hot run Sam carried 1.0 litre, sipped it through, and finished 1.3% down.
This is for healthy adults training for an hour or more. If a doctor, nurse or dietitian has given you a fluid limit, or you have a heart or kidney condition, follow their advice, not this page. Never drink to put weight on: in a study of marathon runners, finishing heavier was linked to dangerously low blood sodium. Call 999 if someone has signs of dehydration and is confused, finding it hard to breathe, or sleepier than normal or hard to wake (NHS).
The sweat check
Log your sessions, then plan the next one.
1 · Log a session
Weigh before and after the same way. A toilet stop between the two makes the sweat figure too high, so skip that session or note it. Measure the bottle with the refill planner.
2 · Plan the next one
3 · Your sessions
Log a session to start
Your log is kept only in this browser. Nothing is sent anywhere. It takes 1 kg lost as about 1 litre of sweat and ignores what you ate or passed in between, so treat every figure as an estimate.
What the research says
Too little has a line. Too much has a cost.
The American College of Sports Medicine's position on exercise and fluid sets the goal of drinking during exercise as preventing “excessive (>2% body weight loss from water deficit) dehydration”. It also says “there is considerable variability in sweating rates” between people, recommends customised plans, and gives the method: “Individual sweat rates can be estimated by measuring body weight before and after exercise” (Sawka et al., 2007).
The other side is drinking more than you lose. Researchers took blood at the finish of the 2002 Boston Marathon from 488 runners: 13 percent had low blood sodium, and 0.6 percent had it at a critical level. The runners who finished heavier than they started, and the slowest, were more likely to have it (Almond et al., 2005).
So the useful range has two edges, and only the scale shows where you are between them. EFSA adds a ceiling on speed: adult kidneys clear at most about a litre an hour (EFSA), so a plan is sips through the session, not a bottle at the end.
The idea
Why it works.
The refill planner gets an ordinary day right with a bottle and a clock. Training breaks it, because a long session in the heat can cost more water than the rest of the day, and nobody's thirst or guess tells them their own rate.
Two weighings do. One number per session turns into a rate per hour, and a rate turns into a plan with a floor and a ceiling: enough to stay under 2% down, never so much that you finish heavier. If you also warm up before hard sessions, weigh before the warm-up.
When it goes wrong
Habits that help.
- One reading taken as the truth. A single session is noisy. Wait for two or three in the same weather before you trust the rate.
- Weighed in wet kit. Sweat in your clothes counts as body weight and hides the loss. Towel off and weigh in the same dry clothes as before.
- Finishing heavier. That's drinking past your sweat. Cut back next time, whatever the bottle plan says.
- A cool-weather rate used in a heatwave. Rates change with the weather. Keep them apart, and log a hot session before you plan one.
Go deeper
Keep going.
- Simple Hydration Planning
- Deliberate Hydration Planning
- Advanced Hydration Planning
Practise this in HourLife.
In Intake: Reminders per item. A one-tap log of what you had today: drinks, food, supplements, and anything else, with daily totals, reminders, and insights.
