Hydration Planner
Do a one-hour sweat test, pop in the numbers, and get your race-day plan: how much to drink, how much sodium, and what that means at every aid station. Works for marathons, 70.3s and Ironman runs.
- Sweat rate
- 1.20L/h
- From your test, at test conditions
- Drink per hour
- 720ml/h
- 60% of what you sweat
- Sodium per hour
- 650mg/h
- You lose about 1080 mg/h
- Per aid station
- 150ml
- About 1.5 cups at each station
Fine for trained runners. Expected weight loss about 2.4%. Normal for a marathon, especially for faster runners.
Your race-day timeline
Night before
400-500 ml of a salty electrolyte drink with dinner. Pale yellow pee is the goal, not clear.
Race morning
About 350 ml to 490 ml of electrolyte drink 2-3 hours before the start (5-7 ml per kg). Finish it 45-60 minutes before the gun.
During
Around 720 ml and 650 mg sodium per hour, from drinks, gels or salt tabs. Sip at stations, do not chug.
Total for the race
About 2.5 L of fluid across 3.5 hours of running.
The Wingman sweat test
One hour, a bathroom scale and a measuring jug. Do it properly once and you will never guess your race drinking again.
Set it up
- Pick a 60 min run at goal race effort (marathon pace, or your 70.3 or Ironman run effort). A bike sweat test works the same way.
- Wear what you will race in, and choose weather close to race day if you can. Jot down the temperature and humidity.
- Start normally hydrated. No chugging water beforehand to "help" the test.
- Use a digital scale that reads to 0.1 kg (0.2 lb), and the same scale before and after.
Before you go
- Empty your bladder.
- Weigh yourself naked and write it down.
- Fill a bottle and know exactly how much is in it.
On the run
- Run the full 60 min at race effort.
- Drink as you normally would, from your measured bottle only.
- Try not to pee. If you must, estimate how much and enter it above.
- Skip food and gels for this hour if you can. They muddy the numbers.
Straight after
- Strip off the wet kit. Sweat trapped in your shirt still counts as lost.
- Towel dry, hair included.
- Weigh yourself naked again before you eat, drink or pee.
- Measure what is left in the bottle to work out how much you drank.
The maths (the planner does it for you): sweat lost = weight lost + what you drank - any pee. 1 kg of weight is roughly 1 litre of sweat.
Make it count: repeat the test in cool, warm and hot weather, and keep a little log. Heat, humidity and fitness all change the number. Then glance at your kit: white salt marks mean pick "Salty" in step 3.
Two numbers, two different jobs
How much you sweat and how salty your sweat is are two separate things. You need to know both.
Sweat rate: how much
Litres lost per hour. It sets how much you drink. Your sweat test above gives you this number, and it changes with heat, humidity and pace.
Sweat sodium: how salty
Milligrams of sodium in each litre of sweat. It sets how much salt you take. It ranges from about 200 to over 2,000 mg per litre between people, and it is largely down to your genes.
They do not travel together
A big sweater can have watery sweat. A light sweater can be very salty. Two runners losing 1 litre an hour can lose 400 mg or 2,000 mg of sodium. Same water, five times the salt.
How to find your salt
A sweat sodium test gives the exact number. Our partner Precision Fuel & Hydration runs them: find your nearest Precision sweat test. No test yet? Read the signs: white marks on your kit, stinging eyes, cramps or salt cravings mean pick "Salty" in step 3.
Adjust for the weather
| Conditions | Fluid per hour | Sodium per hour | Extra tactics |
|---|---|---|---|
| Cool, under 10°C (50°F) | 200-400 ml | 300-500 mg | Thirst is muted, so sip anyway. Do not overdrink. |
| Mild, 10-18°C (50-64°F) | 400-600 ml | 400-600 mg | The sweet spot. Stick to your plan. |
| Warm, 18-25°C (64-77°F) | 500-700 ml | 500-800 mg | Water over the head, slower start. |
| Hot, over 25°C (77°F) | 600-800 ml | 700-1000+ mg | Ice in the cap, dilute your fuel, adjust goal pace. |
Too much water is the real danger
Low blood sodium (hyponatremia) hits runners who drink more than they sweat, usually slower finishers on cool days.
- Never drink past the point of feeling full or sloshy.
- Puffy hands, bloating, headache or confusion: stop drinking and get medical help.
- Gaining weight during a race is the red flag.
Train your gut
Your stomach is trainable, just like your legs. Practise on every long run:
- Weeks 1-6: 150-200 ml every 15-20 min.
- Weeks 7-12: add electrolytes and your race-day products.
- Weeks 13+: full race-day rehearsal, including at marathon pace.
Do the test again
Your sweat rate changes with heat, humidity and fitness. Repeat the test when the weather warms up, and always before a hot race. Nothing new on race day. Ever.
Want a coach to plan your race day?
A Wingman coach builds your fuelling and hydration into your plan, so you rehearse it in training and race day holds no surprises.
A guide, not a medical prescription. Questions? Ask Coach Stephan .