Race day
Marathon Hydration Calculator
Quick answer
For a marathon, most runners start with 400–800 ml per hour during the race, then adjust for sweat rate, heat, pace, and stomach comfort. Drink to a practiced plan, not panic. Use the daily calculator below for pre-race baseline hydration, then measure sweat rate on long runs to set your race range.
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Your daily goal: 123 ounces, 14.5 glasses.
Your daily goal
Updates instantlyof water a day · about 14.5 glasses or 7.5 half-litre bottles
Typical range: 78-116 fl oz. Use it as a flexible target, not a medical limit.
Start
2.5 glasses
7:00 AM
Taper
9:00 PM
for better sleep.
Food fluid
+0.9 L
typical food water
- 3.7
- Litres
- 123
- Ounces
- 14.5
- Glasses
Why this number
We start from a weight-based range, then adjust the point estimate for activity, climate, life stage, and GLP-1 settings. Food water is shown separately so the drink goal stays actionable.
Full methodologyNext step
Your sip schedule
- 7:00 AM · Start the day2.5 glasses
- 9:48 AM · Top up2.5 glasses
- 12:36 PM · Top up2.5 glasses
- 3:24 PM · Top up2.5 glasses
- 6:12 PM · Top up2.5 glasses
- 9:00 PM · Wind down2.5 glasses
Ease off after 9:00 PM for better sleep.
Electrolytes? Worth it today
Hard training in the heat means heavy, salty sweat — electrolytes help you hold onto the water you drink.
A friendly estimate for healthy adults, not medical advice. Your needs rise with heat, exercise, illness, pregnancy, and some medications. Don't drink more than ~1 litre per hour.
Daily goal
123 fl oz
The link and preview image include your weight and settings.
Marathon fluid starting ranges
These ranges use 400, 600, and 800 ml/hour as simple starting points. Replace them with your measured sweat rate once you have long-run data.
| Finish time | Conservative | Moderate | Heavy sweater |
|---|---|---|---|
| 3:00 | 1.2 L | 1.8 L | 2.4 L |
| 4:00 | 1.6 L | 2.4 L | 3.2 L |
| 5:00 | 2.0 L | 3.0 L | 4.0 L |
| 6:00 | 2.4 L | 3.6 L | 4.8 L |
Practice the plan before race day
Marathon hydration is not only about the right number; it is also about what your stomach can tolerate while running. Test the same drink, sodium, and timing during long runs. A plan that looks perfect on paper but causes sloshing at mile 18 is not your plan yet.
Use aid stations as checkpoints
Aid stations help you execute a plan. They should not make every runner drink the same amount. In cool weather you may skip or sip lightly; in heat you may need steady fluid and sodium. Watch for swelling fingers, nausea, confusion, or a headache that worsens as you keep drinking: those are reasons to stop forcing water and seek medical help.
Frequently asked
How much should I drink during a marathon?
Most marathoners start around 400–800 ml per hour, then adjust to sweat rate, temperature, thirst, and stomach tolerance. Smaller runners in cool weather may need less; heavy sweaters in heat may need more plus sodium.
Should I drink at every aid station?
Not automatically. Aid stations are prompts, not commands. Sip according to your plan, thirst, and conditions; forcing fluid at every table can lead to stomach problems or over-drinking.
Do marathon runners need sodium?
Often yes, especially in warm races, races over 3 hours, or for salty sweaters. Sodium helps replace sweat losses and makes fluid easier to retain, but the amount should be practiced in long runs.
Sources
- 1.American College of Sports Medicine, Exercise and Fluid Replacement — Athletes should start exercise euhydrated, limit body-mass losses during training, and replace fluid and sodium after heavy sweat losses.
- 2.Sawka et al., ACSM Position Stand — Sweat rate varies widely by athlete, heat, intensity, clothing, and acclimation, so pre/post body-weight checks are the practical way to individualise fluid plans.
- 3.Mayo Clinic — General guidance of roughly 2.7–3.7 L of total fluids a day, with thirst and pale-yellow urine as everyday checks.