21.1 km race-day guide
Half-Marathon Hydration Plan & Calculator
Half-marathon hydration is a logistics problem as much as a physiology problem. Your plan needs to fit your expected time, the course aid stations, the weather and what your stomach has tolerated in training.
Key points
- Build the plan from realistic finish time, representative sweat rate and the organiser’s current aid-station map.
- Practise fluid, carbohydrate and sodium together; changing one can change stomach tolerance for the others.
- Use a range and course checkpoints rather than trying to replace every drop of sweat.
- Never follow a schedule through bloating, nausea, unexpected weight gain or worsening neurological symptoms.
Planning at a glance
- Distance
- 21.0975 km / 13.1 miles
- Typical planning horizon
- Often about 1½–3 hours for recreational runners
- Main logistics
- Aid-station spacing, cup volume and carried fuel
- Best personal input
- Long-run sweat rate in comparable conditions
- Key safety boundary
- Do not plan to gain body mass during the event
Start with duration, not distance alone
Expected finish times commonly span well over an hour. Use your own predicted duration, not an elite runner’s plan. A measured sweat rate from a long run in comparable weather can narrow the planning range.
A calculator can organise the arithmetic, but it cannot observe thirst, stomach comfort, acclimatisation, medical history or a last-minute course change. Treat the result as a starting range to rehearse. It should remain easy to reduce when the day is cooler or your stomach feels full.
Before
- Practise the full breakfast, drink and start-time routine on a long-run day.
- Check the organiser’s aid-station map and which drinks are supplied.
- Avoid attempting to bank excess water immediately before the start.
During
- Plan a range rather than an exact mandatory volume. Thirst, comfort and changing weather still matter.
- Map portions to aid stations or bottle capacity so the arithmetic works on the course.
- Electrolyte products may be useful for longer, hot or high-sweat sessions, but sodium does not make overdrinking safe.
After
- Use thirst, food and measured body-mass change to guide gradual recovery.
- Include sodium and carbohydrate through normal food or a familiar recovery option when appropriate.
- Log what you actually drank, where the plan failed and whether you gained or lost weight.
Build one integrated fluid and fuelling plan
Half-marathon runners often plan gels separately from drinks, yet the stomach receives both. A gel taken with too little water may feel highly concentrated; a sports drink plus gels may deliver more carbohydrate than expected.
Write the sequence as one timeline. Note which stations provide water, which provide sports drink and where each fuel portion will be taken. Rehearse the same brands and approximate timing on long runs.
- Record prepared drink concentration, not just powder scoops.
- Check carbohydrate and sodium per serving.
- Keep an alternative if the stomach becomes unsettled.
Use sweat rate without demanding full replacement
A representative sweat test can narrow the starting range, but a result of 1.0 L/hour does not automatically create a 1.0 L/hour drinking target. The planner starts below measured loss and caps the range because full replacement may be impractical or promote overdrinking.
Repeat the test across seasons. Use body-mass change as one signal alongside thirst, urine, pace, temperature and gastrointestinal comfort.
Plan the start and first station
The first station often arrives before much fluid is needed. Decide in advance whether it is a planned stop, an optional sip or one to pass. This reduces crowd-driven decisions.
Avoid carrying an aggressive pre-race intake into the opening miles. Feeling fluid slosh, repeated urination or bloating at the start suggests the pre-race routine needs review.
Adjusting when race day changes
If the forecast becomes hotter, revise pace, clothing, cooling and warm-up as well as drink opportunities. If it becomes cold and wet, thirst and sweat loss may be lower than the rehearsed warm-weather plan.
A delayed start also changes the timeline. Keep enough familiar fluid and food for the delay, but do not keep drinking continuously merely because you are waiting.
Turn station spacing into an elapsed-time schedule
Write the distance of each station beside the time you expect to reach it. At 5:00 per kilometre, a 4 km gap lasts about 20 minutes; at 7:30 per kilometre it lasts about 30 minutes. This conversion shows whether taking the same cup at every station creates a sensible hourly total for your pace.
Mark stations as planned, optional or pass. A planned station supports a fuel or refill decision, an optional station responds to conditions, and a pass prevents crowd behaviour from silently increasing intake. Recheck the schedule if your realistic finish time changes.
- Use the current organiser map and drink list.
- Estimate the amount actually swallowed after spillage.
- Identify where course congestion may make drinking difficult.
Rehearse race pace, not only easy long-run pace
Stomach tolerance can change when breathing and intensity rise. A plan that feels effortless on a slow long run may be harder to execute near half-marathon pace, particularly when fluid, gels and caffeine arrive close together.
Use at least one appropriate rehearsal to test the exact container, product concentration and timing at a controlled section of expected race effort. The purpose is not to run the whole race in training; it is to reveal handling, swallowing and gastrointestinal problems while changes are still possible.
Plan the finish and recovery window
The finish area may involve queues, a walk to baggage and only unfamiliar drinks. Know what is supplied and keep a familiar recovery option available when practical. Begin gradually and combine fluid with normal food rather than rapidly chasing the largest estimated deficit.
Record actual intake, leftover volume, weather, finish time, stomach comfort and any carefully measured body-mass change. If recovery time before the next demanding session is long, ordinary meals and drinking over several hours may be more appropriate than an aggressive replacement formula.
How the plan changes by situation
Use this table to decide which part of the calculation deserves attention. These are planning directions, not diagnoses or mandatory drinking rates.
| Situation | Practical adjustment | Reason |
|---|---|---|
| Cool race near 90 minutes | Use the lower part of a rehearsed range; some stations may be optional | Shorter exposure and lower heat load can reduce practical need |
| Two-plus hours in mild weather | Map several modest portions to stations | Time makes regular opportunities more useful |
| Warm or humid race | Recheck pace, cooling, fluid range and sodium/fuel strategy | Heat management must be integrated |
| Aid stations far apart | Carry enough to bridge verified gaps | A good hourly estimate fails if access is impossible |
| Nausea or bloating develops | Reduce or pause intake and seek help for concerning symptoms | Following the schedule blindly can worsen overdrinking |
Calculation in context
Worked example: a two-hour half marathon
A runner measured 0.8 L/hour on a similar long run. The planner gives a starting fluid range of 475–725 ml/hour after rounding to 25 ml. Six stations are spaced about 3 km apart, and the runner carries one 500 ml soft flask.
- Across two hours, the displayed total is 0.95–1.45 L—not the full 1.6 L estimated sweat loss.
- The runner starts with the flask, taking comfortable portions rather than emptying it on a timer.
- They plan refilling only if the flask is genuinely low at a suitable station.
- Gels are aligned with water opportunities and tested during long runs.
- The plan is reduced if the race is colder or the runner develops fullness.
How to interpret it: The range becomes a practical one-to-three-bottle problem, supported by stations. It remains a rehearsal target, not a promise that either end is correct on race day.
Step-by-step
How to use the HydrateWise planner for this event
The quality of the result depends more on honest inputs than extra precision. Work through these steps, then revise the plan after a rehearsal.
- 1
Select half marathon and enter your own expected duration.
- 2
Use a saved sweat rate only if it came from comparable intensity and weather.
- 3
Enter the bottle or soft-flask capacity you will genuinely carry.
- 4
Approximate station spacing, then compare generated portions with likely cup volumes.
- 5
Copy the link and rehearse the exact sequence on at least one long run.
Common planning mistakes
Using an elite runner’s plan
A much longer finish time changes total exposure and the number of opportunities.
Replacing 100% of measured loss
Observed sweat loss is not automatically a drinking prescription.
Separating gels from fluid
Combined concentration and total intake determine gastrointestinal tolerance.
Trusting last year’s course map
Station locations and products can change; use current organiser information.
Confusing salt marks with a sodium test
Visible salt gives context but not a measured milligram-per-litre result.
Important limits
- Never keep drinking through nausea, bloating or unexpected weight gain.
- A sweat sodium value from a lab is different from guessing based on salt marks.
- Use the organiser’s current medical and weather advice on race day.
Frequently asked questions
How much water should I carry for a half marathon?
Compare your calculated range with course access. Some runners rely on stations; others carry 250–1000 ml for control. Test the exact arrangement.
Do I need electrolytes?
They become more relevant as duration, heat and sweat loss rise. Need and dose cannot be inferred from the race distance alone.
How often should I drink?
Use manageable opportunities that fit stations and comfort. A strict minute-by-minute instruction is not necessary for most runners.
Can I use my sweat rate from a treadmill?
It is useful context for similar indoor conditions but may not match an outdoor race with airflow, sun and different intensity.
Should I drink at every station?
Not necessarily. Label each station planned, optional or pass based on your rehearsal and conditions.
What should I record after the race?
Finish time, conditions, actual fluid and fuel, leftover volume, stomach comfort, symptoms and any carefully measured body-mass change.
Keep improving the plan
Test it in training, measure what you actually drank and note the conditions. A plan is most useful when it changes with evidence rather than remaining fixed all year.