Race Week Hydration: A Day-by-Day Plan to Start Your Marathon Optimally Hydrated

You've trained for months. Your long runs went well. Your gel plan is dialled in. But if you show up to the start line under-hydrated, none of that preparation matters.

Most marathon guides cover what to drink during the race. Almost none cover the 7 days before it. That's a mistake, because your hydration status on race morning is the result of everything you did — or didn't do — in the week leading up to it.

Here's a day-by-day hydration protocol for the final 7 days before your marathon or half marathon. It's designed for the self-coached endurance athlete who wants a structured plan, not another vague instruction to "drink plenty of water."

Why Race Week Hydration Matters More Than You Think

Your body can't store water the way it stores glycogen. You can't "load" hydration on Friday and expect it to hold until Sunday morning. But you can do something arguably more important: optimise your baseline hydration status so your body starts the race with a full tank rather than playing catch-up from kilometre one.

Two things happen when you're even mildly dehydrated at the start line:

1. Your heart rate runs higher. Reduced blood volume means your heart has to work harder to deliver oxygen to your muscles. Studies consistently show a 3-5% increase in heart rate for every 1% of body mass lost through dehydration.

2. Your perceived effort climbs. The same pace feels harder when you're dehydrated. If your goal pace is 5:30/km, mild dehydration can make it feel like 5:15/km effort — which means you either slow down or burn through your reserves faster.

The goal of race-week hydration isn't to drink as much as possible. It's to arrive at the start line with optimal plasma volume, balanced electrolytes, and a gut that's adapted to absorbing fluid under stress.

The 7-Day Protocol

Day 7 (Sunday before race) — Establish Your Baseline

This is your final long run — or more likely, a short shakeout at the end of your taper. Use it as your last sweat rate check.

Weigh yourself before and after the run. Subtract any fluid you drank during the session. Divide the difference by the number of hours you ran. That's your sweat rate in litres per hour.

If you've been testing sweat rate across your training block, compare today's number to your average. If you've been training through summer, your sweat rate may have increased by 20-40% since spring. Your race-day hydration plan needs to reflect your current rate, not the one you measured in April.

For the rest of the day, aim for your normal daily fluid intake — roughly 35-40ml per kilogram of body weight. For a 70kg runner, that's about 2.5-2.8 litres across the day, including what you get from food.

Day 6 (Monday) — Normal Hydration, Track What You Actually Drink

Most athletes have no idea how much fluid they actually consume on a normal day. Today, track it. Every glass of water, every coffee, every sports drink.

You're not trying to hit a magic number. You're establishing awareness. If your normal intake is well below 30ml/kg, you've found a problem worth fixing before race day — not on race morning.

Keep caffeine intake at your normal level. Race week is not the time to cut caffeine (you'll feel terrible) or increase it (you'll over-stimulate and potentially dehydrate).

Day 5 (Tuesday) — Maintain and Monitor

Continue your baseline fluid intake. If you're running a short taper session, drink 400-600ml of an electrolyte drink in the 2 hours before the run and sip water as needed afterward.

Pay attention to your urine colour as a rough guide — pale straw is the target. But don't obsess over it. Urine colour is a lagging indicator and can be misleading after taking vitamins (which turn it bright yellow) or after a heavy training session.

The more reliable signal: are you urinating every 2-3 hours during the day? If yes, your baseline hydration is in good shape.

Day 4 (Wednesday) — Begin Electrolyte Awareness

Today, start paying deliberate attention to your sodium intake. You're not loading yet — that comes in 48 hours — but you're making sure you're not running a sodium deficit heading into the loading window.

Include sodium-rich foods at meals: salted nuts, olives, broth-based soups, salted rice, or pretzels. Athletes who eat a very clean diet (low processed food, minimal salt) often arrive at race week with lower-than-ideal sodium levels without realising it.

A reasonable sodium target for today is 2,000-3,000mg across the day from food and drinks combined. That's not loading — it's just making sure you're not starting from a deficit.

Day 3 (Thursday) — Final Hard Check

Two days before the race, do an honest assessment:

• Are you urinating pale and regularly?
• Have you been hitting your daily fluid target consistently this week?
• Have your bowel movements been normal? (Overhydration can cause loose stools — a sign to dial back slightly.)

If everything checks out, you're in good shape. If you realise you've been under-hydrating, don't panic-drink. A sudden spike in fluid intake can cause bloating, disturbed sleep, and frequent bathroom trips — none of which you want 48 hours before a race.

Instead, modestly increase your intake by 500ml across the day, spread evenly. Your body absorbs fluid better in small, consistent amounts than in large boluses.

Day 2 (Friday) — Sodium Loading Begins

This is the most impactful day in the protocol.

Sodium loading — also called hyperhydration or pre-loading — increases your blood plasma volume, giving your body a larger fluid reserve to draw from during the race. Research shows it can delay the onset of dehydration by 20-30 minutes in hot conditions.

Here's the protocol:

Target: 1,500-2,000mg of additional sodium above your normal intake, spread across the day.

You can get this from:

• Electrolyte tablets or powder (check the sodium content — many "electrolyte" products are embarrassingly low in sodium)
• Salted broth or bouillon (one cup of chicken broth has ~800mg sodium)
• Salted foods added to meals
• A dedicated sodium-loading product like a pre-race electrolyte sachet

Increase your water intake by 500-750ml above your baseline to match the extra sodium. The sodium pulls fluid into your bloodstream — but only if the fluid is there to pull.

Avoid alcohol completely today and tomorrow. Even a single drink impairs your body's ability to retain fluid.

Day 1 (Saturday — Race Eve) — Sodium Loading Continues, Fluid Caps

Continue the sodium loading protocol from yesterday. Same target: 1,500-2,000mg additional sodium, spread across the day.

But now add a fluid ceiling: stop drinking large volumes by 7pm (or 3 hours before you plan to sleep). The last thing you need is a night of broken sleep from bathroom trips.

Sip as needed through the evening, but your main hydration work is done.

Lay out your race-day hydration gear tonight: your bottles, your electrolyte mix, your gels. Know exactly what you'll drink, when, and how much. If your race provides on-course hydration, know what brand of sports drink they're using — and whether you've trained with it.

Race Morning — The Final Window

You have a 2-3 hour window between waking up and the gun. Use it.

2-3 hours before start: Drink 500-600ml (16-20oz) of water with electrolytes alongside your pre-race breakfast. This gives your body time to absorb the fluid and clear any excess through urination before you start.

30-45 minutes before start: Drink 200-300ml (7-10oz) of an electrolyte drink. This tops up your fluid levels without risking a full bladder at the start.

In the corral: Small sips only. You're topping up, not loading.

If you followed this protocol, you'll start the race with optimised plasma volume, balanced electrolytes, and a gut that's been steadily absorbing fluid all week — not one that's been slammed with water in the last 12 hours.

What This Protocol Doesn't Cover

This is a hydration protocol. It's not your fueling plan, your carb-loading schedule, or your during-race drinking strategy. Those are separate plans that should be built alongside this one, not instead of it.

The best race-week protocols integrate hydration, sodium, carbohydrate loading, and race-day fueling into a single, personalised plan. That's hard to do manually because the variables — your sweat rate, your body weight, the forecast conditions, your goal pace — all interact.

Sweatr builds this integrated plan automatically. Connect your Apple Watch or Garmin, input your race details, and it generates a day-by-day race-week protocol that covers hydration, sodium loading, and carb loading — calibrated to your personal data, not population averages. Try it free on the App Store.

Common Race-Week Hydration Mistakes

Mistake 1: Chugging water on race morning. If you've followed a 7-day protocol, race morning is a top-up, not a rescue mission. Drinking a litre of water in the hour before the start will slosh in your stomach, dilute your blood sodium, and send you to the portaloo queue.

Mistake 2: Ignoring sodium. Water alone doesn't hydrate you optimally. Sodium is the key electrolyte that drives fluid absorption and retention. Without it, the water you drink passes straight through.

Mistake 3: Using urine colour as your only metric. It's a useful signal but it's delayed, easily fooled by vitamins and food colouring, and tells you nothing about electrolyte balance.

Mistake 4: Cutting caffeine during race week. If you normally drink coffee, keep drinking it. Caffeine withdrawal causes headaches, fatigue, and irritability — symptoms you'll wrongly blame on taper madness.

Mistake 5: Thinking "hydration" means "drink more water." Hydration is a system: fluid volume, sodium balance, timing, and gut readiness all matter. A good race-week protocol manages all four.

The Takeaway

Race-week hydration isn't complicated, but it does require intention. Seven days of steady, sodium-aware fluid intake will do more for your race-day performance than any last-minute water binge.

Start tracking your intake on the Sunday before your race. Begin sodium loading on Friday. Execute your race-morning protocol. And let your months of training do their job on a body that's actually ready for the effort.

If you want a plan that adjusts all of this to your personal sweat rate, body weight, and race conditions, Sweatr does exactly that — powered by the data your watch is already collecting. Download Sweatr free and build your race-week plan.