How Much to Drink on a Long Workout: A Formula Without Guesswork

When it comes to drinking while running, you usually hear two types of advice:

Type 1: "Drink every 15 minutes, 150-200 ml, whether you want to or not"

Type 2: "Drink to thirst, your body knows"

Funny thing is, both pieces of advice are correct. And both are incomplete. Let's figure it out.

Your Personal Sweat Rate: The Key Number

Sweat rate is how fast you lose water through sweat. Measured in liters per hour. And it's the most individual thing in hydration.

Two runners of the same weight, same speed, in the same weather can lose:

First one: 0.8 L/hour
Second one: 1.8 L/hour

More than double the difference. Universal advice "drink X ml every Y minutes" will be too much for one, too little for the other.

How to Find Your Sweat Rate

The test takes an hour and only requires a scale:

Use the bathroom
Weigh yourself naked → Weight BEFORE
Run 60 minutes (pace and conditions like your target race/workout)
Record how much you drank → Drank
Dry off, weigh yourself naked → Weight AFTER

Formula:

Sweat Rate = (Weight BEFORE - Weight AFTER) + Drank

Example:

Weight before: 72.0 kg
Weight after: 71.3 kg
Drank: 400 ml (0.4 kg)
Sweat Rate = (72.0 - 71.3) + 0.4 = 1.1 L/hour

Now you know your number. Write it down. This is your baseline sweat rate for these conditions.

Factors That Change Sweat Rate

Your 1.1 L/hour is not a constant. It fluctuates based on:

Air Temperature

Rough adjustment:

Cold (<10°C): sweat rate × 0.7
Comfortable (10-20°C): sweat rate × 1.0
Warm (20-30°C): sweat rate × 1.3
Hot (>30°C): sweat rate × 1.5-2.0

If your baseline sweat rate is 1.1 L/hour at 15°C, expect about 1.4-1.5 L/hour at 28°C.

Humidity

Humidity <60%: no adjustment
Humidity 60-80%: +20%
Humidity >80%: +30%

Humidity is tricky. You sweat the same amount (or even more), but sweat doesn't evaporate — it just drips. No cooling, body sweats even more. Vicious cycle.

Intensity

Easy run (zones 1-2): baseline sweat rate
Tempo run (zone 3): +20-30%
Intervals, race (zones 4-5): +50-70%

Body Weight

About +0.1 L/hour for every 10 kg above 70 kg. If you weigh 90 kg, add 0.2 L/hour to average values.

How Much of This to Replace

Here's where practice begins. You know your sweat rate. What's next?

Golden rule: replace 70-80% of losses, not 100%

Why not 100%? Because:

Stomach can't absorb it all (max ~1-1.2 L/hour)
Some water is formed from fat and carb oxidation
Mild dehydration (up to 2-3%) isn't critical for performance
Excessive drinking is more dangerous than insufficient (hyponatremia)

Practical Numbers by Duration

Workout under 60 minutes:

In cool weather: you can skip drinking entirely
In heat: 200-400 ml for the workout
Just drink before and after

Workout 60-90 minutes:

300-500 ml for the workout
Can take it in one go in the middle

Workout 90-180 minutes:

400-800 ml/hour
Drink every 15-20 minutes, 100-200 ml
Or by thirst, but have water handy

Workout 3+ hours (long runs, ultra):

500-800 ml/hour
Must include electrolytes (sodium!)
Plan fueling/drinking stations in advance

Upper Limit: Never More Than 1000-1200 ml/hour

This is the physiological ceiling. Stomach simply can't process more. Excess water will slosh around inside, cause nausea, and provide no benefit.

Formula for the Lazy

Don't want to do a sweat test? Here's a simplified formula for estimation:

Drinking (ml/hour) = Weight (kg) × Coefficient

Coefficients:
- Cold + easy run: 5-7
- Comfortable + medium pace: 8-10
- Warm + intense: 11-14
- Hot + race: 14-18

Example: 70 kg runner, warm, tempo workout

70 × 12 = 840 ml/hour

It's a rough estimate, but better than nothing.

When "Drink to Thirst" Works

The "drink to thirst" strategy works great when:

Workout under 2 hours
Moderate intensity
Water available (bottle, fountains)
You're not racing for time

The "drink to thirst" strategy works poorly when:

Racing for a result (adrenaline suppresses thirst)
Ultra distances (accumulated deficit)
Extreme heat
Water unavailable (nowhere to refill)

In these cases, you need a plan: how much, when, where to get it.

Long Workout Preparation Checklist

Day before:

Check weather forecast (temperature, humidity)
Estimate sweat rate for these conditions
Decide: bottle, belt, vest, water stations?

Morning:

Drink 300-500 ml 1-2 hours before start
Check urine color (light yellow = OK)
Don't drink right before start — it'll slosh

During workout:

First water intake — after 20-30 minutes
Then — every 15-20 minutes, 100-200 ml
Or by thirst, if running without a timer

After:

Weigh yourself (if you want to refine your sweat rate)
Replace losses over 2-3 hours
Don't chug liters at once — spread it out

The Main Point

The formula is simple:

Find your sweat rate (one-hour test)
Adjust for conditions (hotter/colder, more/less intense)
Replace 70-80% of losses
Don't drink more than 1 L/hour

Everything else is details. But these four points are what separate a conscious approach from guesswork.