Stellar Lift Blog

Hydration

Sweat Rate Calculator: Your Workout Hydration Plan (Water + Sodium per Hour)

Most hydration advice is useless because it’s generic.

You don’t need “drink more water.”
You need one number:

your sweat rate (liters per hour).

Once you know it, you can build a plan that actually works for:

  • normal gym sessions
  • hot gyms / conditioning
  • and yes — a Muay Thai version that fits round breaks without making you feel sick.

This is gym-first (more searches), but the method covers both.

The 60-second idea

Your body weight drops during training mostly because you lose fluid (sweat + breathing).

So if you measure:

  • weight before
  • weight after
  • and how much you drank

…you can estimate how many liters per hour you’re losing.

That’s your “hydration number.”

Sweat Rate Calculator (step-by-step)

You need:

  • a scale
  • your bottle (so you know how much you drank)
  • a note on time

Step 1) Weigh in (before training)

  • Weigh yourself right before you start
  • Best: minimal clothing, dry skin
  • Write it down (kg)

Step 2) Track what you drink

  • Note total fluids during the session
    Example: 750 ml = 0.75 L

Step 3) Weigh out (after training)

  • Towel off sweat first (don’t weigh a wet shirt)
  • Weigh again (kg)

Step 4) Calculate sweat loss

Use:

Sweat loss (L) = (weight before − weight after) + fluids drank − urine

Notes:

  • 1 kg ≈ 1 liter
  • If you didn’t pee during the session, urine = 0

Step 5) Convert to liters per hour

Sweat rate (L/h) = sweat loss (L) ÷ duration (hours)

Example (gym session)

  • Weight before: 87.0 kg
  • Weight after: 86.2 kg
  • Fluids: 0.60 L
  • Duration: 75 min = 1.25 h
  • Urine: 0

Sweat loss = (87.0 − 86.2) + 0.60 = 0.8 + 0.60 = 1.40 L
Sweat rate = 1.40 ÷ 1.25 = 1.12 L/h

So in these conditions you’re losing about 1.1 liters per hour.

That’s a high-sweat session — and now you can plan it.

Turn sweat rate into a drinking plan (simple + safe)

During training, the goal is usually not to replace 100% instantly.

A practical starting point:

  • Target intake = ~60–80% of your sweat rate
  • Keep it within what your stomach tolerates
  • Don’t consistently drink more than you lose

Many guidelines land typical “during exercise” intake around ~0.4–0.8 L/h for a lot of people (but your sweat rate can be outside that range). Your test tells you where you are.

Using the example (1.12 L/h)

  • 60–80% = 0.67–0.90 L/h
  • Choose something you can execute: 0.75 L/h

Now it’s actionable.

Gym plan (strength / hypertrophy)

In the gym you want hydration that:

  • doesn’t bloat you
  • doesn’t interrupt sets
  • is easy to follow

Easy rule

150–250 ml every 15–20 minutes

That gives you:

  • ~0.45–1.0 L/h depending on your needs

Quick templates

45–60 min workout

  • If you sweat moderately: 300–600 ml total
  • If you sweat a lot / hot gym: 500–800 ml total

75–90 min workout

  • Typical target: 0.5–0.8 L/h
  • If your sweat rate is high: push toward the upper end only if your stomach tolerates it

Muay Thai version (between rounds, without feeling sick)

Muay Thai tends to be:

  • more continuous movement
  • more heat
  • more sweat
  • and drinking at the wrong time can cause nausea

So the math is the same, but the execution changes:

Between-round rule

Instead of constant sipping:

  • take 100–200 ml in short repeats between rounds
  • avoid chugging huge amounts at once

If your measured sweat rate is high (1.2+ L/h in hot rooms happens), you still don’t want to slam a full bottle mid-session.

Small repeats beat big chugs.

Sodium: when it matters (and how to calculate it)

If you train longer than ~1 hour and sweat heavily, sodium becomes more relevant.

A practical sodium target often used in guidance for longer exercise is:

~500–700 mg sodium per liter (0.5–0.7 g/L)

Sodium math (the part most people skip)

If you drink 0.75 L/h and aim for 500–700 mg/L:

  • 0.75 × 500 = 375 mg sodium/hour
  • 0.75 × 700 = 525 mg sodium/hour

So a clean range is roughly: ~400–500 mg sodium per hour (for long, sweaty sessions)

What this means in real life

  • Sports drinks vary a lot
  • Electrolyte tablets vary a lot

Read labels and use sodium when it actually fits the session:

  • long session (60–90+ minutes)
  • heavy sweat / hot room
  • salty sweat marks on clothes/skin
  • headaches/cramps when you only drink plain water

The biggest mistake: drinking more than you lose

More water isn’t always better.

A simple safety rule: Don’t consistently drink more than your sweat loss.

If your test shows you lose ~1.0 L/h and you’re drinking 1.5 L/h, that’s a red flag.

Signs you’re overdoing it

  • sloshing stomach / nausea
  • swollen fingers
  • headache + confusion after heavy drinking
  • frequent clear urination mid-session

Quick 3-line setup (copy/paste logic)

1) Measure your sweat rate (L/h)
2) Drink ~60–80% of that (start simple)
3) If >1 hour + heavy sweat, add sodium ~500–700 mg/L

That’s your plan.

If you want the “full picture” around hydration timing and electrolytes:

Sources (high-quality)

Track hydration with Water Tracker Back to Hydration