Every Sodium Guide Gives You a Range. Here's How to Find YOUR Number.

The Problem With Every Sodium Guide You've Ever Read

Search "how much sodium do I need when running" and you'll get the same answer everywhere: somewhere between 300 and 1,500 milligrams per hour.

That's not advice. That's a five-fold range masquerading as a recommendation. It's like a running coach telling you to run your marathon "somewhere between 3 hours and 15 hours." Technically true, practically useless.

The range exists because sodium needs are genuinely individual. Two runners standing on the same start line, at the same weight, running the same pace, can have sodium requirements that differ by 400%. One loses 200 mg of sodium per litre of sweat. The other loses 2,000 mg. Neither can tell by looking in the mirror.

So how do you find your number?

Why Sodium Matters More Than You Think

Before we get into the how, let's settle the why.

Sodium is the primary electrolyte you lose in sweat. Potassium, magnesium, and calcium show up in the conversation, but sodium accounts for roughly 90% of the electrolyte content of your sweat. When it drops too low relative to your fluid intake, three things happen:

1. Your muscles lose their ability to contract properly. This is the cramp you feel at mile 18 — the one that arrives even though you've been drinking at every aid station.
2. Your gut slows down. Sodium drives fluid absorption in the small intestine. Without enough sodium in your drink, the water you're consuming sits in your stomach rather than reaching your bloodstream. This is why some runners feel bloated and sloshy despite drinking steadily.
3. In extreme cases, you develop hyponatremia. Drinking large volumes of plain water without replacing sodium dilutes your blood sodium concentration. It's rare in training but happens every major marathon.

The reason most cramping and GI advice feels contradictory is that it ignores sodium entirely, or treats it as a fixed number. It's neither.

The Two Variables That Determine Your Number

Your personal sodium need during exercise comes down to exactly two things:

Variable 1: Your sweat rate (how much you sweat per hour)

This is the total volume of fluid you lose through sweat. It's affected by your body size, fitness level, exercise intensity, air temperature, humidity, and how heat-acclimatised you are. A lean 60 kg runner in 15°C weather might lose 0.5 litres per hour. A larger runner in 30°C heat might lose 2.5 litres.

Variable 2: Your sweat sodium concentration (how salty your sweat is)

This is the amount of sodium dissolved in each litre of your sweat. It ranges from about 200 mg/L to 2,000 mg/L across the population. Unlike sweat rate, your sodium concentration is largely genetic — it doesn't change much with fitness or acclimatisation. You're born a salty sweater or you're not.

Your hourly sodium loss = sweat rate × sweat sodium concentration.

A runner who sweats 1 litre per hour with a concentration of 500 mg/L loses 500 mg of sodium per hour. A runner who sweats 1.5 litres per hour with a concentration of 1,200 mg/L loses 1,800 mg. Same sport, same distance, wildly different needs.

This is why the guides give you a range. They can't give you a number without knowing both variables.

Step 1: Find Your Sweat Rate (The Weigh-In Test)

You can estimate your sweat rate with nothing more than a bathroom scale and a one-hour run.

What you'll need:

• A digital scale accurate to 0.1 kg
• A one-hour run at your normal training pace
• Conditions similar to what you'll race in (temperature, humidity)

The protocol:

1. Empty your bladder, strip down, and weigh yourself. Record the number.
2. Run for 60 minutes at your typical long-run or race pace. Do not drink anything during the run. Do not use the toilet.
3. Come back, strip down again, towel off any visible sweat, and weigh yourself.
4. Subtract your post-run weight from your pre-run weight. Every kilogram lost equals approximately 1 litre of sweat.

If you lost 0.8 kg, your sweat rate in those conditions is about 0.8 L/hr. If you lost 1.4 kg, it's 1.4 L/hr.

Repeat this test in different conditions. Your sweat rate in 12°C drizzle and your sweat rate in 28°C sunshine are not the same number. Most runners find their hot-weather sweat rate is 30–50% higher than their cool-weather rate. This is exactly why your spring hydration plan fails in summer.

Step 2: Estimate Your Sweat Sodium Concentration

This is harder to measure precisely without lab equipment or a wearable biosensor patch. But you can get a practical estimate using two approaches.

The observation method:

After a hard, sweaty session, check your kit and skin:

• Heavy white residue on your clothing, hat, or skin? You're likely a salty sweater — 1,000 mg/L or higher.
• Some visible salt lines on dark clothing? You're probably in the moderate range — 500–1,000 mg/L.
• Sweat feels "watery" with no visible residue? You're likely on the lower end — 200–500 mg/L.

This isn't precise, but it puts you in a ballpark. And a ballpark is infinitely better than guessing somewhere between 300 and 1,500.

The taste method:

Lick your forearm after a hot training session. If it tastes distinctly salty — not just damp, but salt-on-chips salty — you're likely above 800 mg/L. If it's barely noticeable, you're likely below 500 mg/L.

The professional route:

Companies like Precision Hydration offer in-person sweat composition tests (~$100) that measure your exact sodium concentration. Nix Biosensors sell a wearable sweat patch (~$200 for the starter kit) that reads concentration in real time. Both are accurate. Neither is necessary if you're willing to test and iterate.

Step 3: Do the Maths

Once you have both numbers, multiply them.

Example 1 — The Light Sweater:

• Sweat rate: 0.7 L/hr (cool conditions, moderate pace)
• Sodium concentration: 400 mg/L (low salt residue)
• Hourly sodium loss: 0.7 × 400 = 280 mg/hr

This runner barely needs more than a standard electrolyte tab in their bottle. Over-supplementing would waste money and potentially cause GI issues.

Example 2 — The Moderate Sweater:

• Sweat rate: 1.2 L/hr (warm conditions, race pace)
• Sodium concentration: 800 mg/L (visible salt on kit)
• Hourly sodium loss: 1.2 × 800 = 960 mg/hr

This runner needs a serious electrolyte strategy — a standard sports drink (200–300 mg sodium per 500 ml) won't come close. They'll need a higher-sodium drink mix or supplemental salt tabs.

Example 3 — The Heavy Salty Sweater:

• Sweat rate: 2.0 L/hr (hot day, high intensity)
• Sodium concentration: 1,400 mg/L (heavy salt crust)
• Hourly sodium loss: 2.0 × 1,400 = 2,800 mg/hr

This runner needs an aggressive, deliberate sodium strategy and should be testing specific products and doses in training well before race day. Most off-the-shelf electrolyte products don't come close to this level.

Why Your Number Changes With the Season

Your sweat sodium concentration stays relatively stable — it's mostly genetic. But your sweat rate does not.

When summer arrives and temperatures jump from 15°C to 30°C, your sweat rate can increase by 30–50% almost immediately. If your sodium plan was built on spring data, it's now 30–50% too low. This is the single most common reason runners who "did everything right" in spring start cramping in June.

Heat acclimatisation helps. After 10–14 days of consistent heat exposure, your body learns to start sweating earlier, sweat more efficiently, and conserve some sodium. But you still sweat more in absolute terms. Your number in July will not be your number in March.

The fix: re-test your sweat rate when conditions change. It takes one run and a bathroom scale.

Putting Your Number Into Practice

Once you know your approximate hourly sodium loss, you can build a plan:

1. Check the sodium content of your hydration products. A Nuun tab has about 300 mg. A Precision Hydration 1500 sachet has 750 mg per 500 ml. A SiS GO Electrolyte has roughly 250 mg per 500 ml. Most products label this clearly.

2. Do the gap analysis. If you're losing 900 mg/hr and your drink provides 300 mg per bottle, and you're drinking one bottle per hour, you're running a 600 mg/hr deficit. That deficit compounds over two, three, four hours.

3. Close the gap with drink mix, salt tabs, or gel choice. Some gels contain 50–100 mg sodium per sachet. Salt tabs like SaltStick provide 215 mg per capsule. You can mix and match to hit your number.

4. Test in training before racing. Your gut has an opinion about sodium concentration. Some runners tolerate high-sodium drinks; others find them nauseating. You'll only know by practising on long runs.

Why Most Athletes Never Find Their Number

The process above works, but it requires multiple tests across different conditions, manual calculations, and product cross-referencing. Most athletes start with good intentions and end up defaulting to "I'll just take a salt tab every hour" — which might be too much, too little, or perfectly fine, depending on data they've never collected.

This is the problem Sweatr was built to solve.

Sweatr pulls your workout data from your Apple Watch, Garmin, or Strava — heart rate, duration, intensity, pace, and environmental conditions. It uses these inputs to estimate your sweat rate for each session and build a sodium and hydration profile that adapts as conditions change. When summer hits and your sweat rate jumps, Sweatr recalculates automatically. When you travel to a hotter race venue, it adjusts.

The output isn't a range. It's a number. Your number. With specific product recommendations and a timing plan you can follow mid-race without doing mental arithmetic at mile 15.

A Note on Cramping

If you came to this article because you keep cramping during races, sodium is worth investigating — but it's not the only factor. Current research suggests that exercise-associated muscle cramps have both an electrolyte component and a neuromuscular fatigue component. Replacing sodium won't prevent cramps caused by going out too fast, under-training, or running beyond your fitness level.

But if you're well-trained, well-paced, and still cramping after two hours, a sodium deficit is the first place to look. Finding your number and testing it in training is the fastest way to rule it in or out.

The Bottom Line

Generic sodium guides give you a range because they don't know your sweat rate or sodium concentration. Now you know how to find both. Test your sweat rate with a bathroom scale. Estimate your sodium concentration from your salt residue. Multiply. That's your starting point.

Then test it, adjust for conditions, and refine over several long runs. Or let Sweatr calculate it from the data your watch is already collecting — and spend your mental energy on the run, not the maths.