Sweat sodium loss calculator: measured concentration

Sweat rate measures fluid loss per hour under observed conditions. It does not measure the sodium concentration of that sweat. Sodium-loss arithmetic requires both a sweat-rate estimate and a measured concentration. Use repeated observations and product labels to discuss a plan; a body-weight change alone cannot produce your personal sodium requirement.

Calculate sodium loss from your measurements

Use a sweat-rate estimate and a measured sweat sodium concentration from your test report. Leave concentration blank if you do not have it; body weight and salty clothing cannot supply that number.

Enter your measurements to see estimated loss.

Formula: L/hour × mg/L = mg/hour. Session loss = mg/hour × minutes ÷ 60. This is estimated loss, not a replacement target or a diagnosis. Measurement method and conditions affect the estimate. Inputs stay on this page and are not saved.

Sweat volume cannot tell you sodium concentration.

Two runners can lose the same volume of sweat but different amounts of sodium. A fluid-loss estimate is only one part of the calculation.

At 0.8 L/hour and a measured 700 mg/L, how much sodium is lost in 90 minutes?Think it through, then reveal the explanation

0.8 × 700 = 560 mg/hour. Multiply by 1.5 hours: 840 mg over the session. These are example measurements, not a recommended intake or replacement target.

Separate sweat loss from a drinking target

0.8 L/hour × 700 mg/L = 560 mg sodium/hour. Over 90 minutes: 560 × 1.5 = 840 mg lost. This requires a measured concentration and comparable conditions. Estimated sodium loss is not an instruction to replace every milligram during exercise.

Use this in your next training review

Record exercise duration, starting and finishing mass, fluid consumed and urine passed, using consistent units.
Record measured sweat sodium concentration if available. If unknown, leave it unknown rather than guessing from sweat marks.
Note conditions and what you actually tolerated. Discuss an intake strategy using those observations; the loss estimate alone is not a prescription.

Download the worksheet · PDF

Free, no email required. Source links and space for your notes included.

Watch the worked example · 24 seconds

Silent visual lesson. Full text: 0.8 L/hour × 700 mg/L = 560 mg sodium/hour. Over 90 minutes: 560 × 1.5 = 840 mg lost. This requires a measured concentration and comparable conditions. Estimated sodium loss is not an instruction to replace every milligram during exercise.

Download the video

Put it into practice. Use a measured concentration. If you do not have one, leave it blank rather than borrowing another athlete’s number.

Barnes et al. 2019: measured sweat-rate and sodium variability

How do I use the calculator above?

Enter your sweat rate in liters per hour, measured sodium concentration in mg/L and the session duration. It reports estimated hourly and session loss without guessing concentration or turning that loss into a replacement prescription. The worked-example button fills 0.8 L/hour, 700 mg/L and 90 minutes: 560 mg/hour and 840 mg for the session.

What is the arithmetic when I have a measured concentration?

Multiply liters per hour by milligrams of sodium per liter. For example, 0.8 L/hour and a measured 700 mg/L give 560 mg/hour of estimated loss in those conditions. That does not automatically prescribe an equal replacement amount.

What can I measure without a sodium test?

Estimate sweat rate using pre- and post-session body mass, fluid consumed and urine, with consistent measurement conditions. Repeated sessions are more useful than one number.

What should I do with a product label?

Separate sodium from total salt and count what is actually in the serving. Combine drinks and other products rather than overlooking one source.

Where does this guidance come from?

See the source below for the underlying rules or evidence. Practical training advice should be adapted to your experience and circumstances.

Read the source

Use the related calculator

Fueling and hydration context

Breno Melo

Boston endurance coach since 2015. RRCA, USA Triathlon and TrainingPeaks Level 2 certified. Background & experience.

Published 11 October 2026. Editorial source check: 11 October 2026. Personal coaching review has not been represented as completed.

Put the numbers into a training week.

Choose the support that fits how you train. Every option starts with your goal; the difference is who adjusts the plan.

1:1 coaching

For athletes who want Breno to review their training, make decisions with them and adjust the plan personally.

Apply for 1:1 coaching

Apex

For independent athletes who want software to adapt their training as their week changes.

Build my plan in Apex

Training plans

For self-guided athletes with a consistent schedule who want to follow a fixed, coach-written block.

Find a training plan