Sweat loss in litres equals the weight you lost during the session plus every litre you drank while training. This is the gravimetric method used in sports science labs, validated by Shirreffs and colleagues in 2004.

920 mg per litre is the population mean sweat sodium concentration in the ACSM position stand. Individuals range from 400 to 2000. A lab patch test is the only way to get your own number, and salty-sweaters sit well above the average.

Chasing 100% replacement while you are still moving is a reliable way to upset your gut. The research supports 60 to 70% during activity, with the balance made up afterwards when your stomach can handle it.

Heat and humidity push sweat rate up by 15 to 50%. The multipliers here run from 1.00 in cool conditions to 1.50 in hot and humid, drawn from ACSM guidance and Kenefick and Cheuvront's 2012 work.

Children sweat less overall and their sweat carries less sodium, roughly 600 mg per litre against 920 for adults. They also thermoregulate less efficiently, so the honest answer is usually shade and a shorter session rather than another sachet. The kids profile reflects that and caps what it will ever recommend.

No. Sweat rate moves with heat, fitness, intensity and acclimatisation. Run this three or four times across different conditions and use the spread, not a single result.