Sodium & Electrolyte Calculator
Fluid and sodium per hour for a single session. Thirst is the main cue — conservative planning ranges, not a prescription. Not medical advice.
Over-drinking during long sessions can cause exercise-associated hyponatraemia (low blood sodium). Drink to thirst, do not aim to gain weight during a session, and do not drink just because a calculator told you to.
Optional: sweat-test mode (weigh in before and after)
Methodology
Simple mode: fluid 0.3–1.2 L/h by environment and sweat level; sodium = fluid × 500–700 mg/L (ACSM 2007), rounded to the nearest 10 mg/h. Sessions under 60 minutes in cool/normal conditions return "water and normal meals are usually enough". Bodyweight, age, height and sex are kept with the shared profile for consistency across calculators; this session estimate is driven by duration, environment and sweat level. Sweat-test mode: sweat loss = weight loss + fluid drunk − urine; replace 50–80% in-session. Finishing heavier than you started suppresses the figure and shows a hyponatraemia warning. The 1996 ACSM 0.5–0.7 g/L figure is historical; ACSM 2007 is the current reference. 1 g salt = 393 mg sodium; 1 g sodium = 2.54 g salt.
How much sodium do you lose in training?
Sweat carries roughly 500–700 mg of sodium per litre for most people, and sweat rates in normal conditions run around 0.4–0.7 L per hour — which is why this calculator's per-hour sodium figures are built from fluid rate × concentration (the ACSM fluid-replacement approach) rather than a single magic number. Short, easy sessions in cool weather genuinely don't need added sodium: water and your next meal cover it.
When electrolytes start to matter
Long sessions, hot or humid conditions, heavy sweating, and back-to-back training days are when replacement makes a difference — that's when the tool shows a sodium range and its electrolyte-tablet equivalent. Test any plan in training first, never on race day, and drink to thirst rather than to a schedule.
The over-drinking warning is the important one
Finishing a session heavier than you started means you drank more than you sweated — the setup for exercise-associated hyponatraemia (dangerously low blood sodium). No sports drink fixes over-drinking; the sweat-test mode here exists precisely so your fluid plan is based on what your body actually loses.
Sources
- Sawka MN et al. ACSM Position Stand: Exercise and Fluid Replacement. MSSE 2007. PubMed 17277604. link
- Hew-Butler T et al. 3rd International Exercise-Associated Hyponatraemia Consensus. BJSM 2015. PubMed 26227507. link
- NHS — Salt in your diet. Adults: no more than 6 g salt (2.4 g sodium) per day. link FDA — Sodium in your diet. Adults should limit sodium to less than 2,300 mg per day. link
- Food Standards Scotland — Salt. 6 g salt = 2.4 g sodium per day. link
Frequently asked questions
How much should I drink during exercise? How much should I drink during exercise?
Let thirst lead. As a planning range, many people need somewhere around 0.3–1.2 litres of fluid per hour depending on heat and sweat rate. Aim to finish a session about the same weight you started — not heavier. Let thirst lead. As a planning range, many people need somewhere around 0.3–1.2 litres of fluid per hour depending on heat and sweat rate. Aim to finish a session about the same weight you started — not heavier.
Do I need electrolytes for every workout? Do I need electrolytes for every workout?
No. For sessions under about an hour in cool or normal conditions, water and normal meals are usually enough. Added sodium is more relevant for long, hot, or heavy-sweat sessions. No. For sessions under about an hour in cool or normal conditions, water and normal meals are usually enough. Added sodium is more relevant for long, hot, or heavy-sweat sessions.
What is exercise-associated hyponatraemia? What is exercise-associated hyponatraemia?
It is a drop in blood sodium that can happen when people drink far more than they sweat during long events. It can be serious, which is why the advice is to drink to thirst and avoid over-drinking. It is a drop in blood sodium that can happen when people drink far more than they sweat during long events. It can be serious, which is why the advice is to drink to thirst and avoid over-drinking.
How much sodium should an electrolyte drink contain? How much sodium should an electrolyte drink contain?
A common planning figure is about 500–700 mg of sodium per litre of fluid (ACSM). Test any drink in training before relying on it in an event. A common planning figure is about 500–700 mg of sodium per litre of fluid (ACSM). Test any drink in training before relying on it in an event.
How do I do a sweat test? How do I do a sweat test?
Weigh yourself before and after a session, note the fluid you drank and any urine passed, then the optional sweat-test mode estimates your sweat rate. It is a rough guide, not a precise measurement. Weigh yourself before and after a session, note the fluid you drank and any urine passed, then the optional sweat-test mode estimates your sweat rate. It is a rough guide, not a precise measurement.
The NHS says cut down on salt — isn’t extra sodium bad? US guidance says cut down on sodium — isn’t extra sodium bad?
The NHS daily maximum of 6 g salt (2.4 g sodium) is about your overall diet. Replacing some sodium lost in sweat during long or hot sessions is a different situation, so the two are not in conflict. The Dietary Guidelines for Americans advise adults and teens 14+ to aim for no more than 2,300 mg sodium per day overall. Replacing some sodium lost in sweat during long or hot sessions is a different situation, so the two are not in conflict.
How do I convert between salt and sodium? How do I convert between salt and sodium?
About 1 g of sodium is 2.54 g of salt, and 1 g of salt contains about 393 mg of sodium. Food labels in the UK usually list salt rather than sodium. About 1 g of sodium is 2.54 g of salt, and 1 g of salt contains about 393 mg of sodium. US Nutrition Facts labels list sodium in milligrams.
Who should be cautious with added sodium? Who should be cautious with added sodium?
If you have high blood pressure, heart or kidney disease, are on a salt-restricted diet, take diuretics, or have had hyponatraemia before, talk to your GP or pharmacist before adding sodium. This tool is not medical advice. If you have high blood pressure, heart or kidney disease, are on a sodium-restricted diet, take diuretics, or have had hyponatraemia before, talk to your physician or pharmacist before adding sodium. This tool is not medical advice.