Why the scale is the instrument
Over an hour or two of exercise, almost everything leaving the body is sweat and almost everything entering it is what you drank. That makes the accounting simple: sweat lost = weight lost + fluid drunk − urine passed. A runner who starts at 170 lb, finishes at 167 lb and drank a 20 oz bottle on the way lost 48 fl oz through weight plus the 20 they replaced, so 68 fl oz of sweat in total. Over ninety minutes that is 45 fl oz per hour.
The conversion from pounds to ounces is the one assumption worth naming. This page treats 1 lb of weight change as 16 fl oz of fluid, which is the working convention in the field. Water is slightly denser than that implies — a US pint weighs about 1.04 lb rather than exactly one — so the convention runs about 4% high. Against a bathroom scale that reads in tenths of a pound and a bottle you estimated by eye, 4% is not the source of your error.
What the percentage means and the absolute number does not
A litre lost is 2% of body weight for a 110 lb runner and 1.1% for a 200 lb one, and the consequences track the percentage rather than the volume. That is the whole reason the calculator reports both.
| Body weight lost | What is generally observed |
|---|---|
| Under 1% | Ordinary day-to-day variation. Nothing to change. |
| 1 to 2% | Noticeable thirst, heart rate drifts up a little at the same pace. Replacing afterwards handles it. |
| 2 to 3% | The band where endurance performance starts to measurably decline, more sharply in heat. |
| Over 3% | Thermoregulation gets harder and heat illness risk rises on a hot day. This is a planning failure, not a hydration slip. |
These bands come from group means and individuals scatter around them. Some people hold pace past 3%; others feel poor at 1.5%. Heat acclimatisation, humidity, wind and intensity move the same person around week to week.
The spread between people is the point
Two runners on the same course at the same pace on the same morning can lose 17 and 85 fl oz per hour respectively. Body size, acclimatisation, intensity, air temperature and humidity all compound. Advice of the form "a cup every fifteen minutes" is therefore roughly double what one of them needs and half what the other does.
Measuring twice in different conditions is more useful than measuring once precisely. One cool indoor session and one hot outdoor session bracket your range. If you are planning for a race, the measurement that counts is the one taken in weather resembling race day.
The sodium figure is the least reliable number on the page
Sweat sodium runs from about 200 to 2,000 mg per litre between individuals — a tenfold spread. Determining yours takes a patch test analysed in a lab. The three bands offered here are inferred from visible clues, white residue on dark kit and stinging when sweat runs into your eyes, which is a crude proxy and nothing more.
Practically, sessions under an hour rarely require any attention to sodium. Past two hours, or several hours outdoors in heat, or a repeated pattern of cramping, is where it becomes worth addressing. A typical mixed diet already carries plenty of sodium, so most ordinary training is covered by meals alone.
Drinking more is not automatically safer
The opposite failure exists and is more dangerous. Drinking well past thirst over a long event dilutes blood sodium, producing exercise-associated hyponatraemia. Its early symptoms — nausea, headache, dizziness — overlap with dehydration, which makes drinking more the intuitive and wrong response. Swollen fingers, and finishing heavier than you started, point at this side rather than the other.
A meaningful share of the collapses reported at marathons are attributed to this rather than to dehydration. Taking a full cup at every aid station regardless of need is the habit that produces it. Knowing your own rate is what turns drinking from a reflex into a plan.
Where this stops being useful
Weight change is not purely fluid. Water leaves through the lungs, water is produced by metabolism, and glycogen consumed during a long effort takes bound water with it. Across an hour these are small enough to ignore. Across a five-hour ultra they are not, and part of the weight you have lost is genuinely fuel rather than fluid — replacing all of it produces overhydration. In very long events, finishing 1 to 2% down is often described as the expected outcome rather than a problem to solve.
Nothing here diagnoses anything. If you have a kidney or heart condition, or take medication that affects fluid balance, the numbers your clinician has set for you replace these entirely. The same goes if you have ever had a hyponatraemia episode, where the sensible drinking plan is a medical decision rather than an arithmetic one. Sodium in the wider diet is covered by the sodium intake checker, and total body water by the body water estimate.
Questions people ask
I lost two pounds during a workout. Did I lose fat?
Effectively none of it. Burning a pound of fat takes around 3,500 kcal, and an hour of hard training rarely exceeds 700, so the fat contribution to that two pounds is well under an ounce. The rest is water and it returns within the day once you drink. If you are tracking body composition, weigh at the same time each morning before eating and read the multi-day trend rather than anything either side of a session.
Should I drink back everything I sweat out during the session?
No, and with a high sweat rate you could not. The gut absorbs roughly 34 to 41 fl oz per hour during exercise; drinking past that leaves fluid in the stomach where it does nothing except make you uncomfortable. The workable target during the session is to keep weight loss inside about 2%, then replace the remainder over the following two to four hours. That is the reason the drinking plan on this page caps itself below high sweat rates rather than matching them.
Is water enough, or do I need a sports drink?
For sessions under an hour, water is enough and the sports drink is adding calories rather than solving a problem. Past two hours, in heat, or if your sweat is visibly salty, sodium starts to matter and either an electrolyte drink or salted food covers it. Repeated cramping is a reasonable prompt to look at sodium, though the evidence linking cramps to electrolytes specifically is weaker than the marketing suggests.
My weight went up. Is the calculator wrong?
Not if you drank more than you lost, which is entirely possible. It is usually unremarkable. It is worth a second thought if the session was long and the intake was plain water, since that is the setup for hyponatraemia. Headache, nausea and puffy hands together mean stopping fluids and taking salt, and seeking care if it worsens. With no symptoms at all, drink less next time and move on.
Can I measure once and use that figure forever?
Only within similar conditions. Ten degrees warmer, or humid instead of dry, changes the rate substantially for the same session. Heat acclimatisation itself shifts things: adapted people start sweating sooner and sweat more, while sweat sodium concentration falls. Measure once cool and once hot to get a range, and base any race plan on the measurement taken in the closest weather to the race.