How to Use the Running & Marathon Hydration Calculator
Enter your body weight, how long you expect to run and the race-day conditions, and the calculator tells you how much to drink per hour and every 20 minutes. If you know your sweat rate, add it for a much more personal plan.
- Body weight: your normal weight in kg or lb. It sets both your estimated sweat rate and how much fluid you can safely lose (2% of body weight).
- Expected finish time: hours and minutes for the run or race. Use a realistic goal, not a best-case one, because slower finishes mean more time sweating.
- Race-day conditions: pick the temperature band you expect at the warmest part of the race. Add a band if it's humid or sunny.
- Measured sweat rate (optional): if you've done a weigh-in test in similar weather, enter it in liters per hour and the calculator will use it instead of an estimate.
The headline figure is your hourly intake, and the line below breaks it into sips every 20 minutes, which roughly matches aid-station spacing in most marathons. Total to drink is the amount for the whole run, and projected weight loss shows where you'll finish if you follow the plan. A result of zero means you can finish within 2% without drinking; you can still drink to thirst.
How the Running & Marathon Hydration Calculator Works
The calculator estimates how much you'll sweat over the run, subtracts the fluid you can afford to lose (2% of body weight), and spreads the rest evenly across the hours you'll be running.
Running hydration formula
Sweat rate (L/hr) = Measured rate, or Base rate × (Weight kg ÷ 70)Total sweat loss (L) = Sweat rate × HoursAllowable loss (L) = 0.02 × Weight kgDrink per hour (L) = max(0, Total sweat loss − Allowable loss) ÷ Hours- Base rate: a typical sweat rate for a 70 kg runner in each condition: 0.5 L/hr (cool), 0.8 L/hr (mild), 1.1 L/hr (warm) and 1.4 L/hr (hot). These sit inside the 0.5–2.0 L/hr range reported for endurance runners.
- Weight ÷ 70: scales the estimate to your size, since heavier runners produce more heat at the same pace and usually sweat more.
- Allowable loss: 2% of body weight, the level at which endurance performance generally starts to decline. One kilogram of weight equals about one liter of fluid.
Worked example: A 70 kg runner aims for a 4:00 marathon in warm weather. Sweat rate = 1.1 × (70 ÷ 70) = 1.1 L/hr, so total sweat loss = 1.1 × 4 = 4.4 L. Allowable loss = 0.02 × 70 = 1.4 L. Fluid to drink = 4.4 − 1.4 = 3.0 L, or 0.75 L per hour (about 250 ml every 20 minutes). The runner finishes about 2.0% lighter.
Because hourly intake is never more than the sweat rate, the plan won't tell you to finish heavier than you started, which is the main driver of exercise-associated hyponatremia. Results are rounded to the nearest 10 ml. If your plan exceeds about 0.8 L/hr, practise it in training first, since many runners struggle to absorb more than that.
Running & Marathon Hydration Calculator FAQs
How much water should I drink during a marathon?
Most marathoners do well on roughly 400–800 ml (about 14–27 fl oz) per hour, but the right number is whatever keeps your body-weight loss under about 2% without exceeding your sweat rate. For a 4-hour marathon in warm weather, that often works out to 2.5–3 L in total.
Do I need to drink during a 5K or 10K?
Usually not. Most runners lose well under 2% of body weight in races shorter than an hour, so the calculator often shows zero planned intake; sipping at a water station on a hot day is fine, but it isn't required.
How accurate is the estimated sweat rate?
It is a starting point, not a measurement. Real sweat rates for runners at the same pace and temperature can differ by a factor of two or more, so entering a measured sweat rate from a weigh-in test makes the plan far more reliable.
Should I drink to thirst or follow a schedule?
Both approaches work for many runners. Drinking to thirst is safe for most races up to about 90 minutes, while a schedule based on your sweat rate helps in longer or hotter events where thirst can lag behind losses.
Do I need electrolytes when running a marathon?
For races longer than about two hours, adding sodium is sensible, typically 300–600 mg per hour from a sports drink, gels or salt tablets. Heavy, salty sweaters may need more, and the main purpose is to help you retain fluid and lower the risk of hyponatremia.
What are the limitations of this running hydration calculator?
It does not know your sweat sodium, gut tolerance, the race's aid-station spacing or how weather will change on the day. Use the result as a plan to rehearse in long training runs, then adjust it before race day.