Beach Day Water & Electrolyte Intake Calculator

How it works

The starting point is a widely used everyday baseline of about 35 ml of water per kilogram of body weight per day. For a 70 kg person that is 2,450 ml before any beach-day extra is added. This baseline covers ordinary indoor needs, not a long hot day in the sun.

On top of the baseline, the calculator adds an hourly amount for time outdoors. That hourly amount has two parts: a temperature part (0 ml in mild weather, 150 ml in warm weather, 300 ml in hot weather) and an activity part (100 ml for gentle activity, 250 ml for moderate activity, 450 ml for vigorous activity). Multiply the combined hourly amount by your hours outdoors and add it to the baseline to get the day total.

The sodium guidance is a range, not a prescription. On a short mild day most of it can come from ordinary food. On a long, hot, sweaty day you will need more salt from food and drinks, so the range rises with hours outdoors and with heat and activity. If you have blood pressure, kidney, heart, or other conditions that affect salt or fluids, use your clinician's advice instead of this range.

How to use your result

Spread the total across the day rather than drinking it all at once, and start before you feel thirsty. Take a bottle you can refill, and plan a salty snack or an electrolyte drink for the hottest part of the day. If you feel dizzy, nauseous, confused, or unusually tired, stop, get into shade, and seek help — those can be signs of heat illness, not just thirst.

Related guides

• A Beach Day Hydration Plan for Hot Weather
• Electrolytes for Beach Days: What You Actually Need
• Dehydration Signs at the Beach