Bring enough water for the time you will actually spend away from a reliable refill point, then add a reasonable reserve for delays and heat. There is no single bottle count that fits every airsoft day. A short open-play round with frequent staging breaks needs a different plan from a long scenario in sun and humidity. Ask the organizer about potable water, refill access, and the schedule before you choose a container size.
Estimate the drinking window
Start with the longest period between dependable refills, not the total event duration. If your field runs brief rounds and returns everyone to staging, you can keep most water at a safe, accessible base and carry a smaller amount during play. If an event holds players on a distant part of the field for hours, plan for that entire interval and a reserve. Check whether you may leave and reenter play to refill.
The U.S. National Institute for Occupational Safety and Health recommends about one 8-ounce cup every 15 to 20 minutes for moderate work in heat lasting less than two hours. That is workplace guidance, not an airsoft-specific prescription. It gives a planning reference for a hot, active interval, not a fixed amount everyone should force down. NIOSH also warns against excessive drinking and recommends appropriate breaks and electrolyte-containing drinks when sweating continues for several hours. Your pace, weather, health, clothing, and access to shade all matter.
For example, a two-hour hot-weather period at that workplace reference rate would imply roughly 48 to 64 ounces consumed. That arithmetic is only a planning illustration, not a target for every player. If the weather is cool and the activity light, needs may be lower. If conditions are more demanding or you have a medical reason to limit fluids, get individualized advice rather than copying someone else's bottle count.
Make the water reachable
Store refill water where the field permits and where it will stay clean. Bring a bottle or reservoir you can drink from without removing required protective equipment in an active area. Practice reaching it while fully dressed. A container that is hard to access can leave you with unused water when you need a drink. Mark your own bottle, and do not count on another player's supply.
One well-engaged r/airsoft discussion describes several arrangements: small bottles during short games, larger reserves in staging, and on-body water for long rounds. These are player experiences, not measured hydration recommendations. A HopUp event listing with long game blocks is another reminder that schedules vary. Confirm the actual plan for your event.
Recheck during the day
Drink at regular breaks, refill before a long round, and tell staff if you feel unwell. Rest in a cooler place when needed. Headache, dizziness, weakness, or confusion call for stopping play and following the field's medical response; severe symptoms need urgent medical help. Do not try to solve a heat problem with water alone while continuing to exert yourself.
The useful answer is a repeatable calculation: time between refills, expected conditions, practical drinking access, and a modest reserve. The carrier can be simple. Having water available when you need it matters more than the style of pack.