Water Tank Turnover Time Calculator
Old water in a tank loses disinfectant residual, grows disinfection byproducts and — in chloramine systems — risks nitrification. Enter tank volume and average demand to see your turnover time.
Tank Turnover
| Item | Value |
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How the calculation works
Formulas
Turnover (days) = tank volume (gal) / average daily demand (gal/day)
1 MG = 1,000,000 gal · 1 MGD = 1,000,000 gal/day · gpm × 1440 = gal/day
Approximate average water age ≈ turnover / 2 (rough indicator only).
Turnover (days) = tank volume (gal) / average daily demand (gal/day)
1 MG = 1,000,000 gal · 1 MGD = 1,000,000 gal/day · gpm × 1440 = gal/day
Approximate average water age ≈ turnover / 2 (rough indicator only).
Notes & limitations
Many utilities target full turnover within 3–5 days (AWWA M42 guidance). Turnover time is a screening indicator — dead zones and short-circuiting can make the oldest water much older than the average.
- Above 5 days is elevated water age: expect disinfectant residual loss, higher disinfection byproducts, and — in chloraminated systems — nitrification risk.
- Run the calculation with seasonal low demand: turnover is worst when demand is lowest.
- Turnover time is not the maximum water age. Poor inlet/outlet configuration or stratification leaves pockets of much older water — confirm with field residual profiling.
- If turnover is persistently long, options include lowering the operating band, seasonal tank cycling, or mixing improvements — coordinate with your distribution supervisor.
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