Turnover rate is the hidden variable behind most pool problems. Cloudy water that won't clear? Chemistry that won't hold? Algae that keeps coming back? In a startling percentage of cases, the real issue is that the water isn't moving through the filter fast enough.
The formula
Turnover time (hours) = Pool volume (gallons) ÷ Flow rate (gpm × 60)
Or: Turnover time = Gallons ÷ (gpm × 60).
Code-required turnovers (typical)
| Pool type | Required turnover |
|---|---|
| Residential pool | 6–8 hours |
| Public / commercial pool | 6 hours |
| Competition pool | 6–8 hours |
| Wading pool | 1–2 hours |
| Spa / hot tub | 30 minutes |
| Splash pad / interactive | 30 minutes or less |
| Therapy pool | 30 minutes |
Your local code may be stricter. Always verify with your state/county. Florida's public-pool code (DOH) runs close to these.
Why 4 turnovers ≈ 98% filtered
Pool water mixes continuously, so filtration is exponential rather than linear:
- 1 turnover: ~63% of water has passed through the filter
- 2 turnovers: ~86%
- 3 turnovers: ~95%
- 4 turnovers: ~98%
- 5 turnovers: ~99.3%
That's why “run the pump for 24 hours after a shock” translates to roughly four turnovers for most pools — it's not arbitrary, it's the diminishing-returns point.
Verifying actual flow rate
Your pump's nameplate flow is not your actual flow. System resistance, filter pressure, and plumbing geometry all cut real flow below the rated number. Methods to verify:
- Flow meter on discharge line — best, required on commercial pools
- Pump curve + system curve intersection — engineering estimate, plus or minus 15%
- Timed bucket test at a hose bib — crude, but better than nothing for small systems
Worked example
Pool: 20,000 gal. Required turnover: 6 hours.
Required flow: 20,000 ÷ (6 × 60) = ~56 gpm.
Your pump plate says 100 gpm at 50 ft TDH. Your system's actual TDH measures 65 ft. Real flow: ~75 gpm (from the pump curve). You're above code. ✓
Signs of inadequate turnover
- Persistent cloudy water despite correct chemistry
- Algae returning in corners even with good FC
- Filter pressure drops low without a clean
- Skimmer water not moving briskly
- Uneven chemical distribution (readings vary by pool zone)
Fixing low turnover
- Clean or backwash the filter (first and easiest)
- Check for air leaks on the suction side (the pump will lose prime or run hot)
- Clean skimmer and pump baskets
- Inspect for kinked hoses, closed valves, debris
- Verify impeller condition (wear/scaling)
- If nothing helps: hydraulic inspection. Possible undersized plumbing or wrong pump.