Detention Time Calculator (Tanks, Clarifiers & Ponds)
Hydraulic retention (detention) time of a tank, clarifier, lagoon, pond or contact pipe from its size or volume and the flow, in any unit (m³, gallons, MGD, gpm, L/s) — with surface overflow rate, weir loading and horizontal velocity for clarifiers, peak-flow and parallel-unit checks, chlorine CT with EPA baffling factors, and the flow that gives a target detention time.
How it works
Detention time = volume ÷ flow (× 24 for hours when flow is per day); surface overflow rate = flow ÷ surface area; weir loading = flow ÷ weir length; horizontal velocity = flow ÷ (width × depth); T₁₀ = detention time × baffling factor; CT = residual (mg/L) × T₁₀ (min)
Hydraulic retention time — how long, on average, water spends in a basin — is the first check on any treatment unit, because every process needs a minimum contact time: a few minutes for chemical mixing, 20–60 minutes for flocculation, two to four hours for particles to settle in a clarifier, hours for chlorine to disinfect, days for a lagoon. It is simply volume divided by flow. Enter the tank as a rectangle, a circle, a pipe or a known volume, and the flow in whatever unit you have — m³/day, m³/h, L/s, US gpm, gallons per day or MGD. For settling tanks two more ratios matter: the surface overflow rate (upflow velocity) that a particle must beat to settle, and the loading on the outlet weir, which if too high drags floc over the edge; rectangular tanks also show the mean horizontal velocity. Under More options you can split the flow between identical tanks in parallel, check the peak flow, and enter the real weir length. Real tanks never behave as ideal plug flow; density currents and inlet jets short-circuit part of the flow, which is why disinfection rules use the T₁₀ time. Switch on the CT check to multiply the theoretical time by an EPA baffling factor (0.1 for an unbaffled tank up to 1.0 for a pipe) and the outlet residual, and compare it with the CT your regulator requires.
Worked example
A 20 × 8 m rectangular clarifier 3.5 m deep (560 m³) treating 4,500 m³/day: detention 560 ÷ 4,500 × 24 = 2.99 h, surface overflow rate 4,500 ÷ 160 = 28.1 m³/m²/day and end-weir loading 4,500 ÷ 16 = 281 m³/m/day — the weir is on the high side. At a 2.5 peak factor the detention drops to 1.19 h and the overflow rate rises to 70 m/day. For a 2-hour detention the tank could take 6,720 m³/day. Used as a chlorine contact basin with average baffling (0.5) and 1.0 mg/L residual, T₁₀ = 179 min × 0.5 = 90 min and CT = 90 mg·min/L.
Sources: US EPA — Disinfection Profiling and Benchmarking Technical Guidance Manual (baffling factors, CT)
Frequently asked questions
What detention time does a septic tank or pond need?
Septic tanks 24–48 h; facultative wastewater ponds 20–40 days; stormwater detention basins are sized by volume to release over 24–72 h.
Why is chlorine contact time quoted as CT?
Disinfection depends on concentration × time (mg/L·min). Regulators use the T₁₀ (time for 10% of a tracer to exit) rather than the theoretical HRT because baffling determines the effective contact. Without a tracer study, the EPA guidance multiplies the theoretical detention time by a baffling factor: 0.1 unbaffled, 0.3 poor, 0.5 average, 0.7 superior and 1.0 for plug flow in a pipe.
How do I calculate detention time in gallons and gpm?
Divide the volume in gallons by the flow in gallons per minute to get minutes, then divide by 60 for hours. A 100,000-gallon tank at 700 gpm holds water for 143 minutes, or 2.4 hours. Choose gallons and gpm in the unit menus and the calculator does the conversion.
What is the surface overflow rate of a clarifier?
Flow divided by the water surface area, in m³/m²/day (m/day) or gallons per day per square foot. It equals the upward velocity of the water, so only particles that settle faster than it are removed. Typical design values are 30–50 m/day for primary and 16–32 m/day for secondary clarifiers at average flow.
Should I use average or peak flow?
Both. Detention time is usually specified at average flow, but clarifier overflow rates and disinfection CT must also be met at peak flow, when detention is shortest. Enter the peak factor under More options to see both.
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This calculator is for educational purposes. Double-check important results before acting on them.