# Detention Time Calculator URL: https://speedycalculator.com/calculators/chemistry/detention-time/ Last updated: 2026-10-30 > The detention time calculator finds how long water stays in a tank, clarifier, basin or separator from its volume – entered directly or from rectangular or circular dimensions – and the flow rate, plus the surface overflow rate and horizontal velocity used to check settling tanks. ## Inputs - Tank volume - Tank volume (US gal, ft³, m³, L, million US gal, bbl (oil)) - Length (ft, m, in, yd) - Width (ft, m, in, yd) - Diameter (ft, m, in, yd) - Water depth (ft, m, in, yd) - Flow rate (MGD (million US gal/day), US gal/min (gpm), US gal/day, m³/h, m³/day, L/s, ft³/s, bbl/day) ## Results - Water volume in the tank - Detention time - Surface area - Surface overflow rate - Horizontal (flow-through) velocity - Tank volume turnovers per day ## Formula ``` DT = V ÷ Q · rectangular V = L·W·D · circular V = π·d²·D ÷ 4 · surface overflow rate = Q ÷ surface area · horizontal velocity = Q ÷ (W·D) ``` ## Explanation Detention time is the average time a parcel of water spends in a tank, assuming the flow passes through evenly. It is one of the first numbers a water or wastewater operator checks, because nearly every treatment step needs time: particles need time to settle in a clarifier, floc needs time to grow in a flocculator, and disinfectant needs contact time to inactivate pathogens. In oil and gas, the same idea – often called residence or retention time – sizes separators so that oil, gas and water have time to part. The calculation is simply volume divided by flow, but the units trip people up. Operators in the US often work in gallons and million gallons per day, while designers use cubic feet, cubic metres and m³/day. This calculator lets you mix any of them and gives the answer in hours, minutes, days or seconds. If you do not know the volume, enter the tank dimensions – length, width and water depth for a rectangular basin, or diameter and side-water depth for a round clarifier. Detention time alone does not guarantee good settling. For sedimentation basins the surface overflow rate (flow per unit of surface area) controls which particles are removed, and the horizontal velocity shows whether settled sludge risks being scoured. Both are reported for tanks entered by dimensions. ### How to use the Detention Time Calculator - **Tank volume** - **Tank volume** - **Length** - **Width** - **Diameter** - **Water depth** - **Flow rate** Results update as you type, every field has its own unit menu, and everything is calculated in your browser. ### How it works Pick how to describe the tank and enter its volume or dimensions together with the flow rate. The calculator converts everything to cubic metres and seconds, divides volume by flow and converts back to your chosen time unit; for dimensioned tanks it also divides the flow by the surface area and by the cross-section. ### Worked example A rectangular settling basin is 80 ft long, 20 ft wide and holds 10 ft of water: 16,000 ft³ = 119,688 US gal. At 1 MGD the detention time is 119,688 ÷ 1,000,000 × 24 = **2.87 h** (172 minutes). The surface area is 1,600 ft², so the surface overflow rate is 1,000,000 ÷ 1,600 = 625 gpd/ft², and the horizontal velocity is 133.7 ft³/min ÷ 200 ft² = 0.46 ft/min. Doubling the flow to 2 MGD would halve the detention time to 1.44 h and double the overflow rate. ### Typical design ranges ProcessTypical detention timeRapid (flash) mixing10–60 secondsFlocculation20–40 minutesSedimentation in water treatment2–4 hoursPrimary clarifier (wastewater)1.5–2.5 hoursChlorine contact tank (wastewater)15–30 minutes at peak flowRanges are indicative; the design standard that applies to your plant takes precedence. ### Tips - Read the short explanation under each result. - Change one input at a time to see how the result reacts. - Everything runs in your browser, so you can experiment freely. - Check the worked example if a result surprises you. ### Common mistakes to avoid - Mixing units between fields. - Using a rule of thumb outside the range where it applies. - Treating an estimate as an exact answer. ### Inputs explained FieldWhat to enterTank volumeChoose one: I know the volume, Rectangular tank – length × width × depth, Circular tank – diameter × depthTank volumeUnits: US gal, ft³, m³, L, million US gal, bbl (oil)LengthUnits: ft, m, in, ydWidthUnits: ft, m, in, ydDiameterUnits: ft, m, in, ydWater depthUnits: ft, m, in, ydFlow rateUnits: MGD (million US gal/day), US gal/min (gpm), US gal/day, m³/h, m³/day, L/s … ### What each result means - **Detention time** = [Water volume in the tank]÷[Flow rate] - **Surface overflow rate** = [Flow rate]÷[Surface area] - **Horizontal (flow-through) velocity** = [Flow rate]÷([Width]×[Water depth]) - **Tank volume turnovers per day** = 86400×[Flow rate]÷[Water volume in the tank] ## Worked example Inputs — Tank volume: Rectangular tank – length × width × depth; Length: 80 ft; Width: 20 ft; Water depth: 10 ft; Flow rate: 1 MGD (million US gal/day). Results — Water volume in the tank = 119,688.3117 US gal; **Detention time = 2.8725 h**; Surface area = 1,600 ft²; Surface overflow rate = 625 gpd/ft²; Horizontal (flow-through) velocity = 0.4642 ft/min; Tank volume turnovers per day = 8.355. ## Key terms - **Detention time**: Average time water spends in a tank, V ÷ Q. - **Surface overflow rate**: Flow per unit of tank surface area; equals the settling velocity of the smallest particle fully removed. - **Short-circuiting**: Flow that bypasses part of the tank and leaves sooner than the average. - **T10**: Time for 10% of a tracer to appear at the outlet, used for disinfection contact time. ## FAQ **What is the detention time formula?** DT = V ÷ Q – tank volume divided by flow rate, in consistent units. 500 US gal at 100 gpm gives 5 minutes. **How do I convert gallons and MGD to hours?** DT (h) = volume (gal) × 24 ÷ flow (gal/day). 120,000 gal at 1 MGD gives 120,000 × 24 ÷ 1,000,000 = 2.88 h. **What is a typical detention time for a sedimentation basin?** Conventional water-treatment settling basins are often designed for about 2–4 hours; primary clarifiers in wastewater plants typically 1.5–2.5 hours at average flow. **How long should flocculation and rapid mixing take?** Rapid mixing lasts seconds (roughly 10–60 s); flocculation usually 20–40 minutes, depending on the design standard you follow. **Is detention time the same as hydraulic retention time?** They are calculated the same way. "Detention time" is the usual term for clarifiers, basins and contact tanks; "hydraulic retention time" is used for biological reactors. **Why is the real contact time shorter?** Short-circuiting and dead zones mean some water leaves much faster than the average. Tracer studies give T10 – the time for 10% of a tracer to pass – which is often 30–70% of the theoretical detention time and is what disinfection CT credits use. **What does the surface overflow rate mean?** Flow divided by surface area, in gpd/ft² or m³/(m²·day). Particles that settle faster than this rate are removed in an ideal basin, regardless of depth. **What formula does the Detention Time Calculator use?** DT = V ÷ Q · rectangular V = L·W·D · circular V = π·d²·D ÷ 4 · surface overflow rate = Q ÷ surface area · horizontal velocity = Q ÷ (W·D). ## Sources - [US EPA – Water research and drinking water](https://www.epa.gov/ground-water-and-drinking-water) - [USGS – Water science school](https://www.usgs.gov/special-topics/water-science-school) - [LibreTexts Chemistry](https://chem.libretexts.org/)