Free Calculator · Published October 2026
Contact Tank Volume Calculator: Gallons and Minutes
Measure the straight side. Get gallons, nominal detention, and a planning effective contact time at the flow you enter.
Built by SubcontractorHub — the software plumbing contractors use to quote, finance, and run every job.
Book a DemoA Contact Tank Volume Calculator Starts from the Shell
This contact tank volume calculator is the tank you can measure. Chemical feed is the chlorine injection system sizing calculator. A one-time shock pour is the chlorine dose calculator.
Inside of the shell, not the outside of the fiberglass.
The straight wall only. Do not include the domes unless you mean to.
Usually the well pump rate through the tank, not the average daily use.
A planning label for short-circuiting. A tracer study replaces it.
Used only to show residual times effective minutes. Not a required CT.
Important: All calculator results are ballpark estimates
All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.
What the Contact Tank Volume Calculator Is Measuring
A cylinder, not a sticker
Gallons are the inside diameter and the straight-side height, as a cylinder, times 7.4805 gallons per cubic foot. A 24 by 54 inch straight side is on the order of a hundred gallons, not the nominal size printed on a different tank. Domes, a retention plate, and water that does not fill the shell are outside the formula. If the manufacturer publishes an operating volume at the low water line, use that volume for minutes and keep this page as the check on the shell you measured.
Nominal minutes are not effective minutes
Nominal detention is gallons divided by peak gallons per minute. Water short-circuits. The baffling factor is the share of that nominal time you are willing to claim as effective contact, often called T10 in public-water guidance. An unbaffled factor of 0.1 means a tank that looks like ten minutes on paper is about one minute of effective contact. Average baffling at 0.5 claims half. A long pipe at 1.0 claims the nominal time. Pick the factor that matches the vessel, or replace it with a tracer study. Do not treat the factor as a law for a private well.
Worked examples
Same math as the form. CT appears only when a residual was entered.
| Case | Gallons | Nominal | Effective |
|---|---|---|---|
| 24×54 in, 10 gpm, unbaffled, 1 ppm | 105.8 | 10.6 min | 1.06 min |
| Same tank, average baffling, 1 ppm | 105.8 | 10.6 min | 5.29 min |
| 12×48 in pipe-like, 5 gpm, no residual | 23.5 | 4.7 min | 4.7 min |
Where this sits in a treatment train
Oxidation of iron often wants time after the chemical and before the filter. The iron filter sizing calculator flags high iron that may need that chemical path. This page does not choose the chemical. It only says how many minutes the shell can support at the flow. Bacteria, sulfur, and iron are different jobs. Confirm the residual after the tank with a test kit. A clear glass is not a passed test.
How to measure the shell
Use the inside diameter if you can get it, not the outside jacket. Use the straight side, not the overall height that includes the domes. Inches in, gallons out. If the tank is already piped and you cannot see the straight side, do not invent a height from a catalog cousin. Leave the field blank until you measure, or type an operating volume you trust and use gallons divided by gpm for the minutes. A public-water contact-time rule, such as the ones described by EPA under the surface water treatment rules, is a different program. A house well is not a surface-water plant. Borrow the baffling idea as a label. Do not borrow a required CT credit.
Residual is optional on purpose
CT on this page is free chlorine, in ppm, times effective minutes. Leave residual blank when you only need gallons and minutes. Filling it in does not size a pump or a bleach jug. Dose is the chlorine dose calculator for a one-time shock, or the injection calculator for a continuous feed. This page answers a narrower question: how much water the straight side holds, and how many minutes that water stays in the shell at the flow you typed.
What to write on the job ticket
Diameter, straight side, the gallons you used, peak gpm, the baffling factor and why you picked it, and the residual if you computed CT. A water treatment dealer who saves that note can explain a short contact time without remeasuring the tank from memory. SubcontractorHub keeps the note on the customer record with the rest of the install.
Frequently Asked Questions
What does a contact tank volume calculator measure?
It measures the cylinder of the straight side you enter: inside diameter and straight-side height, converted to gallons. Peak flow turns those gallons into nominal detention minutes. A baffling factor you pick turns nominal minutes into a planning effective contact time. It does not size a chlorine dose or a metering pump.
Why is the gallon count different from the sticker on the tank?
Manufacturers often print a nominal gallon that includes domes or a marketing size. This calculator uses only the straight side you measure, as a cylinder. Domes, internals, and a tank that is not full are not in the number. If you know the operating gallons, you can still use the minutes formula: gallons divided by gpm.
What baffling factor should I use?
The factors here are the familiar public-water planning set: 0.1 unbaffled, 0.3 poor, 0.5 average, 0.7 superior, and 1.0 for plug flow in a pipe. They estimate short-circuiting. They are not a tracer study of your tank. A private well is not a public water system. If a state or a manufacturer specifies a factor, use that factor.
Does this replace the chlorine injection calculator?
No. The injection calculator starts from a dose and a desired contact time and picks a catalog tank, and it also sizes chemical feed. This page starts from a tank you can measure and tells you the minutes that shell can claim at a flow. Use both when you are designing. Do not treat them as the same answer.
What is CT on this page?
If you enter a free chlorine residual, CT is that residual in ppm times the effective minutes. It is a planning product, not a required public-water CT value and not a log-inactivation credit. Leave the residual blank if you only want gallons and minutes.
Save the Tank With the Water Test
Diameter, minutes, and the factor you claimed belong on the job, next to the chemical settings.
Get a Free DemoAll calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements. It does not size a chlorine dose, a metering pump, or a public-water CT credit.