Why a dosed system exists at all
A gravity drainfield takes whatever the tank passes, whenever it passes it, and the first few feet of the first trench do most of the work until they stop being able to. That is tolerable on ground that will accept it. On sites where it is not tolerable — shallow soil, a high water table, a slope, a mound, a sand filter, anything where the whole absorption surface has to be used evenly — the design puts a pump between the tank and the field and sends the effluent in measured slugs.
The slug has to be large enough and fast enough that the laterals fill and pressurise before much comes out of the near orifices. Once the pipe is full and under pressure, every orifice discharges at roughly the same rate and the field is loaded across its whole area. Then the pump stops, the laterals drain, and the soil gets several hours of air before the next dose. That resting period is doing real work; a continuously wet field behaves differently from one that dries between loadings.
The dose is a property of the pipe, not the house
This is the part people get backwards. The size of the dose is set by how much pipe is in the ground, because the dose has to fill it and then some. The house sets how often a dose happens, not how big it is.
Volume in a pipe is 0.0408 gallons per foot per square inch of bore — so a foot of 1.25 inch pipe holds about 0.064 gallons, and 320 feet of it holds around 20 gallons. Multiply by whatever multiple your design uses and you have the dose. Change the field layout and the dose changes; add two occupants and it does not, you just get more doses a day.
That is why a table of dose volumes against bedrooms, which does exist on the internet, is meaningless. It is answering with the wrong variable.
Drainback, and why the floats are further apart than the dose
Most force mains are installed so that they empty back into the tank when the pump shuts off, usually through a small weep hole drilled in the discharge. The reason is freezing: a buried line full of standing effluent all winter is a line that bursts.
The consequence is that the pump has to refill the main at the start of every cycle before a drop reaches the field. So the volume that leaves the tank per start is the dose plus the main volume, and the floats have to be separated by enough inches to allow for both — then the level comes back up by the main volume a minute later. The net fall per cycle, which is what matters for how often the pump runs, is just the dose.
The page shows both because people measuring float separation in a tank and comparing it to a stated dose volume find they do not match, and conclude something is wrong when nothing is.
Getting the tank dimensions right
Gallons per inch is inside length times inside width times 0.6234. Outside dimensions of a concrete tank overstate the inside by the wall thickness on both sides, which on a five by four foot tank is a several percent error in the same direction every time. If the tank is fibreglass or polyethylene the taper matters too and the cross-section is not constant with depth.
Best case you do not measure at all: the tank documentation, the permit file at the health department, or the panel commissioning record will state the dose and the float settings that were set at installation, and those are the figures to use.
Where this page stops
Not one design figure appears on this page, and that is deliberate. Dose volumes, orifice sizes, loading rates, sand depths, trench widths, separation distances, reserve areas and pumping triggers are set by the county or state health department and worked out for your particular ground by a licensed onsite designer, engineer or installer. Every figure the page uses is one you typed in off your own approved design or your own measurement. The page arranges arithmetic; it does not decide anything, it does not check anything against a rule, and it will never tell you whether what you have is right.
Nothing here is a verdict. The page does not say a system is compliant, adequate, undersized, failing or working, because it cannot see your site, your soil, your permit or your local rules, and because that judgement belongs to people who are licensed to make it and who carry the responsibility for it.
Septic and pump tanks are confined spaces and they kill people. The atmosphere above the liquid has no oxygen worth breathing and the hydrogen sulphide in it destroys the sense of smell in seconds, so there is no warning once you are over the opening. The recurring pattern is somebody reaching in for a dropped phone or float and a second person dying trying to pull them out. Nobody leans over an open tank and nobody enters one, ever, for any reason. An open or cracked lid is separately a fall-in hazard that has killed children, so a lid comes off only when a crew is standing over it and it goes back secured. Raw sewage is a disease exposure and effluent on the skin or in a cut is treated as one.
Related arithmetic on the same system: the effluent pump head calculator works out the total dynamic head the pump sees and reads it against a curve you enter, the orifice flow calculator gives the gallons a minute the field will actually take, and the drainfield area organizer arranges flow and loading rate into area. For the flow figure itself, start at the household wastewater flow calculator. The pipe volume arithmetic on its own is at the pipe volume calculator, and the cycle-counting logic for a groundwater sump, which works in the opposite direction, is at the sump pump sizing calculator.
Questions people ask
What should my dose volume be?
That question has an answer and it is on your design drawing, not here. The dose is worked out from the volume of lateral pipe installed on your site, the layout of the laterals, and the way the design intends the field to be loaded, and it is set into the control panel at commissioning. This page will multiply your pipe volume by your multiple and show you what that comes to, which is useful for understanding a panel setting or checking a proposal, but the multiple itself belongs to the designer. If you do not know what your system was set to, the panel documentation, the installer, or the permit file at the health department will have it.
Why does the tank drop more than the dose volume when the pump runs?
Because the pump is also refilling the force main, which drained back into the tank after the previous cycle. If the main holds fifteen gallons and the dose is sixty, seventy-five gallons leave the tank on each start, the level falls accordingly, and then fifteen come back a minute after the pump stops. The floats are set for the seventy-five; the field gets the sixty; the net fall per cycle is the sixty. Systems that keep the main charged with a check valve behave differently, and there are reasons a design might do it either way, mostly to do with freezing.
Is a high number of pump starts a problem?
Motor starts are the main wear mechanism on a small pump, considerably more so than running hours, so the number is worth knowing. Whether the number your system produces is high for that pump is a question about that specific pump and its duty rating, which is in its documentation and not in a general calculator. What the arithmetic here does show clearly is the shape of the tradeoff: dose size and start count are inversely proportional, so a design that halves the dose doubles the starts, and that is one of the things being balanced when a designer picks the multiple.
Can I change the float settings myself to reduce the cycling?
Moving floats changes the dose, and the dose is a design parameter. It was chosen to fill the laterals in the way the field needs to be loaded, and shortening it can mean the far end of the field stops receiving effluent while the near end takes everything. In many jurisdictions altering a permitted onsite system is also a permit matter in its own right. Beyond that, the settings live in a tank that is a confined space and a fall-in hazard. If the cycling looks wrong, the call is to whoever services the system, with the numbers from this page in hand as a starting point for the conversation.
The pump runs but the alarm keeps sounding. What does that tell me?
That the level reached the alarm float, which means over some period more went in than went out. That is all it means and the causes are numerous: a pump that is no longer making its curve, a partly blocked or frozen force main, a check valve stuck shut, a field that is accepting less than it was, a float that has fouled or hung up, a control problem, or simply far more water arriving than usual because of a leaking toilet or a wet week finding its way into the tank. Sorting those apart is service work. Meanwhile the honest useful number is the one this page gives for the reserve: roughly how many hours of household flow the space above the pump-on level represents, so you know whether you have an evening or a day.