Three terms, and the one everybody forgets
Total dynamic head is what the pump has to produce, and on an onsite system it is the sum of three things.
Static lift is the vertical distance from the water surface in the tank when the pump shuts off up to wherever the effluent leaves the pipe. Not to the top of the tank, and not to the ground: to the pump-off level, because that is the worst case the pump has to work against on every cycle.
Friction is what the force main and its fittings take out. It rises roughly as the square of the flow and falls extremely fast as the pipe gets fatter, which is why the same job on 1-1/4 inch and 2 inch pipe can look like two different problems.
Residual head is the term people leave out, and on a pressure distribution system it is the whole point. The orifices only discharge because there is pressure behind them. If the design calls for a certain head at the distal orifice, the pump has to still be producing it after the lift and the friction have taken their share. A gravity-fed system does not have this term at all, which is where the habit of ignoring it comes from.
The friction arithmetic
This page uses Hazen-Williams, which is the usual choice for water and effluent at ordinary temperatures in pipes of this size. In the form that takes gallons per minute and inches of bore:
head loss in ft per 100 ft = 0.2083 × (100 ÷ C)1.852 × Q1.852 ÷ d4.8655
Two features of that expression are worth carrying around. The flow exponent of 1.852 means friction is a bit under the square of flow — twenty percent more flow is about forty percent more friction. The diameter exponent of 4.8655 means friction is savagely sensitive to bore: at the same flow, going from 1.38 inches to 1.61 inches of bore leaves about 47 percent of the friction, and going on to 2.07 inches leaves about 14 percent — a seventh of what it was, for two nominal sizes.
C is the roughness coefficient and it is a field left for you to fill in rather than a constant baked into the page, because the honest value depends on the pipe you actually have and how long it has been there. If you are checking a designer figure, use the C they used.
Velocity comes out separately, as 0.4085 × GPM ÷ d2 in feet per second. It matters on a force main because a line carrying solids has to move fast enough to keep them moving.
Reading it against a curve
A pump does not have a capacity, it has a curve: it delivers less as the head rises, and where it settles is the point at which its curve crosses the head the system demands. That is why a pump advertised at a flow figure can deliver something quite different once it is in your tank at the end of your force main.
Enter two or three points read off the curve in your own pump documentation and this page will draw a straight line between them and find where it crosses the system curve. Two caveats, both real. A straight line between three points is coarser than the printed curve, so the crossing is approximate. And the page will not extrapolate beyond the points you typed — if the crossing is outside them it says so rather than inventing a number, because a made-up point on a pump curve is worse than no point.
What the number does not settle
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.
A crossing point tells you where the hydraulics land. It says nothing about whether the pump is rated for the duty cycle your dosing produces, whether it handles effluent of that quality, whether the main will stay clear at that velocity, whether the electrical supply and controls suit it, or whether any of it matches the permit. Those are the parts a designer and an installer are for.
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.
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.
Where the other numbers come from
The flow you need at the field is the sum of what the orifices will pass, which is at the pressure distribution orifice calculator. The dose volume and how often the pump starts are at the dose volume and cycle calculator. For friction on ordinary household supply piping in psi rather than feet, with a full fittings table, use the water flow pressure drop calculator; for a surface water feature where the flow comes off a weir instead, the waterfall pump sizing calculator does the same head arithmetic with a power and running cost line attached.
Questions people ask
Why is my calculated head so much higher than the vertical rise?
Because the rise is only one of three terms, and on a long run of small pipe it is often the smallest. A hundred and sixty feet of 1-1/2 inch main at thirty gallons a minute takes real head to push through, and if the design also calls for pressure remaining at the far orifice then that gets added on top. The breakdown at the top of the results is there precisely so you can see which term is doing the damage. If friction is the big one, the sensitivity is in the pipe bore rather than anywhere else, because the friction term goes as diameter to the power of about 4.87.
What Hazen-Williams C should I use?
Whatever fits the pipe you have, which is why it is an input rather than a constant. New smooth plastic is normally taken at a high value; older pipe, pipe that has carried effluent for years, and metal pipe are taken lower, and the drop over the life of a line is not trivial. If you are checking somebody else arithmetic, the only useful thing to do is use the same C they used, otherwise you are comparing two different calculations. If you are trying to be cautious about a line that will age, running the numbers at a lower C as well shows you how much margin the difference costs.
Do I measure the lift to the top of the tank or the water level?
To the water level at the moment the pump shuts off, which is the lowest the level goes and therefore the highest lift the pump has to work against. Measuring to the top of the tank overstates it, sometimes by several feet, and measuring to the pump-on level understates it. The other end is wherever the effluent actually leaves the pipe and becomes free of the system, which on a pressure field is the orifice elevation rather than the ground surface above it.
The curve does not cross the system curve. What does that mean?
It means the crossing lies outside the flow range covered by the points you entered, or the two curves do not meet in that range at all. Add points further along the curve from the pump documentation, particularly a shutoff head point at zero flow and a point out at the high-flow end, and run it again. The page deliberately refuses to extend either curve past your data, because extrapolating a pump curve produces a confident-looking number that has no basis, and people act on it.
Can I use this to pick a pump?
No, and the distinction matters. It works out the head the hydraulics demand, which is one input among several into selecting a pump for an onsite system. The others include the duty the dosing regime imposes in starts per day, whether the pump is rated for effluent of the quality leaving your tank or a filtered version of it, the minimum velocity the force main needs, the electrical supply, the control panel, freeze protection, and above all whether the whole arrangement matches the permitted design. Effluent pumps on onsite systems are specified by the designer and installed by licensed people, and a head figure is something to bring to that conversation rather than a substitute for it.