The equation, and why it is worth knowing by heart
Water leaving a hole under pressure follows the orifice equation. Discharge is the coefficient times the area times the velocity, and the velocity comes from the head. Converted into the units anybody actually works in — gallons per minute, inches of diameter, feet of head — it collapses to:
Q = Cd × 19.637 × d2 × √h
A 1/8 inch hole with a coefficient of 0.6 at five feet of head gives 0.6 × 19.637 × 0.015625 × 2.236, which is 0.41 gallons a minute. Eighty of those holes in a field is about 33 gallons a minute, and that is the flow the pump has to produce before anything else is considered.
The two exponents are the whole story. Diameter squared, so a small change in drill size is a large change in flow. Square root of head, so a large change in pressure is a small change in flow. Anybody who has spent time on these systems has absorbed that asymmetry, and anybody who has not tends to reach for the pressure when the problem is the holes.
What the coefficient is doing there
Water does not leave a hole as a clean cylinder the diameter of the hole. It contracts to a narrower jet just downstream, and it loses a little to friction on the way through. The discharge coefficient bundles both effects into one multiplier, and its value depends on how the hole was made: sharp-edged and clean in thin material behaves differently from a hole with a burr, a ragged edge, or a long bore through thick wall.
It is a field on this page rather than a constant because the right value belongs to the design and to the way the laterals were actually drilled. If you are checking somebody arithmetic, use the coefficient they used.
Every orifice does not flow the same, and the design knows that
The arithmetic here treats every hole as seeing the same head. In a real lateral it does not: pressure falls along the pipe as water leaves it, so the first orifice sees more head than the last one and discharges more. Distribution designs manage this deliberately, through the pipe size relative to the total discharge, the number of orifices, the position of the manifold, and the residual head specified at the far end.
The square-root relationship is what makes it workable. If the far orifice sees eighty percent of the head the near one does, it flows at the square root of 0.8, which is about ninety percent — a ten percent spread from a twenty percent pressure difference. That forgiveness is the reason the whole approach works at all, and it is why the residual head at the distal end is a specified design figure. Getting the head at the far end is the total dynamic head problem.
Squirt height
Head in feet and the height a jet rises out of a vertical tube are the same number, less a little to air resistance and turbulence. That is a useful accident, because it means the pressure in a lateral can be verified in the field with a piece of clear tube rather than a gauge, and a set of laterals can be compared end to end by eye. It is a commissioning and service check performed by whoever is responsible for the system, and the height it is read against comes from the design and not from anywhere else.
What this page is not
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.
Call 811 and have the utilities located before any digging, and allow days rather than an afternoon for it. Trench walls collapse without warning and buried people are recovered, not rescued. Once anything is in the ground, keep vehicles, trailers, stock and stored material off the field and off the reserve area: the soil structure is the working part of the system and compaction from one truck on wet ground is not repairable.
The rest of the arithmetic on the same system: dose volume and pump cycles, total dynamic head, aggregate and pipe take-off for what goes in the trench, and the drainfield area organizer for turning a flow and a loading rate into an area. Orifice discharge in a completely different setting — a stormwater basin emptying through an outlet over time rather than a lateral running at steady pressure — is at the basin drawdown calculator.
Questions people ask
What size should the orifices be?
That is on your design, and it is one of the figures that varies most between systems and jurisdictions. The size interacts with the head, the lateral diameter, the number of orifices, the spacing, the pump and the way the field is meant to be loaded, and changing one of those in isolation changes what the others do. This page will show you what any size you type produces at the head you type, which is useful for reading a design or understanding why a change matters, and it will not suggest a size.
Why does the far end of my lateral squirt lower than the near end?
Because pressure falls along a pipe as water leaves it through the holes, so the last orifice always sees less head than the first. Some difference is expected and designed for. The forgiving part is that flow follows the square root of head, so a twenty percent drop in pressure is only about a ten percent drop in flow. A large difference is a different matter and can point to a lateral that is partly blocked, orifices that have fouled or been enlarged, air trapped at a high point, or a design being run outside what it was drawn for. Sorting that out is service work on a permitted system, not a homeowner adjustment.
Can I drill the holes bigger if the field is not taking water?
No, on several counts. Flow goes as the square of the diameter, so a small increase in drill size is a large increase in the flow out of every hole in the field at once, which changes the loading of the soil, the run time, the dose and the head the pump has to hold. It also cannot be undone. And on a permitted system the laterals are part of a design that a jurisdiction approved, so altering them is a permit question before it is a hydraulic one. A field that has stopped accepting water is a call to the health department and to a licensed contractor.
How do I convert psi to feet of head?
A foot of water is 0.4333 psi, so feet times 0.4333 gives psi and psi divided by 0.4333 — which is the same as multiplying by 2.31 — gives feet. The page takes either; the selector at the top of the form switches which one it expects. It is worth being careful with the direction of that conversion, because five psi and five feet are very different things and both look plausible written on a page: five feet is about 2.2 psi, and five psi is about 11.5 feet.
Does this tell me if my pump is big enough?
It tells you the flow the orifices will pass at the head you entered, which is one of the two numbers you need. The other is what the pump will deliver against the total head the system imposes, and that is the total dynamic head calculator read against the curve for your actual pump. Even with both numbers in hand the conclusion belongs to the designer, because pump selection on an onsite system involves duty cycle, solids handling, force main velocity, controls and the permit, none of which are hydraulics.