Half an inch of error, a third of the flow
Put the defaults in — 30 GPM arriving, four 4-inch outlets sitting at 0, 1/8, 1/4 and 1/2 inch above the lowest — and the water in the box stands 1.29 inches over the bottom outlet. Trench 1 takes 10.45 GPM, which is 34.8 percent. Trench 4, half an inch higher, takes 4.05 GPM, or 13.5 percent. An even split would be 7.5 GPM and 25 percent each, so the low trench is getting nearly forty percent more than its share and the high one is getting about half of its share.
Half an inch is roughly the thickness of two stacked quarters, and it is not much of a target to hit in a hole in the ground with a box set on bedding that will be walked on before it is backfilled.
Why the head is so small
Everything follows from the fact that the box only ever fills an inch or so. Flow through a partly full opening rises roughly as the square of the depth over the invert while the opening is still shallow, because both the width of the wetted slot and the velocity through it are growing at once. So the box only needs a little over an inch of water to pass 30 GPM through four 4-inch stubs — and once the total head is one inch, an eighth of an inch is an eighth of everything.
Reverse it and the same mechanism is reassuring. Push more flow through and the head rises, and the fixed level error becomes a smaller share of it. The page prints this: with three outlets level and one lifted a sixteenth of an inch, the high outlet takes 22.0 percent at 15 GPM, 22.9 at 30, 23.6 at 60 and 24.1 at 120. It never reaches 25, but it stops mattering much. A pumped system running a fixed dose rate is therefore working in a friendlier part of this curve than a gravity box handling a trickle.
The outlet that gets nothing
The sharp case is an outlet sitting above the water surface entirely. At low flows that is easy to arrange with a quarter inch of settlement, and the page says so plainly rather than reporting a very small number: the invert is above the water and it takes nothing at all. Meanwhile the trenches that are low take everything, which is how a system with plenty of field on paper concentrates its entire loading into one run.
What the arithmetic is not
It is an orifice model of stubs discharging freely, and it describes a box that is doing its job badly rather than a box that has stopped working. If a trench line has backed up and drowned its outlet, the level in that pipe sets the flow and nothing here applies. If the inlet arrives as a jet across the box, the water surface is not flat and nothing here applies either. And whether any of this matters for your system, whether a box needs levelling devices, and what should be done about a field taking uneven flow are all questions for whoever holds the permit — not for a page that multiplies areas by velocities.
Questions people ask
How level does a distribution box have to be?
That is set by whoever permits and inspects the system where you live, and this page does not state it. What it can tell you is the consequence of whatever you measure: at 30 GPM through four 4-inch outlets, a half-inch spread across the inverts turns a 25 percent share into 34.8 percent for the lowest and 13.5 percent for the highest.
Why does a small level difference matter so much?
Because the total head driving the box is small. Passing 30 GPM through four 4-inch stubs takes only about 1.3 inches of water over the lowest invert, so an eighth of an inch is roughly a tenth of the entire driving head. Flow through a partly full opening also grows faster than the depth does, which widens the gap further.
Does the split even out at higher flow?
It gets closer but never even. With one outlet a sixteenth of an inch high and the rest level, that outlet takes 22.0 percent of a 15 GPM flow, 22.9 percent at 30 GPM and 24.1 percent at 120. The head rises with flow, so a fixed level error becomes a smaller share of it. The error itself does not go away.
What flow should I enter for a gravity system?
Not the daily average — the box never sees that rate. A gravity box gets a fixture discharge for a few seconds at a time, so the interesting number is a peak: what leaves the tank when a bath or a washing machine dumps. The household flow calculator gives daily gallons, but the surge rate is what a box actually splits.
Can I check my own box?
This page gives no procedure for opening, entering or working in one, and that is deliberate. A distribution box is a small buried chamber on a wastewater system, with everything that implies about gas and oxygen, and a lid off is a hole in the ground. What is inside one is a job for the people who do it for a living.