Friction is the small half
Work the defaults on the form — 100 emitters at 1 GPH on 2 ft spacing, 0.57 in bore, 20 psi at the header, climbing 4 ft over the 200 ft run — and the pressure at the last emitter is 16.87 psi. Of the 3.13 psi that went missing, 1.73 is the climb and only 1.40 is friction.
That ratio surprises people who have sized water pipe before, and it comes from the shape of the flow. A supply pipe carries its whole flow to the far end, so every foot of it rubs at full rate. A drip lateral is being drained the entire way: the segment at the header carries all 100 emitters, the segment before the end cap carries one. Loss goes as flow to the 1.85 power, so nine tenths of the run is carrying a fraction of the flow and rubbing at a small fraction of the rate. Elevation has no such taper. The last foot of climb costs 0.43 psi exactly like the first one did, which is why a lateral pointed up a slope behaves quite differently from the same lateral laid along it.
The label does not describe the lateral
The emitters on the default run are rated 1 GPH at 15 psi and the header is at 20 psi, so nothing on that lateral is running at 1 GPH. The page returns 109.75 GPH for the run, not 100 — nearly ten percent above what multiplying emitters by the label gives. Set a run time from the label figure and you put on ten percent more water than you meant to, and size the zone from the label and the valve sees more flow than the paper says.
The two ends differ as well. The wettest emitter, right at the header, delivers 1.153 GPH; the driest, at the cap, delivers 1.060. That is an 8.0 percent spread produced by a pressure spread of only 1.18 to 1, because a turbulent path emitter follows the square root of pressure and a square root flattens differences rather than magnifying them.
What the exponent buys
Run the same geometry with a perfectly compensating emitter and the page reports the second solve: 100.00 GPH for the lateral, every emitter at its label, zero flow spread, and 17.06 psi at the far end rather than 16.87. The pressure profile barely moves — the compensating lateral carries slightly less water so it rubs slightly less — but the flow profile flattens completely.
The honest reading of that pair is that on this run, compensation is buying 8 percent. On a lateral with real grade in it, the same 0.5 exponent turns a much wider pressure spread into a much wider flow spread, and that is where compensation stops being a refinement. The page prints both solves rather than telling you which emitter to buy, because the answer depends entirely on how much pressure spread your ground produces.
Length is not the only lever
The shorter-and-longer block at the bottom re-solves the whole lateral at half, three quarters, one and a quarter and one and a half times the emitter count, keeping the grade the same so a longer run climbs proportionally further. On the defaults the 50 emitter version holds 98.8 percent uniformity and the 150 emitter version falls to 92.8 — a slow, well behaved decline rather than a cliff, which is why laterals get pushed further than they should and nothing obvious happens for a season or two.
What the block will not tell you is which trade you should make. Splitting one long lateral into two shorter ones fed from a common header halves the flow in each and cuts the friction sharply, but it costs header, fittings and a second flush point, and it does nothing about the grade. Feeding a single lateral from both ends does something similar for less material. Neither is priced here.
Questions people ask
How long can a drip lateral be?
There is no fixed answer, because the limit is set by the bore, the emitter flow, the spacing, the inlet pressure and the grade together. Put your own figures in and read the far-end pressure and the flow spread. On the defaults here — 1 GPH emitters at 2 ft on 0.57 in tubing, 20 psi in, 4 ft of climb — 200 ft ends at 16.87 psi and 96.6 percent uniformity, and 300 ft ends lower. The tubing maker publishes maximum run tables for their own product at stated conditions, and those are worth reading against this.
Why is the friction loss so much smaller than a pipe calculator says?
Because the flow is being taken away all along the run. The segment at the header carries every emitter, the segment before the cap carries one, and friction goes as flow to the 1.85 power, so most of the length is rubbing at a small fraction of the header rate. A plain pipe calculation sends the whole flow to the far end and overstates a drip lateral by a large factor. This page steps segment by segment instead.
What is the emitter exponent and where do I find it?
It is the power that relates flow to pressure: flow goes as pressure to the power x. Some makers publish it. If yours does not, take two rows off the flow chart and use the derive option on the form — x is the log of the flow ratio divided by the log of the pressure ratio. A plain turbulent path lands near 0.5, a long labyrinth path higher, and a pressure compensating emitter near zero inside the band the maker quotes.
Does a pressure compensating emitter fix a sloped run?
Only inside its band. Compensation holds flow steady between two pressures the maker publishes, and outside that band the emitter behaves like an ordinary one. The band check on this form counts how many emitters on your run fall below the low end or above the high end using the figures you enter. Whether that matters for your planting is not something this page can say.
Should I measure the tubing bore or use the nominal size?
Measure it, or read the spec sheet. Half inch drip tubing is not half an inch inside and the bore differs between makers by enough to matter — friction goes as the bore to the 4.87 power, so a 0.57 in tube and a 0.60 in tube are not close to the same pipe. The nominal name is a name, not a dimension.