Two valves, four and seven tenths of a psi
The defaults are a plain residential manifold: a zone valve whose maker publishes 2.5 psi of loss at 15 GPM, and a master valve publishing 2.2 psi at the same flow. The page converts each to a flow coefficient — 9.49 and 10.11 — and adds the losses at the zone flow to get 4.70 psi.
That is close to five psi gone before the water has reached a single foot of lateral pipe, and it is a share of the total that surprises people who have spent their afternoon worrying about whether to run 3/4 in or 1 in pipe. On a system with a filter and a backflow assembly in the same line, the devices routinely account for more of the loss than everything else put together.
The square is the whole story
Read the block that steps the flow. At 15 GPM the two valves cost 4.70 psi. At 22.5 GPM, half again as much flow, they cost 10.58 psi — not 7.05. At 30 GPM they cost 18.80.
Flow through a fixed opening goes as the square root of the pressure across it, so the relationship read the other way is a square. This is why combining two zones onto one valve so rarely works out the way the flow arithmetic suggests it should: the flow doubles, the pipe friction goes up by a factor of about 3.6, and the valve loss goes up by exactly four. It is also why splitting a zone is often the cheapest pressure you will ever buy, because the same square runs backwards and takes the device loss down to a quarter.
A bigger valve is not a free lunch either
The sizing block at the bottom takes whichever device is costing the most — the zone valve here, at 2.500 psi — and shows what a larger coefficient would do. A Cv 1.25 times bigger takes it to 1.600 psi, saving 0.900. Doubling the coefficient takes it to 0.625 and saves 1.875. The saving is real but it flattens fast, and the second doubling is worth much less than the first, because what you are removing is a fixed fraction of a number that is already shrinking. Nobody should expect a larger valve to buy back a system that is short by ten psi.
It is also worth saying that Cv is not a size. Two one inch valves from different manufacturers publish different coefficients, sometimes by a wide margin, and the only honest way to compare them is to read both data sheets. The form takes either a published coefficient or a published loss at a stated flow, and derives the coefficient from the second, because most irrigation manufacturers publish the table rather than the number.
What this page will not tell you
Whether the pressure left at the head is enough. That depends on the nozzle, and the nozzle chart is the manufacturer document that answers it. Whether your backflow arrangement is right, or even what it should be: that is code-governed permitted work and it appears here only as a loss figure you read off your own published data. And whether the flow you entered is the flow you actually have, which a bucket and a stopwatch at a flush cap will settle in two minutes and no amount of arithmetic will.
Questions people ask
What is Cv on an irrigation valve?
The flow coefficient: the flow in GPM that produces one psi of pressure drop across the valve with water. Loss at any other flow is the flow divided by Cv, squared. A valve with Cv 9.49 loses 2.5 psi at 15 GPM and 10 psi at 30. It is a measured property of one model at one size, published by the manufacturer, and it cannot be inferred from the pipe size.
My valve data sheet gives a loss table, not a Cv. Can I still use this?
Yes, and that is the usual case. Leave the Cv field blank and enter one row of the table — the flow and the loss at that flow. Cv is the flow divided by the square root of the loss, and the page derives it and then re-applies it at your own zone flow. Picking a table row near your actual flow gives the closest answer, because real valves do not follow the square law perfectly across their whole range.
Why did combining two zones cost so much more pressure than I expected?
Because valve and device loss goes as the square of the flow. Doubling the flow through a valve quadruples the loss through it, and pipe friction rises by roughly a factor of 3.6 at the same time. The far heads on the combined zone are the ones that feel it first, usually as a shorter throw and a ring of dry grass at the edge of the pattern rather than as anything dramatic.
Does a bigger valve solve a low pressure problem?
It buys back part of one device loss and then flattens out. On the defaults here, doubling the coefficient of the worst device saves 1.875 psi and taking it further saves progressively less, because you are removing a shrinking fraction of a shrinking number. If a system is short by a lot, the shortfall is usually somewhere else — the supply, the elevation, the flow being higher than the design assumed, or a zone carrying more heads than it can feed.
What about the backflow preventer?
It appears here only as a loss figure you read off your own published data for the device you have. What device is required, where it goes, how it is installed and how it is tested are set by the code your local authority has adopted, and it is permitted work for somebody licensed to do it. This page gives no procedure and makes no judgement about any of it.