How much pressure does a valve lose?
Every irrigation control valve, ball valve or gate valve restricts flow a little, and that restriction shows up as a drop in pressure between its inlet and outlet. The amount depends on the valve's internal design and size, and on how fast water is moving through it, so there's no single figure that applies to every valve. That's why manufacturers rate each valve with a flow coefficient, called Cv in US units and Kv in metric units, that lets you calculate the loss at any flow.
Many residential and light-commercial irrigation control valves lose roughly 2 to 5 psi (about 14 to 35 kPa) at their rated flow. That's a typical range, not a rule: a valve throttled halfway closed, an undersized valve or an unusually restrictive body style can all lose considerably more. Your valve's own Cv, Kv or a manufacturer chart is the only reliable number for your situation.
This calculator is for the valve only. It doesn't include the pipe, fittings, backflow preventer or filter upstream and downstream of it. Add those separately when you're budgeting total system pressure.
How this valve pressure loss calculator works
The calculator uses the standard flow-coefficient equation that valve manufacturers and control-valve standards use to relate flow and pressure loss, then converts the result into a few other useful forms.
1. Pressure loss from Cv or Kv
US units
Pressure loss (psi) = (Flow (gpm) ÷ Cv)²
This is the defining equation for the flow coefficient Cv, for water at typical temperature with fully turbulent flow.
Metric units
Pressure loss (bar) = (Flow (m³/h) ÷ Kv)²
The calculator converts your flow from L/min to m³/h (× 0.06) and the result from bar to kPa (× 100) automatically.
2. Finding a flow coefficient from a chart reading
If you only have one flow-and-loss point from a manufacturer's chart rather than a published Cv or Kv, the calculator rearranges the same equation to solve for the coefficient, then uses it for your own flow.
Effective flow coefficient
Cv = Reference flow ÷ √(Reference pressure loss)
Works the same way for Kv, using m³/h and bar. One good reading is usually enough, since Cv and Kv are properties of the valve, not the flow.
3. Loss expressed as head
Pressure loss is also shown in feet or meters of head, since that's how some other hydraulics calculators on this site express results.
US units
Head (ft) = Pressure loss (psi) × 2.31
From the weight of water: 1 psi over the 144 sq in in a square foot equals 62.4 lb, the weight of a 1 ft³ column, so 144 ÷ 62.4 ≈ 2.31 ft of head per psi.
Metric units
Head (m) = Pressure loss (kPa) ÷ 9.81
From the same relationship using water's density (1,000 kg/m³) and standard gravity (9.81 m/s²).
4. Velocity through the valve
When you pick a valve size, the calculator estimates velocity using that size's inside diameter, based on standard Schedule 40 PVC dimensions, the pipe most commonly used for irrigation mainlines and valves in the US.
US units
Velocity (ft/s) = 0.408 × Flow (gpm) ÷ Diameter (in)²
Derived from Q = V × A, converting gallons per minute and inches into consistent units.
Metric units
Velocity (m/s) = 21.22 × Flow (L/min) ÷ Diameter (mm)²
The metric form of the same continuity equation.
Irrigation designers commonly keep velocity in irrigation pipe and valves at or below about 5 ft/s (1.5 m/s) as a guideline to limit water hammer risk and excessive pressure loss. It's a widely used design target rather than a single universal code number, so check your local plumbing code and the valve manufacturer's literature for any limit specific to your project.
When this calculator isn't the right tool
It's built for a single control, ball or gate valve rated with a Cv or Kv. Use a different tool for:
- Backflow preventers. These use their own published pressure-loss curves. See the backflow preventer pressure loss calculator.
- Straight pipe runs. Friction loss along pipe follows a different equation. See the irrigation pipe size calculator or the PVC friction loss chart.
- Your whole system's pressure budget. To see what's left after every component's loss, use the available irrigation pressure calculator.
- Velocity anywhere else in the mainline. Use the mainline velocity calculator for pipe runs rather than a single valve.
For a broader look at hydraulics across a whole irrigation system, the irrigation hydraulics calculators cover pipe sizing, velocity and pressure budgeting together.
Valve pressure loss calculation example
A 1 in irrigation control valve is rated Cv 10 on its spec sheet, and the zone it feeds runs at 16 gpm.
- Pressure loss: (16 ÷ 10)² = 1.6² = 2.56 psi, shown as about 2.6 psi.
- As head: 2.56 × 2.31 ≈ 5.9 ft.
- Velocity check: at a 1 in nominal size (1.049 in inside diameter), 0.408 × 16 ÷ 1.049² ≈ 5.9 ft/s, above the typical 5 ft/s guideline.
The same calculation in metric, from a chart reading
A valve's spec sheet gives no Kv directly, but its chart shows 2.4 m³/h (40 L/min) at 0.09 bar (9 kPa) of loss. The zone's actual flow is also 40 L/min through a 1 in (DN25) valve:
- Effective Kv: 2.4 ÷ √0.09 = 2.4 ÷ 0.3 = 8.0.
- Pressure loss at 40 L/min: (2.4 ÷ 8.0)² = 0.09 bar = 9 kPa, matching the chart reading exactly, as expected at the same flow.
- As head: 9 ÷ 9.81 ≈ 0.92 m.
- Velocity check: at a 1 in (26.6 mm inside diameter) valve, 21.22 × 40 ÷ 26.6² ≈ 1.2 m/s, within the typical 1.5 m/s guideline.
Using the result
Finding your valve's Cv or Kv
Start with the valve's spec sheet or installation instructions, searching by the exact model number printed on the valve body or its label. Manufacturers usually list Cv (US) or Kv (metric) as a single number for the fully open valve. Some publish a small table or chart of flow against pressure loss instead. Either works with this calculator: enter the number directly, or read one point off the chart and let the calculator work out the equivalent coefficient.
Applying the result
Add this valve's loss to the other losses in the same flow path, such as the pipe, fittings, a backflow preventer and a filter, then subtract the total from your static or service pressure to see what's left at the sprinkler or emitter. The available irrigation pressure calculator walks through that full budget.
Common mistakes
- Using a Cv from a different valve size or model. Cv and Kv are specific to one valve design; a similar-looking valve from another line can have a very different rating.
- Reading a chart point from the wrong flow range. A point taken far outside the flow you'll actually use can give a less reliable coefficient than one closer to your design flow.
- Forgetting the valve isn't the only loss. Pipe, fittings and other components each add their own loss on top of the valve.
- Ignoring a partly closed valve. A published Cv normally assumes the valve is fully open; a flow control valve set partway closed loses more than this calculator will show.
What changes a valve's pressure loss
The Cv or Kv equation gives an accurate estimate once you know the coefficient, but several things change what coefficient you're really working with.
| Factor | Effect on loss | What to adjust |
|---|---|---|
| Valve body style | Globe, angle and ball valves each restrict flow differently, so Cv varies by design even at the same nominal size. | Use the Cv or Kv from that exact valve model, not a similar one. |
| Valve size vs. flow | An undersized valve for the flow gives a low effective Cv relative to flow, raising both loss and velocity. | Check the velocity result; consider a larger valve if it's consistently high. |
| Partial closure | A flow control valve throttled below fully open has a lower effective Cv than its rated, fully-open value. | Use a chart reading taken at the actual opening, if the manufacturer publishes one. |
| Multiple valves in series | Losses across separate valves in the same flow path add together; they don't average out. | Calculate each valve separately and add the results. |
| Fluid other than water | This calculator assumes water (specific gravity 1); a denser fluid changes the loss for the same Cv. | Not applicable for standard irrigation water. |
| Upstream strainer or filter fouling | A partly clogged filter upstream reduces the pressure actually available at the valve, though it doesn't change the valve's own Cv. | Check and clean filters; measure pressure at the valve if losses seem high. |
Velocity matters alongside pressure loss. A valve that shows a modest pressure loss can still run at a high velocity if it's undersized for the flow, which is why this calculator reports both figures rather than pressure loss alone.
Frequently asked questions
What is Cv (or Kv) for a valve?
Cv, the flow coefficient, describes how much water a valve passes for a given pressure drop; Kv is the metric equivalent. A higher Cv or Kv means the valve loses less pressure at the same flow. Manufacturers publish these numbers, usually in the valve's spec sheet or installation instructions, sometimes as a single value and sometimes as a small chart across several flows.
Where do I find my valve's Cv or Kv?
Check the valve's spec sheet, installation instructions or the manufacturer's website, searching by model number. Many list Cv directly; others show a flow-versus-pressure-loss chart instead. If you can only find a chart, read one point off it, a flow and its matching pressure loss, and enter that as a data point instead of a Cv value.
What if I only have a chart, not a Cv number?
Use the data point option and enter one flow and pressure loss reading from the chart. The calculator works backward to find the flow coefficient that produced that point, then uses it to estimate the loss at your own flow rate. Reading the point from the middle of the chart, rather than its very ends, usually gives a more reliable estimate.
How much pressure does a control valve typically lose?
It depends heavily on the valve, its size and how far it's open, so there's no single number that fits every valve. Many residential and light-commercial irrigation control valves lose roughly 2 to 5 psi (about 14 to 35 kPa) at their rated flow, but this is only a typical range: check your valve's own Cv, Kv or chart for an actual figure.
Does pressure loss change a lot with flow?
Yes, and not in a straight line. Because loss follows flow squared, doubling the flow through the same valve roughly quadruples the pressure loss, while cutting the flow in half cuts the loss to about a quarter. A small increase in flow can add more loss than expected, especially through an undersized valve.
What velocity is recommended through an irrigation valve?
Irrigation designers commonly keep velocity in irrigation pipe and valves at or below about 5 ft/s (1.5 m/s) as a guideline to limit water hammer risk and excessive pressure loss, rather than as a fixed code requirement. Local codes and the valve manufacturer's own literature take priority where they specify a different limit.
Does valve size affect pressure loss?
Yes. A larger valve generally has a higher Cv or Kv and loses less pressure at the same flow, but only if the valve is actually sized to match that flow. Fitting an oversized valve to a low flow can cause control problems of its own, so match the valve to the flow it needs to handle rather than always choosing the largest size available.
Should I use this calculator for a backflow preventer or filter?
No. Backflow preventers, filters and other components have their own published pressure-loss curves, which behave differently from a valve's Cv equation. Use the backflow preventer pressure loss calculator for backflow devices, and check the manufacturer's data for filters and other inline components.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Formula, unit conversions and both worked examples verified by hand, including a round-trip check using a sample manufacturer data point.
- Velocity guidance and pipe dimensions checked against the sources below.
- Method follows the standard valve flow-coefficient (Cv/Kv) equation used across the valve industry.
Sources
- ANSI/ISA-75.01.01, Flow Equations for Sizing Control Valves [VERIFY URL]. Defines the flow-coefficient (Cv) equation this calculator uses.
- Irrigation Association, Landscape Irrigation Design Manual [VERIFY URL]. General reference for irrigation system hydraulics and pipe velocity guidance.
- USDA Natural Resources Conservation Service, National Engineering Handbook, Part 652, Irrigation Guide [VERIFY URL]. Reference for irrigation pipeline velocity guidance.
- ASTM D1785, Standard Specification for Poly(Vinyl Chloride) (PVC) Plastic Pipe, Schedules 40, 80, and 120 [VERIFY URL]. Source for the pipe inside diameters used in the velocity check.