Butterfly Valve Dimensions & Cv Flow Chart

Q: What is Cv and why does it matter when selecting a butterfly valve?

Cv is the flow coefficient equal to the number of US gallons per minute of 60 degree F water that flows through a fully open valve with exactly 1 psi of pressure drop across it. A higher Cv means less flow restriction. It matters for butterfly valves because the disc sits in the flow path even when open, and an undersized Cv creates excessive pressure drop while an oversized valve loses throttling authority in the normal operating range.

Q: What face-to-face standard do wafer butterfly valves follow?

Wafer butterfly valves follow API 609 and MSS SP-67 face-to-face dimensions. This means they drop directly into standard ANSI/ASME Class 125 and Class 150 flange gaps without requiring custom spool pieces. Always confirm the exact lay length from the manufacturer datasheet before cutting pipe.

Q: Does Cv change when the butterfly valve disc is partially open?

Yes, significantly. A butterfly valve delivers most of its full Cv rating only in the upper portion of the opening range, roughly 60 to 90 degrees open. Below about 30 degrees open the effective Cv drops sharply. This is why throttling with an oversized butterfly valve is difficult; most of the useful control range gets compressed into a narrow band near closed.

This reference gives typical flow coefficients (Cv) for resilient-seated wafer butterfly valves by nominal size, plus notes on bore and face-to-face dimensions. Use it to confirm a valve will control flow in a useful range — not just near full open.

What Cv means

Cv is the flow coefficient: the gallons per minute of 60°F water that pass through a fully open valve at 1 psi of pressure drop. Higher Cv means less restriction. A butterfly valve reaches most of its rated Cv only in the last ~30–70° of opening, so for throttling you want to operate in that controllable band.

Typical Cv by size

Approximate full-open values for resilient-seated butterfly valves. Always confirm against the manufacturer datasheet for the specific valve.

Nominal size	Typical full-open Cv (approx.)
2"	~120
2-1/2"	~190
3"	~340
4"	~630
5"	~1,000
6"	~1,500
8"	~2,900
10"	~4,900
12"	~7,400

Bore and face-to-face

Wafer butterfly valves have a near-full bore (the disc sits in the flow path). For reference, our 4" stainless-disc valve has a 4.10" bore and the 10" has a 9.87" bore. Face-to-face (lay length) follows API 609 / MSS SP-67 for wafer valves, so they drop into standard ANSI 125/150 flange gaps. Confirm exact lay length on the datasheet before finalizing pipe spool dimensions.

Sizing tips

For on/off service, size the valve to the line. For throttling, pick a size where your normal flow falls in the 30–70° open range — an oversized valve does all its control in a narrow, twitchy band near closed. When in doubt, share your flow rate and pressure drop and we'll help size it.

Shop butterfly valves

See sizes and configurations: Butterfly Valves. Related: selection guide and lever vs gear operators.

Frequently Asked Questions

What is Cv and why does it matter when selecting a butterfly valve?

Cv is the flow coefficient — it equals the number of US gallons per minute of 60°F water that flows through a fully open valve with exactly 1 psi of pressure drop across it. A higher Cv means less flow restriction. It matters for butterfly valves because the disc sits in the flow path even when open, and an undersized Cv creates excessive pressure drop while an oversized valve loses throttling authority in normal operating range.

How do I read the Cv chart to pick the right valve size?

Find your required flow rate and acceptable pressure drop, then use the Cv formula (Q = Cv × √ΔP for water) to calculate the minimum Cv you need. Match that to the chart — for on/off service, pick the valve whose full-open Cv meets or exceeds your requirement. For throttling service, aim for a size where your normal operating flow falls in the 30–70° open range, not near fully open or nearly closed.

What face-to-face standard do these butterfly valves follow?

Wafer butterfly valves from Apex Flow follow API 609 and MSS SP-67 face-to-face (lay length) dimensions. This means they drop directly into standard ANSI/ASME Class 125 and Class 150 flange gaps without requiring custom spool pieces. Always confirm the exact lay length from the manufacturer datasheet before cutting pipe, since even small deviations can affect flange fit.

Does Cv change when the butterfly valve disc is partially open?

Yes, significantly. A butterfly valve is highly non-linear: it delivers very little flow restriction — and most of its full Cv rating — only in the upper portion of the opening range (roughly 60–90° open). Below about 30° open the effective Cv drops sharply. This is why throttling with an oversized butterfly valve is difficult; most of the useful control range gets compressed into a very narrow band near closed.

Shop Related Products

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Technical data disclaimer: Specifications, dimensions, pressure/temperature ratings, and compatibility information on this page are compiled from manufacturer publications and are provided for reference only. Always verify suitability for your application against the manufacturer’s current documentation before purchase or installation. Apex Flow Solutions assumes no liability for errors, omissions, or misapplication of this information.