In short: a fitting's pressure drop is ΔP = K × ρV²/2 — the fitting's K-factor times the flow's velocity head. Two 90° elbows, a tee and an open gate valve (K = 3.0) in a 1-inch pipe at 10 GPM adds about 0.34 psi. A single fully-open globe valve alone (K = 10) costs more than that whole combination — about 1.12 psi. Use the calculator above to total up your own fittings.
These figures are planning aids based on published K-factors. For code-required system design, confirm with a licensed engineer.
How to calculate pressure drop across a fitting
Every elbow, tee, valve or other fitting resists flow by an amount proportional to the flow's kinetic energy — its "velocity head." That proportionality constant is the fitting's K-factor:
Worked example. 10 GPM through 1-inch (ID) pipe gives a velocity of 1.245 m/s. With two 90° elbows (K = 0.9 each), one branch tee (K = 1.0) and one open gate valve (K = 0.2) — the calculator's default — total K = (2 × 0.9) + 1.0 + 0.2 = 3.0. ΔP = 3.0 × 998.2 × 1.245² / 2 ≈ 2,321 Pa, which converts to 0.337 psi.
Valve choice matters more than you'd think. A fully open globe valve (K ≈ 10) forces flow through an S-shaped internal path even wide open, giving it roughly 50 times the resistance of a fully open gate valve (K ≈ 0.2). Swapping valve types on a critical run can matter more than several ordinary fittings combined — see the table below.
Why fitting type, count and pipe size change the answer
Fitting type sets K. Smoother, straighter flow paths (gate valves, long-radius bends) have low K-factors; anything that forces a sharp turn or a narrow, twisting path (globe valves, sharp tees) has a much higher one.
Count adds up directly. K-factors for fittings in series simply sum, so a run with several elbows can add up to as much resistance as a single high-K valve.
Pipe diameter drives velocity, and velocity is squared. The same K-factor produces very different pressure drops depending on pipe size, because a narrower pipe means higher velocity for the same flow rate — and pressure drop scales with velocity squared. See the pipe pressure drop calculator for how diameter affects straight-run friction the same way.
Reference: typical K-factors (Crane TP-410)
Published resistance coefficients used by the calculator. These are typical values for standard fittings — actual K can vary by manufacturer and fitting design.
| Fitting | K-factor |
|---|---|
| 90° standard elbow | 0.90 |
| 45° elbow | 0.40 |
| Tee, branch flow | 1.00 |
| Coupling / union | 0.08 |
| Gate valve, fully open | 0.20 |
| Ball valve, fully open | 0.05 |
| Swing check valve | 2.50 |
| Globe valve, fully open (highest) | 10.00 |
Worked examples
2 elbows + tee + gate valve
0.337 psi
The calculator's default combination (K = 3.0) at 10 GPM through 1-inch pipe.
One globe valve alone
1.122 psi
A single fully-open globe valve (K = 10) at the same flow and pipe size costs more than the whole four-fitting combination above.
Same fittings, doubled flow
1.347 psi
The default K = 3.0 combination at 20 GPM instead of 10 — pressure drop roughly quadruples as velocity doubles.
What this calculator doesn't include
Straight-pipe friction. This tool covers fittings and valves only. Add the separate friction loss along the pipe runs between fittings using the pipe pressure drop calculator or friction loss calculator.
Partially open valves. K-factors here assume valves are fully open. A partially closed valve has a much higher, and much less predictable, K-factor — often high enough to dominate the whole system.
Fitting orientation effects. Two fittings close together can interact and produce a different combined loss than the simple sum used here, especially at high velocity. For critical designs, consult manufacturer data or a piping engineer.
Frequently asked questions
What is a K-factor?
A K-factor (resistance coefficient) measures how many "velocity heads" of pressure a fitting or valve consumes, independent of pipe size. A 90-degree elbow with K = 0.9, for example, loses pressure equal to 0.9 times the velocity head of the flow passing through it, regardless of whether the pipe is half an inch or six inches across.
Why does valve type matter so much?
Different valve designs force the flow through very different paths. A gate valve, fully open, barely disturbs the flow (K around 0.2), while a globe valve forces the flow through an S-shaped path even when fully open (K around 10) — roughly 50 times more resistance for the same nominal size and flow rate.
How do I combine multiple fittings?
K-factors simply add together for fittings in series carrying the same flow: total K equals the sum of each fitting's K-factor multiplied by how many of that fitting are in the line. That combined K is then used once in the pressure drop equation with the line's flow velocity.
Does pipe diameter affect the K-factor itself?
Not directly — K-factors for standard fittings are dimensionless numbers that stay roughly the same across pipe sizes. But diameter strongly affects the resulting pressure drop, because it changes the flow velocity for a given flow rate, and pressure drop depends on velocity squared.
How does K-factor relate to equivalent length?
Equivalent length converts a fitting's K-factor into the length of straight pipe that would produce the same pressure drop, using the pipe's own friction factor: Le = K × D / f. It's a convenient way to fold fitting losses into a single "total developed length" for a friction loss calculation. See the equivalent length calculator for that conversion.
Does this include straight-pipe friction loss?
No. This tool calculates only the losses from fittings and valves themselves (minor losses). Straight-pipe friction along the run between fittings is a separate calculation — see the pipe pressure drop calculator or friction loss calculator, and add the two results together for a full picture.
Sources & references
- Crane Co. (2013). Flow of Fluids Through Valves, Fittings, and Pipe (Technical Paper No. 410).
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)