How do you calculate pressure drop in a pipe?

Add the equivalent length of the fittings to the length of the run, multiply by the friction loss per foot from the Hazen-Williams formula, then add 0.433 psi for every foot the water climbs. For 5 gpm through 50 ft of 3/4 in Type L copper with four elbows, one tee and a 10 ft climb, that is 5.99 psi.

How do I calculate flow rate in a pipe?

Flow is velocity times the bore area. In gallons per minute, Q = v × d² / 0.4085, with v in feet per second and d the inside diameter in inches. 3/4 in Type L at 5 ft per second carries about 7.5 gpm.

How many gallons are in a pipe?

Multiply the length by the gallons per foot for the tube. The Copper Tube Handbook gives 1/2 in Type L as 0.0121, 3/4 in as 0.0251 and 1 in as 0.0429 gallons per foot, so 100 ft of 3/4 in holds 2.51 gallons.

What velocity is too fast for copper pipe?

The handbook says velocities in excess of 5 to 8 ft per second are not recommended, and it stops tabulating friction loss above 8. This page warns above 8 and notes anything between 5 and 8.

Does Type L or Type M change the pressure drop?

Yes, slightly. The thinner wall of Type M leaves a larger bore: 3/4 in Type M is 0.811 in inside against 0.785 in for Type L, so at the same flow it loses a little less pressure per foot.

Home › Plumbing › Pipe Pressure Drop Calculator

Pipe Pressure Drop Calculator

CDA Copper Tube Handbook, Tables 14.2, 14.6 and 14.7

Pressure drop in a water pipe is friction over the length of the run, plus the fittings counted as extra length, plus the height the water climbs. Most calculators forget the fittings, and in a short run with a few elbows they can be a third of the answer.

Water pressure drop and volume in Type K, L or M copper tube

Tube type

Nominal size

Flow

gpm

Length of the run

90° elbows

Tees, flow through the branch

Height the water climbs (negative if it drops)

Hazen-Williams C (150 in the handbook)

Water at ordinary temperatures, straight tube plus soldered fittings.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

Pressure drop by cause

Worked example

5 gpm through 50 ft of 3/4 in Type L copper with four 90° elbows and one tee branch, climbing 10 ft.

Inside diameter. Table 14.2b gives 3/4 in Type L a bore of 0.785 in.
Velocity. 0.4085 × 5 / 0.785² = 3.3 ft per second.
Friction. 4.52 × 5^1.85 / (150^1.85 × 0.785^4.87) = 2.72 psi per 100 ft.
Fittings. Table 14.7 counts a 3/4 in 90° ell as 2 ft and a tee branch as 3 ft: 4 × 2 + 3 = 11 ft of extra tube.
Over the run. 61 ft at that rate is 1.66 psi, and 10 ft of rise adds 4.33 psi, for 5.99 psi in all.

The fittings add 11 ft to a 50 ft run, 22 percent more friction than the tube alone. The climb costs more than all the friction together, which is why the top floor of a house on a low-pressure main is where the shower goes weak.

The formula

The Copper Tube Handbook's note to Table 14.6:

P = 4.52 Q^1.85 / (C^1.85 d^4.87)

P: friction loss, psi per linear foot
Q: flow, gallons per minute
d: average inside diameter, inches, from Table 14.2
C: constant, 150 for copper
velocity: 0.4085 Q / d² ft per second, which is gallons per minute turned into a speed through the bore
elevation: 0.433 psi per foot, the weight of a foot-high column of water on a square inch

Fittings are added to the length before the friction is multiplied out, using the handbook's equivalent lengths.

Checked against the handbook's own table

The formula reproduces the printed table. Computed at C = 150 with the Table 14.2 bores, 120 of 129 cells checked in Table 14.6 match the handbook to three decimal places. The rest are cells the handbook leaves blank above 8 ft per second or rounds differently in the third place.

It is the 2026 edition. CDA Publication A4015-26/24, read from the Copper Development Association's own site. Older copies circulating online are the 2006 edition, with the tables numbered differently.

Why the fittings matter more than people think

A 3/4 in elbow resists flow like 2 ft of straight tube, and a tee with the water turning through the branch like 3 ft. On a long main that is noise. On a 20 ft run to a bathroom with six elbows and two tees it is 18 ft, nearly doubling the friction.

The equivalent lengths in Table 14.7 are for streamlined soldered fittings. The handbook says to double them for threaded fittings.

How much water is in a pipe

Volume is the bore area times the length, and the handbook prints it per foot. 3/4 in Type L holds 0.0251 gallons per foot, so 50 ft holds 1.26 gallons. That is the water you run off before hot water arrives at a fixture, which is why the smallest pipe that meets the flow is also the one that wastes the least waiting.

Frequently asked questions

How do you calculate pressure drop in a pipe?: Add the equivalent length of the fittings to the length of the run, multiply by the friction loss per foot from the Hazen-Williams formula, then add 0.433 psi for every foot the water climbs. For 5 gpm through 50 ft of 3/4 in Type L copper with four elbows, one tee and a 10 ft climb, that is 5.99 psi.
How do I calculate flow rate in a pipe?: Flow is velocity times the bore area. In gallons per minute, Q = v × d² / 0.4085, with v in feet per second and d the inside diameter in inches. 3/4 in Type L at 5 ft per second carries about 7.5 gpm.
How many gallons are in a pipe?: Multiply the length by the gallons per foot for the tube. The Copper Tube Handbook gives 1/2 in Type L as 0.0121, 3/4 in as 0.0251 and 1 in as 0.0429 gallons per foot, so 100 ft of 3/4 in holds 2.51 gallons.
What velocity is too fast for copper pipe?: The handbook says velocities in excess of 5 to 8 ft per second are not recommended, and it stops tabulating friction loss above 8. This page warns above 8 and notes anything between 5 and 8.
Does Type L or Type M change the pressure drop?: Yes, slightly. The thinner wall of Type M leaves a larger bore: 3/4 in Type M is 0.811 in inside against 0.785 in for Type L, so at the same flow it loses a little less pressure per foot.

Check these numbers yourself

The Copper Tube Handbook, CDA Publication A4015-26/24Copper Development Association, 2026. Read 2026-09-15. Tables 14.2a, 14.2b and 14.2c tube dimensions and volume per foot; Table 14.6 and its note giving the Hazen-Williams formula with C = 150 and the velocity limits; Table 14.7 equivalent lengths of fittings.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-15. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

For water in copper tube. The C factor and the bores are the handbook's for copper; PEX, CPVC and steel have different bores and different C factors, and nothing here converts them. The pressure available at the fixture also depends on the meter, the backflow preventer and any water heater in the path, which this page does not see.