What is friction loss?

Friction loss is the pressure water gives up rubbing against the inside of the hose as it flows. It depends mostly on how much water is moving — the flow enters the formula squared — and on the hose diameter and length.

How do you calculate friction loss?

Multiply the hose coefficient by the flow squared by the length, using the C × Q² × L form where Q is flow in hundreds of gallons per minute and L is length in hundreds of feet. The squared flow term is why a small jump in flow makes a large jump in loss. Hose coefficients vary by department — treat the result as a training estimate.

Why does bigger hose have less friction loss?

Because the friction coefficient falls sharply as the hose diameter grows. A wider line moves the same water at a much lower loss, so a big supply line can carry a high flow while giving up only a little pressure per length.

Hydraulics & Pressure

Friction Loss

Friction loss is the pressure water gives up rubbing against the inside of the hose as it flows. It depends far more on the flow than on the hose, and it grows with the square of the flow — double the flow and the loss roughly quadruples.

Friction loss · interactive

NOTABLE LOSS · 69.8 psiFriction is eating a real share of the line: the pump has to work harder to keep the nozzle pressure up. Watch the square-of-flow effect — a little more flow costs a lot more loss.

Flow150 gpm

Hose length200 ft

Friction loss grows with the square of flow: at this setting, doubling the flow multiplies the loss about 4×. Larger bores carry the same flow with far less loss. The good/notable/ severe call is an illustrative training cue, not an AHJ limit — these are training estimates; follow your SOPs and the hose's own data.

Friction loss grows with the square of flow for a fixed hose. Pick a hose diameter and drag the flow to watch pressure drain across the line — doubling the flow roughly quadruples the loss.

Every length of hose subtracts pressure between the pump and the nozzle. The amount depends on the hose's friction coefficient, the length of the lay, and — most of all — the flow, which enters the formula squared. That squared term is why a small jump in flow produces a large jump in loss.

Larger-diameter hose loses far less pressure at the same flow, so a big line moving a lot of water can need a lower pump pressure than a small line moving less. Trusting the math instead of the apparent size of the line is the habit friction loss teaches.

Formula

FL = C × Q² × L

C = hose coefficient (from the diameter); Q = flow in hundreds of gallons per minute; L = length in hundreds of feet.

Common questions

What is friction loss?: Friction loss is the pressure water gives up rubbing against the inside of the hose as it flows. It depends mostly on how much water is moving — the flow enters the formula squared — and on the hose diameter and length.
How do you calculate friction loss?: Multiply the hose coefficient by the flow squared by the length, using the C × Q² × L form where Q is flow in hundreds of gallons per minute and L is length in hundreds of feet. The squared flow term is why a small jump in flow makes a large jump in loss. Hose coefficients vary by department — treat the result as a training estimate.
Why does bigger hose have less friction loss?: Because the friction coefficient falls sharply as the hose diameter grows. A wider line moves the same water at a much lower loss, so a big supply line can carry a high flow while giving up only a little pressure per length.

Where this shows up

Calculator
Friction Loss
Calculator
Friction Loss Reference Table
Lesson
Friction Loss Basics