Enter Your Hose Parameters

Coefficient (C): 15.5
GPM
Typical 1¾" attack: 150–200 GPM · 2.5": 250–325 GPM
FT
Standard 50-ft sections — enter total working length.
PSI
Friction Loss (FL)
—
PSI
Pressure lost to hose friction (FL = C × Q² × L)
Pump Discharge Pressure (PDP)
—
PSI
Set your pump panel to this value (FL + NP, level ground)
Elevation not included. Add ~0.434 psi per foot of rise (+5 psi/floor), subtract for downhill. Include appliance loss per your SOPs.

What Is Friction Loss in Firefighting?

Friction loss (FL) is the pressure drop that occurs as water flows through a fire hose due to friction between the water and the hose lining. The faster water flows (higher GPM) and the smaller the hose diameter, the greater the friction loss — and the more pressure the pump must generate to compensate.

Understanding friction loss is a core fireground hydraulics skill. If a pump operator under-compensates for FL, nozzle pressure drops below operational minimums — reducing reach, penetration, and suppression effectiveness. Over-compensating wastes pump capacity and can damage equipment or injure personnel from hose whipping at excessive pressures.

The three factors that control friction loss are:

  • Flow rate (GPM) — FL increases with the square of flow. Double the GPM, quadruple the FL.
  • Hose length — FL increases proportionally. Twice the hose = twice the FL.
  • Hose diameter — Larger bore hose has a lower coefficient (C) and far less FL at the same flow.

The Friction Loss Formula (FL = C × Q² × L)

FL = C Q² L
US fire service standard formula (IFSTA / NFPA)
  • FL Friction Loss in PSI
  • C Hose Coefficient (roughness + diameter)
  • Q Flow rate in GPM ÷ 100
  • L Hose length in feet ÷ 100

Why divide by 100?

The ÷100 simplification makes the math fireground-friendly. Instead of working with raw GPM and feet, you work with manageable single and double-digit numbers. For example, 150 GPM becomes Q = 1.5, and 200 ft becomes L = 2.

Note on formula variants: Some older texts use FL = C × Q² × (L/100) with un-divided Q — the result is identical if you apply the formula consistently. This calculator uses the most common US academy form where both Q and L are pre-divided by 100.

Standard Hose Coefficients (C Values)

The coefficient reflects the hose's internal resistance — a function of diameter and lining smoothness. Larger diameter = dramatically lower C = far less friction loss at the same flow.

Hose SizeCoefficient (C)Typical UseMax Recommended GPM
1 ½ inch24Booster, trash lines, wildland125 GPM
1 ¾ inch15.5Primary attack hose (most common)200 GPM
2 ½ inch2Heavy attack, exposure lines325 GPM
3 inch0.8Supply / intermediate LDH500 GPM
4 inch0.2Large diameter supply (LDH)1,000 GPM
5 inch0.08Large diameter supply (LDH)2,000 GPM

* Coefficients based on typical modern fire hose construction per IFSTA references. Always verify against your department's tested pump charts.

What Is Pump Discharge Pressure (PDP)?

Pump Discharge Pressure (PDP) is the total pressure a fire pump must generate at the pump outlet to overcome all resistance and deliver the correct nozzle pressure at the end of the hose lay. On level ground with no appliances: PDP = FL + NP.

In real-world fireground operations, additional factors modify PDP:

Full PDP Formula

PDP = FL + NP ± Elev + AFL
  • FL: Friction loss (calculated above)
  • NP: Nozzle pressure
  • Elev: ±0.434 psi per foot
  • AFL: Appliance friction loss

Common Appliance Friction Loss (AFL)

Gated wye10 psi
Siamese / FDC10 psi
Ladder pipe25 psi
Portable monitor20 psi
Aerial tip25 psi

For elevation: add 0.434 psi per foot of rise (approximately 5 psi per floor above the pumper in high-rise operations), and subtract the same amount for downhill lays. Use our PDP Calculator for complex multi-factor calculations.

Worked Examples — Step by Step

Example 1: 1¾" attack line (most common)

Hose: 1¾" (C=15.5) · 150 GPM · 200 ft · NP=100 psi

  • Q = 150 ÷ 100 = 1.5
  • L = 200 ÷ 100 = 2
  • FL = 15.5 × (1.5²) × 2 = 15.5 × 2.25 × 2 = 69.75 ≈ 70 psi
  • PDP = 70 + 100 = 170 psi

Example 2: 2½" heavy attack line

Hose: 2½" (C=2) · 250 GPM · 200 ft · NP=80 psi

  • Q = 250 ÷ 100 = 2.5
  • L = 200 ÷ 100 = 2
  • FL = 2 × (2.5²) × 2 = 2 × 6.25 × 2 = 25 psi
  • PDP = 25 + 80 = 105 psi

Example 3: Long 1¾" lay (400 ft)

Hose: 1¾" (C=15.5) · 150 GPM · 400 ft · NP=100 psi

  • Q = 1.5 · L = 4
  • FL = 15.5 × 2.25 × 4 = 139.5 ≈ 140 psi
  • PDP = 140 + 100 = 240 psi
  • ⚠ Near max pump pressure — consider 2½" for long lays.

Example 4: 5" LDH supply line

Hose: 5" (C=0.08) · 1000 GPM · 1000 ft · NP=20 psi (hydrant residual)

  • Q = 10 · L = 10
  • FL = 0.08 × 100 × 10 = 80 psi
  • Demonstrates why LDH is critical for long supply lays.

Quick Pump Chart Reference

Pre-calculated FL values for the most common fireground hose configurations (level ground, no appliance loss). Print and laminate for your pump panel.

GPM100 ft150 ft200 ft300 ft400 ftPDP (200ft, NP=100)
1001623314762131
1252436487397148
150355270105140170
175477195142189195
2006293124186248224

Highlighted row = most common fireground scenario. Values rounded to nearest PSI.

GPM100 ft200 ft300 ft400 ft500 ftPDP (200ft, NP=80)
20081624324096
2501325385063105
3001836547290116
32521426385106122
GPM100 ft500 ft1000 ft2000 ft
5002102040
7505234590
100084080160
15001890180360

Friction Loss FAQ

The standard US fire service formula is FL = C × Q² × L, where C is the hose coefficient, Q is flow rate in GPM divided by 100, and L is hose length in feet divided by 100. This formula is derived from IFSTA and NFPA hydraulics references and is the basis for most US fire academy training.

On level ground with no appliances: PDP = FL + NP. Full formula: PDP = FL + NP ± Elevation + Appliance Friction Loss. Add ~0.434 psi per foot of elevation gain; subtract for downhill. Common appliance losses: gated wye (10 psi), ladder pipe (25 psi).

The standard coefficient for 1.75-inch hose is 15.5. Some departments use slightly different values based on tested hose performance. Always verify against your department's pump chart. Low-friction attack hose may have a lower tested C value.

Friction loss is proportional to the square of the flow rate (Q²). Doubling GPM quadruples FL. For example, increasing from 100 to 200 GPM on 1¾" hose multiplies FL by 4×. This is why smooth bore nozzles at lower flows can be more efficient than fog nozzles at high flows on long lays.

Smooth bore handlines operate at 50 psi nozzle pressure. Smooth bore master streams (deck guns, monitors) use 80 psi. Combination/fog nozzles are typically rated at 75–100 psi depending on the manufacturer. Always confirm with your department SOP and nozzle manufacturer specs.

Use standard C values for training and quick mental math. Use a custom C if your department has conducted flow tests and published verified hose coefficients in your pump chart SOPs. Tested C values more accurately reflect your actual hose construction and condition.
Training reference only. Always follow your department's pump charts, SOPs/SOGs, and IFSTA references. Account for elevation and appliance loss on all real-world incidents. Verify all calculations before committing to fireground operations. Editorial policy · Our authors