LDH Friction Loss: Complete Reference Chart for 3″, 4″, 5″ and 6″ Supply Hose

Every pump operator running an LDH supply line needs to know friction loss in the hose. Get the friction loss math wrong, and your pump discharge pressure will be too low to deliver target flow at the attack line — or too high, wasting pump capacity and risking hose failure. This reference guide gives you the formula, the coefficients, and complete pre-calculated friction loss tables for 3″, 4″, 5″ and 6″ supply hose at every realistic flow rate.

Whether you’re sizing a supply hose for a new apparatus build, calculating pump discharge pressure for a relay-pumping evolution, or determining how far you can move a target flow from a hydrant, the math comes down to one formula and one coefficient per hose size. The numbers below let you skip the calculator and read the answer directly.

The NFPA Friction Loss Formula

The standard fire-service formula for friction loss in any hose is:

The only variable that depends on hose construction is the coefficient C. Here are the standard NFPA coefficients for the most common fire hose sizes:

The coefficients tell the LDH story directly: a 5″ supply line has 25 times lower friction loss than a 2½” line at the same flow rate. That’s the math behind why LDH transformed fire-service supply operations from the 1990s onward.

For the complete picture of how these LDH sizes fit into the CA-FIRE product range — including matching rubber-covered LDH and hydrant supply hose — see our product pages.

LDH Friction Loss Reference Tables

The following tables pre-calculate friction loss for the most common combinations of LDH size, flow rate and hose length. All values are in psi total friction loss (not per 100 ft — total loss across the full hose length shown).

3″ Supply Hose Friction Loss (C = 0.8)

For 3-inch hose with 2½-inch couplings. Practical maximum flow: ~600-750 GPM.

4″ LDH Friction Loss (C = 0.2)

For 4-inch large diameter hose with Storz couplings. Practical maximum flow: ~1,000-1,250 GPM.

5″ LDH Friction Loss (C = 0.08)

For 5-inch large diameter hose with NFPA 1963 Storz couplings. Practical maximum flow: ~1,500-2,000 GPM.

6″ LDH Friction Loss (C = 0.05)

For 6-inch large diameter hose (industrial / refinery / port). Practical maximum flow: ~2,000+ GPM.

4″ vs 5″ LDH — The Math Behind the Choice

The friction-loss difference between 4″ and 5″ LDH drives one of the most important decisions in modern fire-service procurement. Here’s the head-to-head comparison at the most common flow rates:

The 5″ advantage is consistent: 5″ LDH has 2.5× lower friction loss than 4″ LDH at every flow rate. In practical terms, this means 5″ can either (a) deliver the same flow over 2.5× the distance, or (b) deliver 1.58× more flow over the same distance for the same friction loss budget.

Real-World Application: How Far Can I Move 1,000 GPM?

A practical procurement question: given a hydrant with 60 psi residual pressure at 1,000 GPM, how far can each LDH size deliver that flow before the friction loss eats all the available pressure?

The math: divide the available friction-loss budget (60 psi) by the friction loss per 100 ft at the target flow. The result is how many 100-ft sections you can run.

• 3″ supply at 1,000 GPM: 80 psi per 100 ft friction loss. 60 psi / 80 psi = 0.75 × 100 ft = 75 ft maximum. Not practical for any real supply lay.
• 4″ LDH at 1,000 GPM: 20 psi per 100 ft friction loss. 60 psi / 20 psi = 3 × 100 ft = 300 ft maximum. Adequate for short urban supply lays, limiting for any longer evolution.
• 5″ LDH at 1,000 GPM: 8 psi per 100 ft friction loss. 60 psi / 8 psi = 7.5 × 100 ft = 750 ft maximum. Comfortable for most municipal supply evolutions.
• 6″ LDH at 1,000 GPM: 5 psi per 100 ft friction loss. 60 psi / 5 psi = 12 × 100 ft = 1,200 ft maximum. Industrial-scale supply, refinery and port operations.

The numbers explain why 5″ LDH became the U.S. municipal fire-service standard. At typical hydrant pressures, 5″ delivers full pumper-rated flow over the supply distances most departments routinely face. 4″ works for short lays but limits operational flexibility. 6″ is overkill for municipal service but essential for industrial fire protection.

Practical Worked Examples

Example 1: Standard Residential Response

Setup: 500 ft of 5″ LDH from hydrant to engine pump. Target flow: 750 GPM.
Friction loss: FL = 0.08 × (7.5)² × 5 = 0.08 × 56.25 × 5 = 22.5 psi total
Implication: Pump operator needs at least 22.5 psi residual pressure at the hydrant after flowing 750 GPM. Most municipal hydrants easily meet this.

Example 2: Relay Pumping

Setup: 1,000 ft of 5″ LDH between two engines in relay. Target flow: 1,250 GPM.
Friction loss: FL = 0.08 × (12.5)² × 10 = 0.08 × 156.25 × 10 = 125 psi total
Implication: The downstream pump needs at least 125 psi residual + 20 psi minimum pump inlet pressure = 145 psi pump discharge pressure at the upstream relay engine. Many pumper apparatus can supply this; smaller pumpers may not.

Example 3: 4″ LDH Limit Scenario

Setup: 600 ft of 4″ LDH from hydrant. Target flow: 1,000 GPM. Hydrant residual pressure at this flow: 50 psi.
Friction loss: FL = 0.2 × (10)² × 6 = 0.2 × 100 × 6 = 120 psi total
Implication: Required pressure exceeds the 50 psi available residual by 70 psi. The supply will not deliver 1,000 GPM through this configuration. Options: shorten the supply line, reduce target flow, or upgrade to 5″ LDH.

Example 4: Industrial 6″ Refinery Supply

Setup: 1,000 ft of 6″ industrial LDH. Target flow: 2,000 GPM.
Friction loss: FL = 0.05 × (20)² × 10 = 0.05 × 400 × 10 = 200 psi total
Implication: Substantial pressure required, but well within industrial fire-pump capacity. The 6″ size is essential — 5″ LDH at the same flow rate would generate 320 psi friction loss, requiring pumps beyond typical municipal capability.

Things That Increase Friction Loss Beyond the Tables

The coefficients and tables above assume ideal conditions — straight hose lay, no kinks, fresh hose, smooth interior, no appliances. Real-world conditions add to the calculated friction loss:

• Sharp bends and kinks. A 90° bend in LDH adds the equivalent of about 25 ft of hose to the friction-loss calculation. Multiple bends compound — a hose lay with three sharp turns might add 75-100 ft of effective length.
• Appliances and adapters. Wyes, gates, valves and adapters introduce additional friction loss. A 2½” gated wye typically adds 5-10 psi of friction loss at maximum flow.
• Hose age and condition. Older hose with rough internal surfaces experiences higher friction loss than new hose. The standard NFPA coefficients assume reasonably new, well-maintained hose. Hose past 8-10 years of service may see 15-30% higher friction loss than the calculated value.
• Elevation changes. Not friction loss, but elevation gain adds approximately 0.434 psi per foot of elevation gain. A supply line running up a 30 ft hill adds 13 psi to the required pump pressure beyond friction loss.
• Lining material and quality. The standard coefficients assume EPDM or similar rubber lining. High-quality rubber-covered LDH with smooth interior lining flows at or slightly below the standard coefficient. Lower-quality construction may see 10-20% higher friction loss.

As a planning rule of thumb: add 10-15% to the calculated friction loss for real-world supply lays with normal hose conditions. This pad gives the pump operator margin if real-world friction loss exceeds the textbook calculation.

Quick-Reference Friction Loss Memorization Tips

Field pump operators don’t have time to do the full FL = C × Q² × L calculation during an emergency response. Memorize these benchmark values for instant recall:

• 5″ LDH at 1,000 GPM: 8 psi per 100 ft. The single most useful number for U.S. municipal fire service.
• 5″ LDH at 750 GPM: 4.5 psi per 100 ft.
• 5″ LDH at 1,250 GPM: 12.5 psi per 100 ft.
• 4″ LDH at 750 GPM: 11 psi per 100 ft.
• 4″ LDH at 1,000 GPM: 20 psi per 100 ft. The threshold where 4″ becomes inadequate for most supply evolutions.

Combined with the rule that 5″ LDH has 2.5× lower friction than 4″ at any flow, and that 6″ LDH has roughly 1.6× lower friction than 5″ at any flow, these numbers cover the vast majority of supply-line decision making.

Frequently Asked Questions

What is the friction loss formula for fire hose?

The standard fire-service formula is FL = C × Q² × L, where FL is friction loss in psi, C is the hose-specific coefficient, Q is the flow rate divided by 100 (so 1,000 GPM = Q of 10), and L is the hose length divided by 100 (so 500 ft = L of 5). For example, 1,000 GPM through 500 ft of 5″ LDH (C=0.08) = 0.08 × 100 × 5 = 40 psi total friction loss.

Why does 5″ LDH flow 2.5× the volume of 4″ LDH?

Friction loss in a hose scales inversely with the fifth power of the diameter — small diameter changes produce large flow changes. The ratio of 5″ coefficient (0.08) to 4″ coefficient (0.2) is 0.4. At equal friction loss, flow scales with the square root of the coefficient ratio: √(0.2/0.08) = √2.5 = 1.58. So 5″ flows 1.58× the volume at equal friction. Conversely, at equal flow, 5″ has 1/2.5 = 0.4× the friction loss of 4″. Both statements describe the same physics from different angles. For more on LDH selection, see our NFPA 1963 Storz couplings guide.

What is the “2Q² + Q” hand method?

The “2Q² + Q” formula is a fire-service shortcut for calculating friction loss in 2½” hose. For 2½”, friction loss per 100 ft = 2Q² + Q (where Q = GPM/100). For other sizes, multiply this 2½” baseline value: 3″ with 2½” couplings = ×0.4, 3½” = ×0.17, 4″ = ×0.1, 4½” = ×0.05, 5″ = ×0.03. The 2Q² + Q method gives slightly different numbers than C × Q² × L for the same conditions because it uses a different baseline assumption — both methods are valid, just different conventions. Use whichever your department’s training uses.

Should I use the calculated friction loss or add a safety pad?

For training and procurement planning, use the calculated values. For real-world operations, add 10-15% to the calculated friction loss to account for bends, fittings, hose age, and minor lay imperfections. The pad gives the pump operator margin to deliver target flow even when conditions aren’t ideal. Some departments train pump operators to round up to the next 5 psi increment for the same reason.

Does hose age affect friction loss?

Yes. New hose with smooth interior lining flows at or slightly below the standard coefficient. Hose past 8-10 years of service may see 15-30% higher friction loss than the calculated value due to wear on the rubber lining and accumulated mineral deposits on the interior surface. NFPA 1962 doesn’t require retirement based on friction loss alone, but elevated friction loss in older hose is a practical reason to plan replacement around the 10-15 year mark. See our Fire Hose Testing & Inspection guide for the complete NFPA 1962 procedure.

What about elevation loss when calculating supply pressure?

Elevation loss is separate from friction loss. Water column produces 0.434 psi per foot of elevation gain. If your supply line runs uphill 30 ft, add 30 × 0.434 = 13 psi to the required pump pressure beyond friction loss. Downhill supply lines provide the reverse — elevation gain reduces required pressure. Most municipal supply evolutions involve modest elevation changes (under 10-15 ft) that produce 5-7 psi of elevation loss. High-rise standpipe operations have substantial elevation loss — every 10 stories adds approximately 50 psi to the required pump pressure.