How does this pipe friction loss calculator work?

It uses the Hazen-Williams equation for water flowing in full pipes. You enter the flow rate in litres per minute, the internal bore in millimetres, the run length in metres and a roughness C factor. The tool converts flow to cubic metres per second and bore to metres, then returns the head lost to friction in metres, the loss normalised per 100 metres and the equivalent pressure drop in bar.

What inputs and C factor should I use?

Use the internal bore of the pipe, not the nominal size, since friction rises steeply as the bore narrows. Enter the total developed length of the straight run. For the roughness C factor use about 150 for plastic pipe like CPVC, PVC or PEX, roughly 130 for copper and around 100 for old galvanised iron. When unsure, check the exact value against the manufacturer data for your pipe.

How accurate is the result and what should I verify?

This is indicative sizing for concept planning and covers straight-pipe friction only. It does not include the losses from bends, tees, valves and meters, which you add separately as equivalent lengths. Real losses also shift with water temperature and pipe age. Confirm the design flow, the C factor and the final pipe size with a qualified plumbing consultant and against NBC 2016 Part 9 before you procure or install.

Interactive Calculator · 2026

Pipe Friction Loss Calculator

Estimate the head lost to friction in a pipe run using the Hazen-Williams equation. Enter the flow rate, the internal diameter, the length of pipe and the roughness C factor — get the head loss in metres, the loss per 100 m and the equivalent pressure drop in bar.

Head loss over 30 m (C = 150)0.00 m≈ 0.00 bar pressure drop · 0.00 m per 100 m

Head loss for this run → normalised to a standard 100 m length

Your pipe run

Flow rate (L/min)

Volume flowing through the pipe per minute.

Internal diameter (mm)

Bore of the pipe, not the nominal size.

Pipe length (m)

Total developed length of the straight run.

Hazen-Williams C factor150

Pipe roughness coefficient — higher is smoother. Use ~150 for plastic (CPVC / PVC / PEX), ~130 for copper and ~100 for old galvanised iron.

Head loss (this run)

0.00 m

Head loss per 100 m

0.00 m

Equivalent pressure drop

0.00 bar

Head loss for this run vs per 100 m

Normalising to 100 m lets you compare pipe sizes and materials on a like-for-like basis.

Your 30 m run loses 1.46 m of head to friction — about 0.14 bar of pressure. Friction climbs steeply as the bore narrows, so if the loss looks high, stepping up one pipe size usually helps far more than a smoother material.

This is straight-pipe friction only. Add the equivalent-length losses of bends, tees and valves separately, and verify the C factor for your exact pipe.

How this is calculated

Flow in SI = 30 L/min ÷ 60000 = 0.00050 m³/s; bore = 25 mm ÷ 1000 = 0.0250 m.
Head loss (Hazen-Williams) = 10.67 × L × Q^1.852 ÷ (C^1.852 × d^4.8704) = 10.67 × 30 × 0.00050^1.852 ÷ (150^1.852 × 0.0250^4.8704) = 1.46 m.
Per 100 m = head loss × 100 ÷ length = 1.46 × 100 ÷ 30 = 4.87 m.
Pressure drop = head loss × 0.0981 = 1.46 × 0.0981 = 0.14 bar.

Indicative sizing for concept planning; straight-pipe friction only — add fitting and valve losses separately. Confirm with a qualified consultant, manufacturer data and NBC 2016 Part 9 before procurement.

Understand the number you just got

Plumbing Pipes Guide (India)Materials, sizes and where CPVC, PVC, GI and copper each belong.Fixing Low Water Pressure (India)How pipe friction and undersized runs sap pressure — and how to fix it.

Frequently asked questions

How does this pipe friction loss calculator work?: It uses the Hazen-Williams equation for water flowing in full pipes. You enter the flow rate in litres per minute, the internal bore in millimetres, the run length in metres and a roughness C factor. The tool converts flow to cubic metres per second and bore to metres, then returns the head lost to friction in metres, the loss normalised per 100 metres and the equivalent pressure drop in bar.
What inputs and C factor should I use?: Use the internal bore of the pipe, not the nominal size, since friction rises steeply as the bore narrows. Enter the total developed length of the straight run. For the roughness C factor use about 150 for plastic pipe like CPVC, PVC or PEX, roughly 130 for copper and around 100 for old galvanised iron. When unsure, check the exact value against the manufacturer data for your pipe.
How accurate is the result and what should I verify?: This is indicative sizing for concept planning and covers straight-pipe friction only. It does not include the losses from bends, tees, valves and meters, which you add separately as equivalent lengths. Real losses also shift with water temperature and pipe age. Confirm the design flow, the C factor and the final pipe size with a qualified plumbing consultant and against NBC 2016 Part 9 before you procure or install.

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