Free · Calculator 6
Pipe sizing and pressure drop
Find the right nominal size from the flow rate and permissible velocity – and estimate the pressure drop of the line including fittings, valves and difference in elevation.
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Basis & sources
What the tool calculates with, which standards and data it relies on and where its limits are. As of September 2026. For practice: Understand it and try it out.
Calculation method
- Velocityv = Q / A with the inside diameter of the selected series
- Frictionλ per Colebrook-White, laminar 64/Re
- LineΔp = λ·L/d·ρv²/2 + Σζ·ρv²/2 + ρ·g·Δh (Darcy-Weisbach)
- Nominal sizesmallest DN with v ≤ permissible velocity
Standards & codes
Reference: the assistant follows Autodesk's file structure and the connection types of these standards. Standard texts and standard tables are not included.
| EN 10220 / DIN EN ISO 1127, ASME B36.10/B36.19 | Pipe dimensions via the Piping Spec Calculator |
|---|---|
| DIN 11850 | Hygienic pipe |
Data & origin
| Pipe dimensions and wall thicknesses | Piping Spec Calculator (current piping spec or standard series) – estimates from manufacturer data |
|---|---|
| Density, viscosity, roughness, ζ values | Guide values, editable in the calculator |
| Typical velocities | Guide values: water discharge line 1.5–2.5 m/s, suction line 0.5–1.5 m/s, compressed air 10–20 m/s |
Assumptions & limits
- Calculated as incompressible – gases only with a small pressure drop (below 10% of the operating pressure), otherwise section by section.
- Steam, two-phase flow and bulk solids are not covered.
- ζ values of valves are guide values – manufacturer values (ζ or Kv) take precedence.
Standards status for this page 3 standards · researched September 2026
| Standard | Contents | Current edition | Replaced / successor |
|---|---|---|---|
| DIN EN 10220 | Seamless and welded steel tubes – General tables of dimensions and masses per unit length | DIN EN 10220:2003-03 | replaces DIN V ENV 10220:1994-02; DIN 2448:1981-02; DIN 2458:1981-02 |
| DIN EN ISO 1127 | Stainless steel tubes – Dimensions, tolerances and conventional masses per unit length | DIN EN ISO 1127:2019-03 | replaces DIN EN ISO 1127:1997-03 |
| ASME B36.10 (until 2018: B36.10M) | Welded and Seamless Wrought Steel Pipe | ASME B36.10-2022 | replaces ASME B36.10M-2018 |
Researched at DIN Media, ISO, IEC and ASME, as of September 2026. Before binding use, please check the current edition with the standards publisher. Overview of all standards: Plant Engineering Infobase · Standards status.
Understand it and try it out
What actually happens in a pipe with water flowing through it? Friction at the wall and every change of direction cost pressure – which the pump has to supply. With the calculator you can try it out: change one value and watch what happens to the others.
The terms in five minutes
- Flow rate Q
- How much passes through the pipe per unit of time, e.g. m³/h or l/s. 1 l/s = 3.6 m³/h.
- Inside diameter di
- What matters is the inside diameter, not the nominal diameter: DN 100 in steel 114.3 × 3.6 has an inside diameter of 107.1 mm. The wall thickness is defined in the piping spec.
- Velocity v
- v = Q / A with A = π/4 · di². Half the diameter → four times the velocity.
- Reynolds number Re
- Re = v · di / ν – tells you whether the flow is orderly (laminar, Re < 2300) or swirling (turbulent, Re > approx. 4000). In piping it is almost always turbulent.
- Viscosity ν
- How "thick" the medium is. Water at 20 °C: around 1.0 mm²/s, at 80 °C only 0.36 mm²/s – warm water flows more easily.
- Friction factor λ
- How much the wall slows the flow. Depends on Re and the roughness k of the wall (Colebrook-White, Moody diagram). Typically 0.015 … 0.03.
- Pressure drop Δp
- Δp = λ · L / di · ρ v² / 2 (Darcy-Weisbach) + Σζ · ρ v² / 2 for bends and valves + ρ · g · Δh for the difference in elevation.
- Head H
- The pressure drop in meters of water column: H = Δp / (ρ · g). 1 bar ≈ 10.2 m for water. The pump manufacturer needs this figure.
Try it out and understand it
Change one value in the calculator above and watch what happens to the others:
| Try it | What happens | Why |
|---|---|---|
| Double the flow rate | Δp rises to about 3.7 times, not double. | Δp grows with v², and λ decreases only slightly with higher Re. That is why "a little more flow" quickly costs a lot of pump power. |
| Choose one nominal size larger (DN 100 → DN 125) | Δp drops to around a third, v from 1.1 to 0.73 m/s. | For the same flow rate, the diameter enters roughly to the 5th power. Larger pipes cost more material but save energy permanently – that is the trade-off when choosing the nominal size. |
| Water temperature from 20 to 80 °C | Δp becomes about 10% smaller. | The viscosity drops to a third, Re increases, λ decreases slightly. The effect is smaller than you might think – unlike with viscous media such as oil. |
| Add ten 90° elbows | The local losses make up a noticeable share, in short lines often more than the straight pipe. | Each bend costs ζ · ρ v²/2. In plant rooms with many fittings, valves and bends make up the main share. |
| Swap steel for plastic pipe (PE) | λ becomes smaller, Δp drops – but the inside diameter is smaller with PE. | A smooth wall causes less friction, the thick plastic wall costs cross-section. Both together decide. |
Typical mistakes
- Calculating with DN instead of the inside diameter – with thick-walled pipes, you end up way off.
- Mixing up units: m³/h and l/s (factor 3.6), bar and m water column (factor ≈ 10.2).
- Using the design temperature of the piping spec as the operating temperature – for the flow, the operating temperature is what counts.
- Calculating gas with constant density even though the pressure drops considerably along the line (more than 10%).
For practice
Exercise: water at 20 °C, 36 m³/h, steel pipe DN 100 (114.3 × 3.6 mm), 100 m straight line. What are the velocity, Reynolds number and pressure drop? Is a pump with 2 m head sufficient for the friction?
Show solution
di = 114.3 − 2 · 3.6 = 107.1 mm; A = π/4 · 0.1071² = 0.00901 m²; Q = 36 m³/h = 0.01 m³/s → v = 0.01 / 0.00901 ≈ 1.11 m/s. Re = 1.11 · 0.1071 / 1.0·10⁻⁶ ≈ 119,000 → turbulent. With k = 0.05 mm, Colebrook-White gives λ ≈ 0.020. Δp = 0.020 · 100 / 0.1071 · 998 · 1.11² / 2 ≈ 11,300 Pa ≈ 0.11 bar ≈ 1.15 m. Yes, 2 m is enough for the friction – but bends, valves and difference in elevation come on top. Reproduce it in the calculator: series "Steel pipe", 36 m³/h, length 100 m, no fittings.
For training and studies: this page may be shown in class. Property values are guide values; for verification, the standards and manufacturer data apply.
What is the calculator for?
Why you need it
The nominal size determines the cost and operation of a line: pipe, valves, insulation and supports get more expensive with every DN step, while an undersized line costs pump energy permanently, becomes noisy and wears out. The pressure drop is the head a pump has to deliver.
Which tasks it solves
- What nominal size does the line need for a given flow rate?
- How high is the pressure drop to the consumer – is the pump sufficient?
- Which of the two routing variants has less loss?
- After modeling: does the DN still match the actual routing?
When in the design process
- Basics / preliminary designLine list: DN per line from the flow rate
- Basic engineeringPump head for the pump enquiry, compare routing variants
- Detail engineeringCheck with actual length and fittings from the 3D model (PCF)
- Installation / commissioning–
- Operation / modification of existing plantsIs the existing line sufficient for a higher flow rate?
Frequently asked questions
Which pipe series should I use?
The series of your piping spec – the wall thickness determines the inside diameter. For your own wall thicknesses, size the class in the Piping Spec Calculator first.
Does the calculator also size pumps?
It provides the pressure drop of the system as head (m). Pump curve and NPSH belong in the pump selection.
Preliminary sizing for design work – not a verification. Dimensions and material properties come from the ASEING Piping Spec Calculator and Support Span Calculator and are estimates from publicly available manufacturer data, generalized; all other values are guide values. They may deviate from the standard – check against the applicable standards and manufacturer data before binding use. You can enter your own values in the calculator.