EngineeringFormulaFluid Mechanics
Pipe Size Calculator
Find the required pipe inside diameter from flow rate and target velocity, or check velocity through an existing pipe.
A pipe size calculation starts with the fundamental continuity relationship Q = Av: volumetric flow rate equals the cross-sectional area times the average flow velocity. This calculator uses that relationship in two directions - given a design flow rate and an acceptable target velocity, it returns the required inside diameter for a circular pipe; and given an existing pipe's inside diameter and a flow rate, it returns the actual average velocity and a broad screening band. The formulas apply to any incompressible liquid at the velocity ranges typical of water-supply, process-piping, and HVAC chilled-water systems.
Application (sets target velocity)
Q · Volumetric flow rate of liquid through the pipe.
v · Design average flow velocity. Typical liquid piping: 0.5–3 m/s.
This is a velocity-method sizing helper. It does not calculate pressure drop, friction loss, pump selection, or pipe schedule - use the Darcy-Weisbach equation and a pipe-dimension table for that.
Solution
Enter a flow rate and target velocity, or a flow rate and pipe diameter, to calculate pipe sizing results.
d = √(4Q / πv)
Formula Sheet
- QFlow Rate
- vAverage Velocity
- AFlow Area
- dInside Diameter
Variables & Units
| Symbol | Variable | Description | Common Units |
|---|---|---|---|
| Q | Flow Rate | Volumetric flow rate of liquid through the pipe. | L/min, L/s, m³/h, US gpm, m³/s |
| v | Average Velocity | Mean fluid velocity across the pipe cross-section. Not the same as peak centreline velocity. | m/s, ft/s |
| A | Flow Area | Internal cross-sectional area of the circular pipe. | mm², cm², in² |
| d | Inside Diameter | Internal bore diameter of the pipe. Depends on both nominal pipe size (NPS/DN) and wall schedule. | mm, in |
How to Use This Calculator
- 01Use Required Pipe Size when you know the design flow rate and an acceptable target velocity. If you are unsure of the target velocity, click one of the application presets (Domestic water, Chilled water, General service, Condensate return) to auto-fill a representative value.
- 02Use Check Existing Pipe when you know a pipe's inside diameter and the flow rate it will carry. The calculator returns the actual average velocity and a screening band.
- 03Enter the pipe's inside (bore) diameter, not the outside diameter or the nominal pipe size - inside diameter varies with pipe schedule (wall thickness).
- 04After calculating in Required Pipe Size mode, review the Nearest Schedule 40 Pipe Sizes table below the result. It shows the 3 smallest standard DN/NPS sizes whose bore is at least as large as the required diameter, with the actual velocity each size would produce.
- 05Use the velocity screening band as a first-pass check only. Final pipe sizing requires friction-loss, pressure-drop, and system-curve analysis.
How the Formula Works
The core relationship is Q = A × v, where Q is volumetric flow rate (m³/s), A is the internal cross-sectional area (m²), and v is the average flow velocity (m/s). For a circular pipe, A = πd²/4, so the required inside diameter is d = √(4Q / πv).
In Check Existing Pipe mode the formula is inverted: A = πd²/4 from the known diameter, then v = Q/A gives the actual average velocity.
Velocity bands used here (m/s) are general liquid-piping screening values: below 0.5 m/s may risk sedimentation in slurries; 0.5–3 m/s covers most general-service piping; 3–5 m/s raises erosion and noise concerns; above 5 m/s poses serious erosion, water-hammer, and noise risk. These are guidelines, not code-compliance criteria - acceptable velocity depends on fluid, pipe material, fittings, and system requirements.
Worked Example 01
Required pipe size for a 100 L/min water line
Known
- Flow rate (Q): 100 L/min
- Target velocity (v): 1.5 m/s
Formula
d = √(4Q / πv)
Substitution
Q = 100/1000/60 = 0.001667 m³/s; d = √(4 × 0.001667 / (π × 1.5))
Result
d ≈ 37.6 mm - consider a DN 40 (1½") pipe (ID ≈ 40.9 mm schedule 40)
At 1.5 m/s the required bore is 37.6 mm. The nearest standard metric size above this is DN 40 (40 mm nominal), whose schedule-40 inside diameter of about 40.9 mm would give an actual velocity of about 1.27 m/s - well within the typical 0.5–3 m/s band.
Worked Example 02
Velocity check through a 25 mm ID pipe at 15 L/min
Known
- Flow rate (Q): 15 L/min
- Inside diameter (d): 25 mm
Formula
v = Q / A
Substitution
A = π × (0.025)² / 4 = 4.91 × 10⁻⁴ m²; v = (15/1000/60) / 4.91 × 10⁻⁴
Result
v ≈ 0.51 m/s - typical band (low end)
A 25 mm bore carrying 15 L/min - a common domestic cold-water supply scenario - runs at about 0.51 m/s, sitting at the low end of the typical velocity band. There is no erosion concern, though very low velocities in slurry or hot-water systems can promote sedimentation or Legionella risk.
Applications
- 01First-pass sizing of water-supply, chilled-water, or process-piping branches
- 02Checking whether an existing pipe can carry a new or increased flow rate at acceptable velocity
- 03Comparing standard nominal pipe sizes against a required inside diameter
- 04Quick screening before a full friction-loss / Darcy-Weisbach pressure-drop analysis
Assumptions
- 01Circular (round) pipe cross-section.
- 02Incompressible, single-phase liquid flow.
- 03Average velocity is uniform across the cross-section (plug-flow assumption for sizing purposes).
- 04Inside diameter is the clear bore - not nominal pipe size, not outside diameter.
Where This Model Stops
- 01Does not calculate friction loss, pressure drop, available system pressure, or pump selection.
- 02Does not model pipe roughness, fittings, bends, reducers, or entrance/exit losses.
- 03Velocity bands are general screening guidance, not code or standard pass/fail criteria.
- 04Does not cover compressible (gas) flow, two-phase flow, slurries, or non-Newtonian fluids.
- 05Does not account for thermal expansion, water hammer, or transient flow conditions.
References
- [1]Pipe Flow - Velocity and Flow Rate
Engineering ToolBox
Reference for Q = Av, pipe velocity bands, and common unit conversions for liquid piping.
- [2]Pipe Sizing - Recommended Velocities
Engineering ToolBox
Velocity band guidance for water, steam, and other fluids in piping systems.
Frequently Asked Questions
What is the difference between nominal pipe size and inside diameter?
Nominal pipe size (NPS or DN) is a trade designation, not an actual dimension. The true inside diameter depends on both the nominal size and the wall schedule (e.g., Schedule 40, Schedule 80). Always use the actual inside diameter from a pipe-dimension table for your specific schedule when calculating flow velocity or pressure drop.
What target velocity should I use?
Common practice for general liquid-service piping is 0.5–3 m/s (roughly 1.5–10 ft/s). Lower velocities are quieter and cause less erosion but require larger, more expensive pipe. Higher velocities can cause noise, pipe erosion (especially with entrained solids), and water hammer. Project-specific standards or codes should take precedence over generic guidance.
How is this different from the Flow Rate Calculator?
The Flow Rate Calculator is a general Q = Av tool that solves for any of the three variables. This Pipe Size Calculator is focused specifically on pipe inside diameter selection: it returns the required bore, compares it to standard nominal sizes, and gives a velocity screening band with pipe-system context and limitations.
How is this different from the Duct Size Calculator?
The Duct Size Calculator is designed for HVAC air ducts: it uses CFM/FPM units, ASHRAE/Manual-D velocity references, and duct-specific geometry like equivalent round diameter. This calculator targets liquid piping with volumetric units common in plumbing and process engineering (L/min, US gpm, m³/h) and liquid-service velocity guidance.
Does this calculate pressure drop?
No. Pressure drop requires pipe roughness, fluid viscosity, pipe length, fitting losses, and elevation change - inputs this calculator does not collect. Once you have a candidate pipe diameter, use the Darcy-Weisbach equation or the Hazen-Williams formula for a full pressure-drop analysis.