How to Use the Water Flow Calculator
- Flow Rate tab: enter the pipe's inner diameter and the water velocity — get Q instantly in L/s, L/min, m³/h, m³/day, US GPM and ft³/min.
- Velocity tab: know the flow rate instead? Enter diameter + flow rate to get the average velocity — useful for checking recommended velocity limits.
- Reynolds tab: add water temperature to determine the flow regime (laminar, transitional, turbulent), which tells you which pressure-drop formulas apply.
- Quick sizes: tap a common diameter pill instead of typing. Copy link saves your inputs in the URL for sharing or bookmarking.
Understanding Pipe Flow: Q, v and the Reynolds Number
For an incompressible fluid like water flowing in a full pipe, the volumetric flow rate is simply the product of average velocity and cross-sectional area: Q = v·A. This continuity relation is the foundation of pipe sizing: pick a design velocity, and the required diameter follows from the demand flow (or vice versa). Because area scales with d², small diameter changes have big effects — going from 25 mm to 32 mm raises capacity by 64% at the same velocity. Always size with the inner diameter: for plastics the DN roughly equals the ID, but for steel pipes the wall schedule shifts the bore (a 1" Sch 40 steel pipe has a 26.6 mm bore, not 25.4 mm).
Whether the flow is smooth or chaotic decides everything about friction losses. The dimensionless Reynolds number Re = ρvd/μ compares inertial to viscous forces. Below ~2,300 flow is laminar — orderly layers with friction governed by viscosity alone (Hagen–Poiseuille). Above ~4,000 it is turbulent — mixing eddies dominate and pressure drop grows nearly with v² (Darcy–Weisbach). Water's low viscosity means real installations are almost always turbulent; a garden hose at 1 m/s already runs Re ≈ 20,000. Temperature enters through water properties: density changes little (999.97 kg/m³ at 5 °C → 958.35 kg/m³ at 100 °C), but dynamic viscosity falls more than fivefold, from 1.52 to 0.28 mPa·s — which is why hot-water circuits exhibit noticeably lower pressure drops.
This calculator evaluates formulas directly in your browser — nothing is sent to a server. It assumes a full, circular pipe with a uniform velocity profile (bulk average). For open channels, partially filled pipes, or compressible gases, different tools and corrections apply.