Flow Rate Calculator

What Is Flow Rate?

Flow rate describes how much fluid moves through a system over time. In engineering, plumbing, HVAC, irrigation, process plants, and laboratories, flow rate is one of the most common quantities you need. It tells you how quickly a tank fills, how much water passes through a pipe, how much air a fan delivers, or how much product moves through a process line.

The most common form is volumetric flow rate, measured as volume per time (such as L/min, m³/h, GPM, or CFM). Another form is mass flow rate, measured as mass per time (such as kg/s or lb/min). Mass flow is especially important when density varies (gases), or when you care about mass balance rather than volume.

Volumetric Flow Rate: Q = V / t

If you know how much volume moves over a known time interval, flow rate is simply:

This method is common in practical measurements: you collect a volume in a container for a measured time, then compute Q. It is also the basis of fill and drain time calculations, where you rearrange to t = V / Q.

Pipe Flow from Diameter and Velocity: Q = A·v

In many systems, you know pipe size and an estimated or measured average velocity. In that case:

For a round pipe, cross-sectional area is:

This calculator uses the diameter and velocity you enter to compute area and then flow. This approach assumes steady flow and uses average velocity, which is the standard engineering interpretation for Q = A·v.

Mass Flow Rate: ṁ = ρ·Q

Volumetric flow does not tell you how much mass moves unless you know density. Mass flow rate is:

For water-like fluids with stable density, mass flow is straightforward. For gases, density changes with temperature and pressure, so mass flow calculations must use the correct density for operating conditions.

Fill and Drain Time

If you know container volume and flow rate, time is:

This is widely used for tanks, pools, dosing vessels, irrigation systems, pumping schedules, and batch processes. Real systems may not have constant flow: pump curves, head changes, valve position, and pressure all affect flow, so use this as a planning estimate unless you have a controlled flow source.

Flow Rate Units and Conversions

Different industries use different flow units. Plumbing commonly uses GPM (US gallons per minute), process engineering often uses m³/h, and ventilation uses CFM. Because unit mismatch causes costly mistakes, this tool includes a dedicated unit converter and a table/export mode for documentation.

The calculator converts through SI flow (m³/s) internally and then formats into your selected unit. This makes conversions consistent and reliable.

What This Calculator Does Not Include

Flow rate alone does not determine pressure drop. In pipes, flow and pressure drop are connected through fluid viscosity, roughness, pipe length, fittings, and the flow regime (often characterized by Reynolds number). If you need pressure loss or pump sizing, use a dedicated pressure drop or head loss calculator alongside this flow tool.

How to Use the Flow Rate Calculator

• Flow from Volume & Time: best for measured flow from a container test or known transfer volume.
• Pipe Flow: best when you know diameter and average velocity.
• Mass Flow: best when you need kg/s or lb/min from volumetric flow and density.
• Fill / Drain: best for scheduling and estimating time-to-fill or time-to-empty.
• Unit Converter: best for quick conversion between flow units (L/min ↔ GPM, m³/h ↔ CFM, etc.).
• Table & Export: best for generating a range table and exporting to CSV for spreadsheets or reports.

Final Notes

Flow rate is a foundational engineering quantity that links equipment size, system performance, and time. Whether you are working on water systems, air systems, or industrial processes, correct flow conversion and consistent units are essential. Use this tool to estimate, compare, document, and convert flow rates quickly—then pair it with pressure drop calculations when system resistance matters.