Flow Rate Calculator

Measure flow from collected volume and time, calculate it from a full pipe’s inside diameter and average velocity, or find velocity from a known flow.

Calculation method

Flow-rate unitsIn velocity mode this also sets the known-flow input unit.

Collected volumeLUsed only in measured-volume mode.

Collected / target volume unitsChanging units converts both the collected volume and target fill volume.

Elapsed collection times

Elapsed-time units

Actual inside pipe diametermmUsed in pipe modes only. Nominal pipe size is not necessarily the inside diameter.

Inside-diameter units

Average fluid velocitym/sCross-section-average velocity, not automatically a centerline sensor reading.

Velocity units

Known volumetric flowL/minUsed only when solving for pipe velocity.

Target fill volume (optional)LSet 0 to omit fill time. Assumes this is the volume still needed and the flow stays constant.

Calculation results

Volumetric flow rate

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How to use this calculator

Pick what you measured

Volume/time mode needs a measured collection volume and elapsed time. The pipe modes need the actual internal diameter and either average velocity or flow.

Keep quantities distinct

Velocity is distance per time, while flow is volume per time. Pipe area connects them; pressure by itself does not determine the answer in this tool.

Use matching units

Choose units for each input. Changing a compatible unit converts its existing numeric value; output can be shown in metric or US customary flow units.

Review conversions and fill time

The result includes several volumetric units. Optional target volume gives a constant-flow filling duration. Zero flow cannot fill a positive target in finite time.

Formula and worked example

Measured volumetric flow Q = collected volume V ÷ elapsed time t
Full circular-pipe area A = π × inside diameter² ÷ 4
Flow from velocity Q = A × average velocity
Average velocity = Q ÷ A
Constant-flow fill time = target volume ÷ Q
1 L = 0.001 m³; 1 US gallon = 3.785411784 L

Collecting 20 liters in 30 seconds gives 0.666667 L/s, or 40 L/min. That is 2.4 m³/h, about 10.5669 US gallons per minute or 1.4126 actual cubic feet per minute. At that constant flow, adding 100 liters takes 150 seconds. Separately, a full 25-mm-inside-diameter pipe at an average 1 m/s carries about 29.4524 L/min.

One measured flow in several units

All rows describe the same volumetric rate at the measured conditions. No correction to standard gas temperature or pressure is applied.
Unit	Equivalent to 40 L/min
Liters per second	0.666667
Cubic meters per hour	2.4
US gallons per minute	10.5669
Actual cubic feet per minute	1.4126

Frequently asked questions

Can I calculate flow from pressure and diameter alone?

Not with this calculator. A pressure-based model also needs the relevant resistance and boundary conditions, such as pipe length, fittings, roughness or an orifice coefficient. Use a model suited to that system.

Should I enter nominal pipe size or inside diameter?

Actual inside diameter. Pipe schedule, wall thickness and lining can change the internal opening even when the nominal size is the same. The area depends on diameter squared.

Does this work for a partially full pipe?

The volume/time method still works for a valid collection measurement. The diameter-and-velocity methods assume a full circular cross-section; they do not calculate the wetted area of a partially filled pipe.

Is CFM the same as SCFM?

No. This CFM output is a direct conversion of actual volumetric flow. SCFM refers to specified standard conditions and requires temperature, pressure and possibly compressibility information. Those adjustments are not made here.

Why does zero flow show no finite fill time?

A positive volume divided by zero flow has no finite filling time. Zero flow is a valid measured rate, so the tool shows zero conversions and explains why a positive target cannot be filled under that assumption.

Methodology and sources

The pipe calculation is a geometric continuity relation using cross-section-average velocity, not a pump, pressure-loss or fluid-network simulation. It assumes a full circular pipe and does not infer fluid properties. It does not validate a safe velocity, pipe rating, equipment size or required air changes. Fill time assumes a constant rate and no concurrent outflow.

OpenStax College Physics 2e — flow rate and velocity — Definitions Q = V/t and Q = A × average velocity.
NIST Handbook 44 — measurement tables — Exact customary and metric unit definitions.

Sources checked September 1, 2026. Referenced organizations do not endorse Efficienco. Calculations retain full precision until display. These are volumetric quantities, not mass-flow or standard-gas-flow conversions.