Volumetric Flow Rate Calculator Updated Aug 26, 2026
Calculate volumetric flow rate from cross-sectional area
and velocity, or from a volume filled over a measured time. Get the result
instantly in GPM, CFM, L/min, L/s, ft³/s, and m³/h — with the formula and
every conversion step shown.
✓ Free, no signup✓ 2 calculation modes✓ 6 output units at once✓ Instant results
Quick answer: Volumetric flow rate is the volume of fluid moving
past a point per unit of time, written Q = A × v (cross-sectional
area times velocity) or Q = V ÷ t (volume divided by time). For
example, water moving at 2 m/s through a pipe with a 0.01 m² cross
section has a flow rate of 0.02 m³/s — about 317 US gallons per minute.
Q = Area × Velocity
Flow Rate (Q)
—
Enter area and velocity
Q = Volume ÷ Time
Flow Rate (Q)
—
Enter a volume and a time
How to use this flow rate calculator
Pick a tab. Use Area × Velocity when you know the
pipe or duct cross-section and the fluid speed. Use
Volume ÷ Time when you've measured how long it takes to
fill a known container.
Enter your values and units. Every field has its own unit
selector — mix imperial and metric freely.
Click Calculate to see the flow rate in m³/s, plus GPM, CFM,
L/min, L/s, ft³/s, and m³/h all at once.
The volumetric flow rate formulas
Volumetric flow rate (Q) measures how much fluid volume passes a given
point per unit time. There are two equivalent ways to calculate it,
depending on what you can measure:
Q = A × v (cross-sectional area × velocity)
Q = V ÷ t (volume ÷ time)
The first form is standard in pipe flow, HVAC duct sizing, and open-channel
hydraulics, where you know the geometry and the fluid speed. The second is
simpler when you can directly measure a fill — for example, timing how long
it takes a 5-gallon bucket to fill from a hose.
Unit
Equal to
Common use
GPM (US gal/min)
0.06309 L/s
Plumbing, pumps, irrigation
CFM (ft³/min)
0.4719 L/s
HVAC, ventilation, air ducts
L/min
0.01667 L/s
Metric plumbing, lab equipment
m³/h
0.2778 L/s
Industrial and process flow
m³/s
1,000 L/s
SI base unit, engineering calcs
Worked example
Example — Area × Velocity. Water flows at 2 m/s
through a pipe with a 0.01 m² cross-section (roughly a 4-inch diameter
pipe):
Q = 0.01 m² × 2 m/s = 0.02 m³/s ≈ 20 L/s ≈ 317 GPM
Example — Volume ÷ Time. A 50-gallon tank fills
in 60 seconds:
Q = 50 gal ÷ 60 s = 0.833 gal/s = 50 GPM
Who uses a volumetric flow rate calculator
Plumbers and irrigation designers sizing pipes and pumps, HVAC
technicians balancing duct airflow in CFM, mechanical and civil engineering
students checking homework, and process engineers converting between
metric and imperial flow units all reach for this calculation regularly.
Tips for accurate flow rate results
Use the inside diameter of a pipe to compute area
(Area = π × r²), not the outer diameter — wall thickness matters.
Measure velocity at the same point where you measure the
cross-section; flow speed can vary across a pipe or duct.
For a quick container-fill test, time at least 10–15 seconds
of fill for a more accurate average than a very short burst.
Match your units to your industry: GPM for plumbing and
irrigation, CFM for HVAC and ventilation, L/s or m³/h for most
metric engineering work.
Volumetric flow rate calculator FAQs
What is volumetric flow rate?
Volumetric flow rate is the volume of fluid that
passes a given point per unit of time, usually written as Q. It's
calculated as Q = A × v (area times velocity) or Q = V ÷ t (volume
divided by time).
How do I convert flow rate to GPM?
Multiply a flow rate in m³/s by 15,850.32 to get US
gallons per minute, or use the calculator above, which converts to
GPM automatically alongside five other units.
What's the difference between GPM and CFM?
GPM (gallons per minute) measures liquid flow, most
common in plumbing and irrigation. CFM (cubic feet per minute)
measures air or gas flow, most common in HVAC and ventilation. They
measure different fluids but use the same underlying formula.
How is flow rate different from flow velocity?
Velocity is how fast the fluid moves (distance per
time, like ft/s). Flow rate is how much volume passes by per time
(like GPM). The two are related by the cross-sectional area: flow
rate = area × velocity.
How do I calculate the cross-sectional area of a pipe?
For a circular pipe, Area = π × r², where r is the
inside radius (half the inside diameter). For a 4-inch diameter pipe,
r = 2 inches, so Area = π × 2² ≈ 12.57 in² ≈ 0.0081 m².
Can this calculator handle both metric and imperial units?
Yes. Each input has its own unit selector, so you can
mix metric area with imperial velocity, or any other combination — the
calculator converts everything internally before computing the result.
Why does my measured flow rate not match the pipe's rated capacity?
Real-world flow is reduced by friction, fittings,
elevation changes, and partial blockages. A flow rate calculated from
geometry alone (Q = A × v) assumes ideal, unobstructed flow.
⚠️ Accuracy note: This calculator provides estimates for
planning and educational purposes using ideal-flow formulas. Real piping
systems involve friction losses, fittings, and other factors — for
critical engineering, plumbing, or HVAC sizing decisions, confirm figures
with a qualified engineer or technician.
This volumetric flow rate calculator from Anchor AI
Tools runs entirely in your browser — your numbers are never sent to a
server.
