Flow Rate Calculator
Solve the volumetric flow rate equation Q = A × v in three directions — flow rate from area and velocity, velocity from flow rate and area, or required diameter from flow rate and velocity — convert between 9 flow units, compute mass flow rate ṁ = ρ × Q, and estimate tank fill time t = V / Q.
Flow rate from area and velocity
Flow rate Q
Velocity from flow rate and pipe size
Average velocity v
Required diameter from flow rate and velocity
Required size
Mass flow rate ṁ = ρ × Q
Mass flow rate ṁ
Tank fill time t = V / Q
Fill time
Frequently asked questions
What is the flow rate equation?
The volumetric flow rate equation is Q = A × v, where Q is the volumetric flow rate, A is the cross-sectional area, and v is the average flow velocity normal to that area. For example, water flowing at 2 m/s through a pipe with a 10 cm inner diameter (area 0.00785398 m²) gives Q = 0.01570796 m³/s = 15.7080 L/s = 942.4778 L/min = 248.98 GPM.
How do I calculate velocity from flow rate and pipe size?
Rearrange the equation to v = Q / A. For example, 5 L/s through a pipe with an 8 cm diameter (area 0.00502655 m²) gives v = 0.005 / 0.00502655 = 0.9947 m/s.
How do I find the pipe diameter for a target flow rate?
Use D = √(4·Q / (π·v)). For example, a target flow of 50 L/min at a velocity of 1.5 m/s needs D = √(4 × 0.00083333 / (π × 1.5)) = 0.02660 m = 26.60 mm.
Which flow rate units does the calculator convert between?
Cubic metres per second, litres per second, litres per minute, litres per hour, cubic metres per hour, cubic feet per second (cfs), cubic feet per minute (CFM), US gallons per minute (GPM) and million gallons per day (MGD). Reference: 1 m³/s = 15,850.32 GPM = 2,118.88 CFM = 1,000 L/s = 60,000 L/min = 3,600 m³/h.
How do I compute mass flow rate?
Mass flow rate is ṁ = ρ × Q, where ρ is the fluid density. For example, 10 L/s of water (ρ = 1000 kg/m³) gives ṁ = 10 kg/s = 36 tonnes/h. The calculator includes density presets for air (1.225 kg/m³), water (1000 kg/m³), seawater (1025 kg/m³), oil (870 kg/m³), gasoline (737 kg/m³), diesel (832 kg/m³) and mercury (13,534 kg/m³), or a custom density.
How long will it take to fill a tank?
Fill time is t = V / Q. For example, filling a 1,000 L tank at 25 L/min takes t = 1000 / 25 = 40.0 minutes (0.6667 hours).
How is this different from the pipe flow, orifice flow and open-channel flow calculators?
This tool solves the general continuity equation Q = A·v and needs no pressure, roughness or channel data. The pipe flow calculator at https://nrtechstudio.com/tools/pipe-flow/ finds flow rate from head loss in a full pressurized pipe using Darcy–Weisbach. The orifice flow calculator at https://nrtechstudio.com/tools/orifice-flow/ handles pressure-driven flow through a restriction via Q = Cd·A·√(2ΔP/ρ). The open-channel flow calculator at https://nrtechstudio.com/tools/open-channel-flow/ handles partially filled channels via Manning's equation.
What assumptions and limits apply?
Steady, incompressible flow with a uniform average velocity over a known cross-sectional area. It does not compute head loss, friction or pressure drop from velocity alone — for that use the pipe flow calculator at https://nrtechstudio.com/tools/pipe-flow/. Results follow your input precision; garbage in, garbage out.
Free tool by NRTechStudio