US gallons per minute
12.5
Free liquid flow calculator
Flow Rate Calculator
Calculate flow from volume and time, estimate fill or drain time, or estimate line flow from pipe diameter and velocity.
Volume and flow planning
Flow rate inputs
Fill, drain, and line flow
Quick examples
Liters per minute
47.318
Liters per second
0.7886
Cubic meters per hour
2.839
Imp gallons per minute
10.408
Cubic feet per minute
1.671
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What is a flow rate calculator?
A flow rate calculator helps you work out how much fluid moves through a line, hose, pipe, tap, pump, or tank over time. In practical terms, people usually want to answer questions like: how many gallons per minute am I getting, how long will this tank take to fill, or what flow should a pipe carry at a given internal velocity.
This tool is built around those common jobs instead of just showing one formula. It lets you calculate flow from volume and time, calculate fill or drain time from a known flow, or estimate line flow from pipe diameter and velocity.
That makes it useful for water tanks, irrigation checks, plumbing estimates, pump comparisons, pool equipment planning, transfer lines, and many other day-to-day fluid handling tasks.
Why flow rate and time both matter
Flow rate by itself is only part of the picture. A system may deliver a certain number of gallons per minute, but the real question is often how that affects fill time, drain time, transfer speed, or whether a pipe size is reasonable for the target movement of water or another fluid.
That is why a practical flow tool should not stop at one unit. It should help you move between liters per second, liters per minute, gallons per minute, cubic feet per minute, and cubic meters per hour while also showing the time impact where relevant.
Useful for tanks and containers
Knowing the flow helps estimate realistic fill and drain times.
Time depends on constant flow
A given volume fills faster or slower depending on the actual sustained flow rate.
Pipe size affects delivered flow
Diameter and velocity together control the line flow that can move through a pipe.
Real systems are not always constant
Pressure loss, fittings, pump curves, and restrictions can change the real-world result.
How the flow rate formulas work
The most common relationship is simple: flow rate = volume ÷ time. If you know how much liquid moved and how long it took, you can solve for the flow. If you already know the flow and the volume, you can reverse the same relationship to solve for time instead.
For pipe-based estimates, the calculator uses area × velocity. Once the internal pipe area is known, multiplying that by the average fluid velocity gives the volumetric flow.
Flow from volume and time
Measure how much liquid moved and how long it took, then divide volume by time.
Time from flow and volume
Divide the total volume by the known flow rate to estimate fill or drain time.
Flow from pipe area and velocity
Use the internal pipe diameter to find area, then multiply by fluid velocity.
Compare practical output units
Results are easier to use when shown in gpm, L/min, L/s, and cubic units at the same time.
Core formulas
Flow rate = Volume ÷ Time
Time = Volume ÷ Flow rate
Pipe flow = Internal area × Velocity
Quick reference examples for flow planning
These examples show how the same flow idea can be used in different practical ways.
| Example | What it tells you |
|---|---|
| 50 gallons in 4 minutes | Lets you calculate the measured flow in gallons per minute. |
| 275-gallon tank at 12 gpm | Lets you estimate how long filling or draining should take if the flow stays steady. |
| 2-inch line at 5 ft/s | Lets you estimate the line flow from pipe area and average fluid velocity. |
| Comparing gpm and L/min | Helps when equipment or manuals use different unit systems. |
| Checking a field-measured hose flow | Useful for irrigation, transfer pumping, or general water delivery checks. |
How to use this flow rate calculator
- 1
Choose the right calculation mode
Use measured volume and time, known volume and flow, or pipe diameter plus velocity depending on what information you already have.
- 2
Enter the units that match the job
Pick gallons, liters, cubic units, seconds, minutes, hours, or line dimensions that match your real measurements.
- 3
Add the known values
Use measured field values whenever possible for better planning accuracy.
- 4
Review both flow and time outputs
Do not just stop at one unit. Compare the result across gpm, L/min, L/s, and time where relevant.
- 5
Use the notes as a reality check
Real systems can change under pressure loss, restrictions, fittings, or pump behavior.
Real-world uses, edge cases, and limitations
Useful for tanks and storage
Helpful when estimating fill time, drain time, or general transfer speed for containers and tanks.
Useful for plumbing and irrigation checks
A quick measured flow rate can help judge whether a line or hose is delivering what you expect.
Useful for equipment planning
Comparing pumps, transfer rates, and operating time is easier when everything is normalized into practical units.
Not a full hydraulic model
This tool does not calculate friction losses, pump curves, head pressure, cavitation risk, or valve and fitting penalties.
Frequently asked questions
- How do you calculate flow rate from volume and time?
- Divide the measured volume by the measured time.
- How do you calculate fill time from flow rate?
- Divide the total volume by the known flow rate, making sure the units match.
- Can pipe diameter and velocity estimate flow?
- Yes. Internal pipe area multiplied by average velocity gives volumetric flow.
- Why might the real flow differ from the estimate?
- Pressure changes, restrictions, fittings, valves, hose conditions, and pump behavior can all affect the actual result.
Estimate flow, time, and line performance in one place
Use this flow rate calculator when you need a practical answer for tank filling, draining, hose delivery, pipe flow, or general fluid transfer planning, with results shown in the units people actually compare on real jobs.