Flow Rate Calculator

What flow rate measures

Flow rate is the volume of fluid that passes a point in a fixed stretch of time, written Q = V/t. Fill a 5 gallon bucket in 30 seconds and the flow is 10 gallons per minute. The same physical flow can be written as gallons per minute, liters per minute, liters per second, cubic meters per hour, cubic feet per minute or cubic feet per second, and every one of those describes the same pipe. That is why a pump box, an irrigation spec sheet and a river gauge rarely agree on the unit.

Hydrologists call the quantity discharge, and the USGS Water Science School (2026) defines it the same way this page does: a volume of water per unit of time, computed in the field as channel cross section area times average velocity. Plumbing and pump work live in gallons per minute, European catalogs quote cubic meters per hour, HVAC and compressed air work uses cubic feet per minute, and stream gauges report cubic feet per second.

None of these units carries more information than another. They differ only by conversion constants, and those constants trace back to exact definitions: the US gallon is 231 cubic inches and 3.785411784 liters exactly, per the NIST Handbook 44 (2026 edition). Once the gallon is pinned down, every flow unit on this page follows by arithmetic alone.

Five units and who uses them

Each unit survives because a whole industry standardized on it. Knowing which one a document means is half the job; the other half is moving between them without dropping a factor of 60.

The full set of constants sits in one table. Read any row as: one of the left column equals all the numbers to its right.

Everything here comes from two exact facts: 1 US gallon is 3.785411784 L and 1 cubic foot is 28.316846592 L (NIST, 2026 edition of Handbook 44). The rest is moving the same volume across different time bases of 60 seconds or 3,600 seconds. One cfs is 60 cfm, which is why the last row reads so large: rivers move a lot of water.

Q = A x v: flow from pipe size and speed

The second way to get a flow rate needs no bucket at all. The Reclamation Water Measurement Manual derives it by dividing distance traveled by time and keeping the area fixed: Q = A x v, flow equals cross section area times average velocity. For a round pipe the area is A = pi x (d/2)^2, with d the inside diameter.

Worked in US units, a 1 inch inside diameter pipe has an area of 0.785 in2, which is 0.005454 ft2. At 4 feet per second the flow is 0.02182 ft3/s, or 1.31 CFM, which converts to 9.79 GPM, 37.07 L/min, 0.618 L/s and 2.22 m3/h.

Diameter does the heavy lifting. Area grows with the square of diameter, so doubling the diameter at the same velocity quadruples the flow, and a move from 1 inch to 3 inches multiplies flow by 9.

The same table runs in reverse. If 9.79 GPM has to pass through a 3/4 inch line instead of a 1 inch line, the velocity climbs from 4 ft/s to 7.11 ft/s, because area and velocity trade places inside the fixed product A x v. Friction loss grows faster still, which is why undersized lines show up as pressure complaints rather than as a wrong flow number.

Worked example: a bucket and a stopwatch

The oldest flow measurement needs no instrument beyond a container and a clock. Suppose a 20 gallon bucket fills in 45 seconds:

• 20 / 45 = 0.4444 gallons per second
• 0.4444 x 60 = 26.67 GPM
• 26.67 x 3.785411784 = 100.94 L/min
• 100.94 / 60 = 1.68 L/s
• 26.67 x 0.2271 = 6.06 m3/h
• 26.67 / 7.4805 = 3.56 CFM

The bucket trick has known error sources worth respecting. A nominal 5 gallon bucket usually holds a little more or less than 5 gallons, reaction time on the stopwatch adds a few tenths of a second on short fills, and the flow should be steady across the test. Filling for a longer time shrinks all of those errors proportionally, which is why a 30 second test beats a 5 second one.

Notice the shape of the arithmetic: every conversion is a single multiplication after the one division that defines the flow. The calculator on this page does the same chain, so 20 and 45 should reproduce the six numbers above exactly.

Worked example: a hose bib on a 1 inch line

Now the two formulas meet. A utility runs a 1 inch service line and measures 4 feet per second at the meter. From the pipe panel of the calculator:

• Area: pi x (0.5 in)^2 = 0.785 in2 = 0.005454 ft2
• Q = 0.005454 x 4 = 0.02182 ft3/s
• 1.31 CFM, which is 9.79 GPM
• Metric form: 37.07 L/min, 0.618 L/s, 2.22 m3/h

If the same utility wants the flow in liters for a billing report, the answer is already there: 0.618 L/s. If a fire flow standard asks for cfs, it is 0.0218. Five units, one measurement, no re-measuring.

Check the flip side: push those 9.79 GPM through a 3/4 inch branch instead and velocity jumps to 7.11 ft/s. Through a 2 inch main the same flow crawls at 1.00 ft/s. Velocity is the number that changes when you change the pipe; flow stays what the source delivers.

Running it backwards: time to fill

Rearrange Q = V/t into t = V/Q and the same inputs answer the question people usually start with: how long will this take? A 15,000 gallon pool fed by a 25 GPM hose needs 15000 / 25 = 600 minutes, which is a 10 hour fill.

The table shows the one rule worth memorizing about fill time: doubling the flow halves the time. Volume never negotiates. If the 10 hour pool fill is unacceptable, the fix is a bigger pipe, a stronger pump or a second hose, not a better calculator.

Reading the unit on the box

Mismatches between documents come from the unit, not the physics. A pump catalog printed in Europe lists 6 m3/h; a plumber sizing pipe thinks in GPM. The bridge is 1 m3/h = 4.4029 GPM, so the pump moves 26.4 GPM. A river gauge reading 2 cfs is carrying 897.7 GPM, because one cfs equals 448.83 GPM. The USGS (2026) explains how those gauge numbers are produced from area and velocity measurements, and the NIST tables (2026) are the upstream authority for every volume factor used here.

Watch the time base as much as the volume. GPM, L/min and CFM share a minute; L/s and cfs share a second; m3/h uses an hour. The classic error is treating L/min and L/s as neighbors off by a small factor when they are off by 60. Converting the time base first, then the volume unit, keeps the mistakes out.

One unit is missing from the drop downs on purpose: US gallons per day and liters per day, used for slow drip and well yield tests. Divide by 1,440 to get GPM from gallons per day, or type the daily volume with hours as the time unit in this calculator and read GPM directly.

Where the estimate drifts

Q = A x v and Q = V/t are exact relationships, but every input carries slack:

• Average velocity is an average. Water moves slower at the pipe wall than at the center, so a single point measurement rarely equals the true mean. The USGS averages many points across a stream cross section for exactly this reason.
• Nominal size is not inside diameter. A 1 inch type L copper tube measures about 1.035 in inside, a 1 inch schedule 40 PVC or steel pipe runs about 1.049 in, and older corroded steel can be well below 1 inch. Area scales with the square, so a 10 percent diameter error is a 21 percent flow error.
• Rated flow is not delivered flow. A pump rated 25 GPM delivers that number at its best efficiency point against its rated head, not at whatever pressure your system presents.
• Open channels are harder than full pipes. Depth changes the wetted area constantly, which is why gauging stations build a stage to discharge curve instead of trusting one measurement.

For household work, a bucket test plus this calculator lands within a few percent of a proper flow meter, and that is enough for sizing a sump, an irrigation zone or a fill time. For permits, water rights or pump procurement, take the measurement with a calibrated meter; the arithmetic here does not replace one.