Gallons Per Minute Calculator
Find flow rate, volume, or time — enter any two of the three and this GPM calculator finds the third instantly. Works with gallons, liters, and every common flow-rate unit.
✓ Solve for volume, time or flow rate ✓ GPM, LPM, GPH and more ✓ Works on mobile
Gallons Per Minute (GPM) Calculator
Fill in any two of Volume, Time and Flow Rate — the third is calculated automatically.
What Is GPM (Gallons Per Minute)?
The standard unit for measuring how fast water — or any liquid — is flowing.
GPM stands for gallons per minute, a unit of flow rate that describes how much liquid volume passes a given point over one minute. It's the standard way to describe how fast water moves through pumps, showerheads, garden hoses, faucets, and pipes, and it shows up on equipment spec sheets, plumbing codes, and irrigation system designs alike.
A "gallon" itself can mean two slightly different things depending on where you are: the US liquid gallon (about 3.785 liters) or the British imperial gallon (about 4.546 liters). This calculator defaults to US gallons, since that's the most common convention for GPM, but supports both alongside metric units.
This calculator is built around one simple relationship — flow rate equals volume divided by time — and lets you solve for whichever of the three quantities you don't already know. Know the volume and how long it took to flow? Get the flow rate. Know the flow rate and the volume you need? Get the time. Know the flow rate and how long it'll run? Get the volume.
Gallons Per Minute Formula
One equation, three ways to rearrange it.
The core formula: divide the volume of liquid that flowed by the time it took, in minutes, to get the flow rate in gallons per minute.
Rearranged to solve for volume — useful for finding out how much water a fixture will use over a set amount of time.
Rearranged to solve for time — useful for estimating how long it will take to fill a container at a known flow rate.
How the GPM Calculator Works
Fill in two, get the third — automatically.
Choose Your Known Values
Decide which two of Volume, Time and Flow Rate you already know, and leave the third field empty.
Pick Your Units
Each field has its own unit selector — mix gallons, liters, seconds, hours, GPM, LPM and more freely.
Automatic Calculation
The calculator converts everything to a consistent base internally, applies the GPM formula, and fills in the missing value.
Compare to Common Fixtures
See your flow rate plotted against typical shower and garden hose flow rates on the live gauge.
Complete Guide to Gallons Per Minute
How to measure it yourself, and what typical flow rates look like.
How to Measure Flow Rate Yourself
The simplest way to measure a real fixture's GPM is with a bucket and a stopwatch: place a container of known volume under the tap, time how many seconds it takes to fill, convert that time to minutes, then divide the volume by the time. For example, filling a 5-gallon bucket in 30 seconds (0.5 minutes) gives a flow rate of 5 ÷ 0.5 = 10 GPM.
US Gallons vs Imperial (UK) Gallons
A US liquid gallon is about 3.785 liters, while a British imperial gallon is about 4.546 liters — roughly 20% larger. GPM figures quoted in the US almost always use the US gallon, but it's worth double-checking which gallon a spec sheet means if you're working with equipment sourced from the UK or another imperial-gallon country, since using the wrong one introduces a meaningful error.
Typical Flow Rates for Common Fixtures
Standard showerheads in the US typically flow at around 2 GPM, and water-saving showerheads are specifically capped at a maximum of 2 GPM to qualify as low-flow; older, pre-regulation showerheads can flow noticeably higher. Garden hoses typically flow somewhere between 9 and 17 GPM, with the exact rate depending on hose diameter, length, and the water pressure (measured in psi) feeding it — shorter, wider hoses generally deliver higher flow rates than longer, narrower ones at the same pressure.
Why Flow Rate Matters for Plumbing and Fixtures
Flow rate directly affects how quickly a sink, tub, or pool fills, how effective a shower feels, and how a home's plumbing system needs to be sized to avoid pressure drops when multiple fixtures run at once. Local plumbing codes often specify maximum GPM limits for fixtures like showerheads and faucets as part of water conservation requirements.
Converting Between Flow Rate Units
Since 1 US gallon equals approximately 3.78541 liters, 1 GPM converts to approximately 3.79 LPM (liters per minute) using that same factor. Other common conversions follow the same pattern: GPM to GPH (gallons per hour) multiplies by 60, and GPM to m³/h (cubic meters per hour) uses the combined gallon-to-cubic-meter and minute-to-hour conversion factors.
Sizing Pumps, Irrigation Systems and Pipes
Flow rate is a central spec when choosing a water pump, designing a sprinkler or drip irrigation zone, or sizing a pipe run, since undersized components can't deliver the flow a system needs, while oversized ones waste energy and money. Many of these design tasks work backward from a target fill time or coverage area — using this calculator's Time or Volume modes rather than Flow Rate directly — to figure out what flow rate a project actually requires.
Step-by-Step GPM Examples
Eight worked calculations solving for flow rate, volume and time.
Example 1 — Basic Flow Rate
A bucket holds 5 gallons and fills in 30 seconds (0.5 minutes).
Example 2 — Shower Flow Rate Check
A shower fills a 1-gallon container in 30 seconds.
Example 3 — Solving for Time
A pool holds 15,000 gallons and is filled by a hose at 12 GPM.
Example 4 — Solving for Volume
A hose runs at 10 GPM for 45 minutes.
Example 5 — Converting GPM to LPM
A pump flows at 15 GPM.
Example 6 — Garden Hose Fill Time
A 100-gallon tank is filled by a garden hose flowing at 12 GPM.
Example 7 — Metric Flow Rate
200 liters flow through a pipe in 4 minutes.
Example 8 — Hourly Flow Rate Conversion
A pump is rated at 300 gallons per hour (GPH).
Quick Reference Tables
Common flow rates, unit conversions, and fixture benchmarks.
| Fixture | Typical Flow Rate |
|---|---|
| Water-saving showerhead | ≤ 2 GPM |
| Standard showerhead | ~2 GPM |
| Bathroom faucet | ~1.0–1.5 GPM |
| Kitchen faucet | ~1.5–2.2 GPM |
| Garden hose | 9–17 GPM |
| GPM | LPM |
|---|---|
| 1 GPM | 3.79 LPM |
| 5 GPM | 18.93 LPM |
| 10 GPM | 37.85 LPM |
| 20 GPM | 75.71 LPM |
| Flow Rate | Approx. Fill Time |
|---|---|
| 8 GPM | ≈ 31.25 hours |
| 12 GPM | ≈ 20.8 hours |
| 17 GPM | ≈ 14.7 hours |
Benefits of Using a GPM Calculator
Why a dedicated tool beats manual unit conversion.
Instant Results
Get flow rate, volume or time the moment you enter your two known values.
Solves for Any Variable
Not just flow rate — solve for fill time or required volume just as easily.
Multiple Units Built In
Mix US gallons, UK gallons, liters, cubic feet and more without manual conversion.
Real-World Benchmarking
See your result compared against typical shower and garden hose flow rates.
Useful for Real Projects
Plan pool fill times, irrigation flow, or pump sizing with a single, reliable calculation.
Mobile-Friendly
Check flow rate figures on-site or in the garden on any phone or tablet.
Practical Applications of GPM Calculations
Where flow rate calculations come up in everyday and professional life.
Plumbing & Fixture Testing
Homeowners and plumbers measure real-world flow rates to check fixtures against water-saving requirements.
Pool & Tank Filling
Estimating how long it will take to fill a pool, tank or trough at a known hose or pump flow rate.
Irrigation & Sprinkler Design
Landscapers size sprinkler zones and drip lines based on the flow rate available from a water supply.
Pump Sizing
Selecting a water pump with adequate GPM capacity for a well, sump, or industrial process.
Pipe & Plumbing Design
Engineers size pipe diameter and layout to deliver required flow rates without excessive pressure loss.
Water Conservation & Auditing
Utilities and building managers audit fixture flow rates to identify water-saving upgrade opportunities.
Common GPM Calculation Mistakes
Avoid these errors when measuring or calculating flow rate.
Forgetting to convert seconds to minutes
GPM is gallons per minute — a time measured in seconds needs to be divided by 60 before applying the formula.
Mixing US and UK gallons
A US gallon (3.785 L) and a UK imperial gallon (4.546 L) differ by about 20% — always confirm which one a spec sheet is using.
Using an inaccurate bucket volume
Many household containers aren't exactly their labeled size — measuring actual volume with a known reference gives a more accurate flow rate.
Timing an inconsistent flow
Water pressure can fluctuate, especially if other fixtures are running at the same time — measure flow rate under normal, isolated conditions for an accurate reading.
Confusing flow rate with pressure
GPM measures volume over time; PSI measures pressure. A high-pressure fixture doesn't necessarily have a high flow rate, and vice versa.
Ignoring unit consistency across all three variables
Combining a volume in liters with a flow rate in GPM without converting first produces a meaningless time or volume result.
Frequently Asked Questions
Everything people ask about gallons per minute.
How many GPM is a shower?
Standard showerheads in the US flow at around 2 GPM. Water-saving showerheads are capped at a maximum of 2 GPM to qualify as low-flow; older showerheads may flow higher.
How do I calculate gallons per minute?
Time how many seconds it takes to fill a container of known volume, convert that time to minutes, then divide the volume by the time: GPM = Volume ÷ Time.
How many GPM is a garden hose?
Garden hoses typically flow between 9 and 17 GPM, depending on hose diameter, length, and the water pressure feeding it.
1 GPM equals how many LPM?
1 GPM equals approximately 3.79 LPM, since 1 US gallon equals about 3.78541 liters.
What is the formula for gallons per minute?
GPM = Volume ÷ Time, where volume is in gallons and time is in minutes.
How do I convert GPM to GPH?
Multiply GPM by 60 to get gallons per hour (GPH), since there are 60 minutes in an hour.
How long will it take to fill my pool?
Divide your pool's volume in gallons by your hose or pump's flow rate in GPM to get the fill time in minutes.
What's the difference between a US gallon and a UK gallon?
A US gallon is about 3.785 liters, while a UK (imperial) gallon is about 4.546 liters — roughly 20% larger. GPM figures in the US almost always use the US gallon.
Can this calculator solve for volume or time instead of flow rate?
Yes. Enter any two of Volume, Time and Flow Rate, and the calculator solves for whichever one you leave blank.
What's a good flow rate for a bathroom faucet?
Bathroom faucets typically flow between about 1.0 and 1.5 GPM, often lower than kitchen faucets due to water-saving aerators.
Does water pressure affect GPM?
Yes. Higher water pressure (measured in PSI) generally increases flow rate for a given pipe or hose diameter, though the two are distinct measurements.
How do I convert GPM to cubic meters per hour?
Multiply GPM by approximately 0.227 to get m³/h, combining the gallon-to-cubic-meter and minute-to-hour conversion factors.
Is this calculator accurate?
Yes. It applies the exact GPM = Volume ÷ Time formula along with standard unit conversion factors for gallons, liters and more.
Is it free to use?
Yes, the Gallons Per Minute Calculator is free, with no signup required.
Does it work on mobile?
Yes. The calculator and flow-rate gauge are fully responsive and work well on phones and tablets.
Calculate Flow Rate Instantly
Enter any two of volume, time and flow rate, and get an exact GPM result in seconds.