Time how long your filter takes to fill a bucket and get its flow rate in L/min, GPM and gallons per day — then check whether it can keep up with your home's peak demand.
Enter the volume filtered and the time taken — we handle all conversions
Step 1 — Volume filtered
Volume Filtered
Peak Demand (optional)
Don't know your peak demand? Estimate it:Uses US maximum flows: showerhead 2.5 GPM (9.5 L/min), kitchen tap 2.2 GPM (8.3 L/min). Low-flow fittings use less.
Step 2 — Time to filter
Hours
hr
Minutes
min
Seconds
sec
⚠ Please enter a volume greater than zero and a total time greater than zero.
WaterTankCalculator.com — Water Filter Flow Rate Results
Your Results
Flow Rate
—
Litres per Minute (LPM)
US Gallons/min
— GPM
Litres/hour
— L/hr
m³ per Hour
— m³/hr
US Gallons/day
— GPD
UK Gal/min
— GPM
Total Time
— min
Filter Sizing Assessment
—
Flow Rate Visualisation
📖 How To Use
How to Use the Water Filter Flow Rate Calculator
You need a container with a known volume — a marked bucket works well — and a timer:
Collect filtered waterOpen the tap after the filter fully, then time how long it takes to fill the container. Enter the volume and the time.
Add your peak demand (optional)Enter the flow your home needs when the most water is in use, or use the estimator — enter how many showers and taps might run at once.
Read the resultYou'll get the flow in L/min, US and UK GPM, L/hr, m³/hr and gallons per day, plus whether the filter keeps up with your demand.
Tip: Test at normal pressure with nothing else running, and repeat two or three times — take the average. For a whole-house filter, test from the tap nearest the filter so pipework doesn't limit the result.
Example: a 10-litre bucket fills in 45 seconds → 10 ÷ 0.75 = 13.3 L/min (3.5 GPM)
Flow rate units
Unit
Equals
1 litre per minute (L/min)
0.264 US GPM · 60 L/hr
1 US gallon per minute (GPM)
3.785 L/min · 1,440 GPD
1 UK gallon per minute
4.546 L/min
1 cubic metre per hour
16.67 L/min
100 US gallons per day (GPD)
0.26 L/min — how RO membranes are rated
How much flow does a home need?
Size a whole-house filter for the most water your home uses at one moment, not the average. US federal law caps fixture flows, which gives a useful upper estimate:
Fixture
US maximum flow
In L/min
Showerhead
2.5 GPM
9.5
Kitchen faucet
2.2 GPM
8.3
WaterSense showerhead
2.0 GPM or less
7.6 or less
WaterSense bathroom faucet
1.5 GPM or less
5.7 or less
Example: two showers and a kitchen tap at once = 9.5 + 9.5 + 8.3 = 27.3 L/min (7.2 GPM). Choose a filter whose rated flow is at or above that, and check the pressure drop the manufacturer lists at that flow.
🔧 Use Cases
When to Check Your Filter's Flow Rate
🏠
Sizing a Whole-House Filter
A filter that can't pass your peak demand causes pressure drops at the taps. Compare its rated flow with your peak.
🔍
Diagnosing Low Pressure
If pressure fell after a filter was fitted, measure flow before and after it. A big difference points to a clogged or undersized cartridge.
📅
Knowing When to Change Cartridges
Write down the flow with a new cartridge. When it falls noticeably below that, the cartridge is clogging — change it, or follow the maker's schedule if that comes first.
💧
Checking an RO System
Measure how fast the membrane fills a container (with the storage tank valve closed) and compare the gallons per day with its rating. Check its TDS rejection with our TDS calculator.
☀️
Before a UV Unit
UV units have a maximum flow for their rated dose. Make sure your peak flow doesn't exceed it — see the UV disinfection calculator.
⚡
Matching a Pump
Check the filter's pressure drop at your pump's flow, and make sure the pump can't exceed the housing's maximum pressure rating.
❓ FAQ
Frequently Asked Questions
How do I calculate water filter flow rate?
Divide the volume of water by the time it took. If a 10-litre bucket fills in 45 seconds (0.75 minutes), the flow rate is 10 ÷ 0.75 = 13.3 L/min, or 3.5 US GPM.
What flow rate does a whole-house filter need?
At least your home's peak demand — the most water used at one moment. Add up the fixtures that might run together: US maximum flows are 2.5 GPM (9.5 L/min) for a showerhead and 2.2 GPM (8.3 L/min) for a kitchen tap. Two showers and a tap need about 27 L/min (7.2 GPM). Choose a filter rated at or above that, with an acceptable pressure drop.
What is the difference between LPM, GPM and GPD?
LPM is litres per minute. GPM is US gallons per minute (1 GPM = 3.785 L/min); UK gallons are larger (1 UK GPM = 4.546 L/min). GPD is gallons per day and is mainly used for RO membranes: 100 GPD is only about 0.26 L/min.
Why has my filter's flow rate dropped?
Usually because the cartridge is clogging with sediment, scale or biofilm — that's what it's designed to catch. Replace it following the manufacturer's schedule or when flow falls noticeably from what you measured with a new cartridge. If flow is low with a new cartridge, check the supply pressure and the filter's rating.
Can I use this for a reverse osmosis system?
Yes. Close the storage tank valve, run the RO tap into a measuring jug and time it — the calculator shows the result in gallons per day to compare with the membrane's rating. Production varies with pressure and temperature, and membrane health is better judged by TDS rejection, which you can check with a TDS meter before and after.
How does water pressure affect filter flow rate?
More pressure pushes more water through the filter, so flow rises with pressure and falls when pressure is low. Manufacturers state flow ratings at a given pressure and list the pressure drop across the filter at a given flow — compare those with your system. A pressure-reducing valve lowers flow; it's only needed if your pressure is above the housing's rating.
How do I measure flow rate accurately?
Use a container with a known volume of at least 5–10 litres so small timing errors don't matter, open the tap fully, start the timer as the water hits the container and stop it when it reaches the mark. Repeat three times and average. Make sure nothing else is using water during the test.
✅ Sources & Review
Sources and review
The fixture flows and conversions on this page come from these references:
US Energy Policy Act of 1992 (10 CFR 430.32) — maximum flow of 2.5 GPM for showerheads and 2.2 GPM for kitchen faucets.
US EPA WaterSense — 2.0 GPM showerheads and 1.5 GPM bathroom faucets.
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
