Use pipe dimensions & velocity, or fill a container and time it
Instant results in L/min, GPM, m³/hr and 6 more units — no manual conversion needed.
Enter pipe dimensions & flow velocity
Inner Diameter
Flow Velocity
⚠ Please enter valid values (greater than zero) for diameter and velocity.
Enter the volume collected and the time taken
Volume Collected
Time Taken
⚠ Please enter valid values (greater than zero) for volume and time.
Your Results
Flow Rate
—
Litres per Minute (L/min)
Litres / Hour
— L/hr
US Gallons / Min
— GPM
US Gallons / Hr
— GPH
UK Gallons / Min
— GPM
Cubic Metres / Hr
— m³/hr
Cubic Feet / Min
— ft³/min
Litres / Second
— L/s
Cubic Metres / Day
— m³/day
Flow Rate Visual
📖 How To Use
How to Use This Calculator
This water flow rate calculator offers two methods. Choose the one that matches your situation:
Method 1 — Pipe Diameter & VelocityEnter the pipe's inner diameter (not the nominal size — a pipe's bore is smaller than its outside diameter) and the average flow velocity. Best for design work, when you know or have chosen a velocity. The results note checks whether that velocity is reasonable for water.
Method 2 — Volume ÷ TimeFill a container of known volume (e.g., a 10-litre bucket) and record how many seconds or minutes it took. Enter both values. Best for on-site measurement of taps, pumps, and irrigation outlets.
Read your resultsThe primary result shows litres per minute. The grid below shows the same flow rate in GPM, m³/hr, L/hr, ft³/min, and more — all calculated in one step.
Copy or printUse the Copy button to save results to your clipboard, or Print for a clean record you can attach to a job sheet or system design document.
Tip: For the bucket method, use a container of at least 10 litres for accuracy — smaller volumes amplify timing errors. Time three fills and average the results. For pipes, a clamp-on or inline flow meter is the practical option when you can't collect the water.
📐 The Formula
Water Flow Rate Formula Explained
There are two standard ways to calculate volumetric flow rate, depending on what you can measure.
Method 1: Pipe Cross-Section × Velocity
Q = A × v
Q = Flow Rate (m³/s)
A = Cross-sectional area of pipe (m²) = π × (d/2)²
v = Average fluid velocity (m/s)
To convert: Q (L/min) = Q (m³/s) × 60,000
This is the continuity equation from fluid mechanics. Water is effectively incompressible, so the same flow passes every point along a pipe. Velocity is not uniform across a pipe — it is fastest at the centre — so v must be the average velocity; with that, Q = A × v is exact.
Method 2: Volume ÷ Time
Q = V ÷ t
Q = Flow Rate (L/min)
V = Volume collected (litres)
t = Time taken (minutes)
This is the most direct measurement — collect a known volume over a measured time. It accounts for any real-world losses and is the method used to calibrate meters and verify pump performance.
Flow Rate Conversion Reference
Unit
= Litres/min
From L/min
1 L/min
1.000
× 1
1 L/hr
0.01667
× 60
1 L/s
60.000
÷ 60
1 US GPM
3.7854
÷ 3.7854
1 US GPH
0.06309
÷ 0.06309
1 UK GPM
4.5461
÷ 4.5461
1 m³/hr
16.667
÷ 16.667
1 ft³/min (CFM)
28.317
÷ 28.317
Typical Flow Velocities for Water
Application
Typical Velocity
Notes
Household supply pipe
0.5 – 1.5 m/s
Too fast causes noise and erosion
Pump discharge
1.5 – 3.0 m/s
Standard design range
Gravity mains
0.3 – 1.2 m/s
Very low velocities let sediment settle
Irrigation laterals
0.5 – 2.0 m/s
Depends on pipe material
Fire suppression
2.0 – 4.5 m/s
High velocity, short duration — follow the fire code
These are common design ranges, not code limits. Local plumbing and fire codes, and the pipe maker, set the actual limits for your system.
🔍 When to Use
Common Use Cases for Water Flow Rate Calculation
🚿
Shower & Tap Flow
Check what your shower or taps actually deliver. A standard showerhead uses about 2.5 US gallons (9.5 L) a minute and a WaterSense model no more than 2.0 (7.6 L); a much lower reading points to low pressure or a blockage.
⚙️
Pump Selection
Specify the right pump for a water tank, irrigation system, or booster by calculating the required flow rate at design conditions.
🏗️
Pipe Sizing
Check that a proposed pipe diameter delivers the target flow rate at an acceptable velocity — avoiding both starved supply and erosion.
🌾
Irrigation Design
Calculate drip or sprinkler system flow to ensure the supply pipe and pump meet peak demand across all zones simultaneously.
🏠
Tank Fill Time
Know your flow rate to estimate how long it takes to refill a household rooftop tank. Pair with our Tank Refill Time Calculator for exact duration.
📊
Water Audit
Benchmark actual flow rates against design values to identify leaks, partially-closed valves, or deteriorating pumps.
🧮 Worked Example
Worked examples: a bucket test and a pipe
Bucket test at an outside tap
A 10-litre bucket fills in 32 seconds. Q = 10 L ÷ 32 s × 60 = 18.75 L/min (shown as 18.8) — about 4.95 US GPM, 1,125 L/hr or 1.13 m³/hr. At that rate a 1,000 L tank takes about 53 minutes to fill.
20 mm bore pipe at 1.5 m/s
Area = π × (0.020 ÷ 2)² = 0.000314 m². Q = 0.000314 × 1.5 = 0.000471 m³/s × 60,000 = 28.3 L/min — about 7.47 US GPM or 1.70 m³/hr. That velocity sits comfortably inside the usual range for water pipes.
❓ FAQ
Frequently Asked Questions
How do I calculate water flow rate?
There are two common methods. For a pipe: Flow Rate (Q) = Cross-Sectional Area (A) × Velocity (v). For a measured fill: Flow Rate = Volume ÷ Time. This calculator does both automatically — just enter your known values and select your units.
What is a good water flow rate for a home?
It depends on how many fixtures run at once, so there is no single figure. For individual fixtures, the US EPA gives 2.5 gallons per minute (about 9.5 L/min) as the standard flow for showerheads and 2.2 gpm (about 8.3 L/min) for bathroom faucets; WaterSense models use no more than 2.0 gpm (7.6 L/min) and 1.5 gpm (5.7 L/min). A whole house needs enough supply for the fixtures likely to run together — a plumber sizes this from local code fixture-unit tables.
What is the difference between US GPM and UK GPM?
A US gallon is 3.785 litres; a UK (Imperial) gallon is 4.546 litres — about 20% larger. So 10 US GPM ≈ 8.33 UK GPM. Always clarify which gallon is being used on pump datasheets and system specs, especially when importing equipment.
How do I measure flow rate without a meter?
Use the bucket-and-stopwatch method: place a bucket of known volume (e.g., 10 litres) under the outlet and time how long it takes to fill. Divide the volume by the time to get flow rate. If you can't collect the water, a clamp-on ultrasonic flow meter or a reading taken from your water meter over a few minutes (with everything else turned off) works too.
How long will it take to fill my water tank at a given flow rate?
Divide the tank volume (in litres) by the flow rate (in litres per minute) to get fill time in minutes. For example, a 1,000 L tank at 20 L/min takes 1,000 ÷ 20 = 50 minutes. For more detail, use our Tank Refill Time Calculator which accounts for simultaneous consumption.
What pipe diameter do I need for a target flow rate?
Rearrange the formula: Diameter = 2 × √(Q ÷ (π × v)). For a target flow rate of 20 L/min (0.000333 m³/s) at a velocity of 1.5 m/s, the required inner diameter is about 16.8 mm. In practice, round up to the nearest standard pipe size. Use our Pipe Size Flow Rate Calculator for direct pipe sizing.
What flow rate does a 1 HP pump deliver?
It depends on the head (the height and pressure it must pump against) and the pump's efficiency. Physics sets the limit: water power = flow × head ÷ a constant (in US units, GPM × feet ÷ 3,960). A 1 HP (746 W) pump that is 40–60% efficient can lift roughly 90–140 L/min against 20 m of head, and about half that against 40 m. Small pumps are often less efficient, so always check the maker's performance curve at your operating head — not just the horsepower. Use our Pump Horsepower Flow Rate Calculator for precise sizing.
Does pipe material affect flow rate?
Pipe material affects friction losses along the pipe's length, which reduces pressure (and therefore flow rate at a given pressure). Smooth materials like PVC and copper have lower friction coefficients (Hazen-Williams C ≈ 140–150) than older galvanized iron (C ≈ 80–100). For short runs this makes little difference; over long runs it becomes significant. This calculator converts a velocity or a measured fill into flow, so friction is already reflected in the velocity or fill you measure. To size a pipe and estimate pressure loss, use the pipe size flow rate calculator.
✅ Sources & Review
Sources and review
The two formulas are standard fluid mechanics; the reference figures on this page come from these sources:
NIST SP 811 — unit definitions and conversions (1 US gallon = 3.785 L, 1 UK gallon = 4.546 L, 1 ft³ = 28.32 L).
US EPA WaterSense — Showerheads (standard 2.5 gpm; WaterSense no more than 2.0 gpm) and Bathroom Faucets (standard 2.2 gpm; WaterSense no more than 1.5 gpm).
Kansas State University Research and Extension — MF-1040: water horsepower = GPM × head (ft) ÷ 3,960.
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.
