How to Use the Pipe Flow Rate Calculator
The calculator has three tabs. Use whichever matches the information you have.
1. From pipe size and velocity. Choose the pipe material and size (or enter an internal diameter), then enter the water velocity. Use this when you're designing a system and want to know how much a pipe can carry at a sensible speed.
2. From pressure drop. Enter the pipe size, the length of the run, and how much pressure you can afford to lose along it. The calculator solves the Hazen-Williams equation for flow. Use this to estimate what a long supply line, a garden hose run, or an irrigation main will actually deliver.
3. Bucket test. Fill a container of known size from the tap or outlet and time it. Enter the volume and the seconds it took. This is the most reliable way to find what your home's supply really delivers, and it's the test boiler installers and well contractors use.
All three tabs show the flow rate in every common unit, plus the velocity if you've given a pipe size.
What Flow Rate Means (and How It Differs from Pressure)
Flow rate is the volume of water passing a point each minute. Pressure is the force pushing it. They're related but not the same:
- Static pressure is what a gauge reads with every tap shut. It might be 60 psi (about 4 bar) and still give a weak shower.
- Dynamic (working) pressure is what's left while water is flowing. Friction in the pipes eats into it, so it is always lower.
- Flow rate depends on how much pressure is available and how much resistance the pipework adds.
A house with high static pressure and narrow, scaled-up pipes can have poor flow. A house with modest pressure and short, generous pipes can have excellent flow. That's why a flow rate calculator is more useful than a pressure gauge alone when diagnosing a weak shower or sizing a combi boiler.
Flow rate units and conversions
| From | To | Multiply by |
|---|---|---|
| US gallons per minute (GPM) | Litres per minute (L/min) | 3.785 |
| L/min | US GPM | 0.264 |
| US GPM | Cubic metres per hour (m³/h) | 0.227 |
| L/min | Litres per second (L/s) | 0.0167 |
| Imperial gallons per minute | L/min | 4.546 |
| US GPM | Gallons per hour (GPH) | 60 |
Watch out for imperial gallons. Older UK pump and boiler literature sometimes quotes imperial GPM, which is 20% bigger than a US gallon. The calculator uses US gallons in USA mode and litres in UK mode.
Formula 1: Flow Rate from Pipe Size and Velocity
The basic flow rate formula is the continuity equation:
where Q is the flow rate, A is the internal cross-sectional area of the pipe (π × d² ÷ 4), and v is the average water velocity.
In practical units:
Always use the internal diameter, not the nominal size. A "1-inch" copper Type L pipe has an internal diameter of 1.025 inches, while 1-inch PEX is only about 0.862 inches. The Pipe Size Calculator lists internal diameters for each material.
Worked example
These are capacities at a chosen velocity, which is how designers size pipework. They don't tell you what your supply will actually push through the pipe. For that, use formula 2 or a bucket test.
Formula 2: Flow Rate from Pressure Drop (Hazen-Williams)
To estimate how much water a pipe will deliver with a given pressure available, use the Hazen-Williams equation. It's the standard empirical formula for water flowing through pipes in buildings, irrigation and fire sprinkler systems. Rearranged to solve for flow:
- USA: Q (GPM) = [ (P ÷ L) × C^1.85 × d^4.87 ÷ 4.52 ]^(1 ÷ 1.85)
- with P = pressure loss in psi, L = pipe length in feet, d = internal diameter in inches, and C = roughness coefficient
| Pipe material | Typical C value |
|---|---|
| PVC, CPVC, PEX, PE / MDPE | 150 |
| New copper | 140 |
| Ductile iron (cement lined) | 140 |
| Cast iron, older | 100–120 |
| Galvanized steel, 30+ years | 80–100 |
Worked example (USA)
You have 100 feet of ¾-inch Type L copper and can lose 10 psi along it.
Q = [ (10 ÷ 100) × 140^1.85 × 0.785^4.87 ÷ 4.52 ]^(1/1.85) ≈ 9.4 GPM, at a velocity of about 6.3 ft/s.
If you allow a 20 psi loss, the same pipe passes about 13.7 GPM, but velocity climbs to 9 ft/s, which is noisy and above the usual 8 ft/s limit. That's the trade-off: more pressure loss buys more flow, up to the point where velocity becomes the problem.
Worked example (UK)
Over a 15-metre run with 0.5 bar to lose:
- 15mm copper: about 16 L/min, at 1.85 m/s.
- 22mm copper: about 46 L/min, though velocity would be 2.4 m/s, so in practice you'd run it slower.
That's why installers upgrade the cold main to 22mm for larger combi boilers: a 15mm pipe struggles to deliver much more than 15 L/min over a typical run.
Don't forget elevation
Lifting water costs pressure: 0.433 psi per foot of rise (about 0.1 bar per metre). A shower 20 feet above the meter loses about 8.7 psi before friction is even counted. In the calculator, subtract elevation from the available pressure before you enter it.
Fittings add resistance too. A rough allowance is to add 25–50% to the straight pipe length for a typical house run with elbows, tees and valves.
Formula 3: Measure Flow Rate with a Bucket Test
The most reliable way to know your actual flow rate is to measure it:
- Turn off every other tap and appliance in the house.
- Fully open the cold tap you're testing. The kitchen tap or the outside tap is usually closest to the incoming main.
- Time how long it takes to fill a container of known volume: a 5-gallon bucket, or a 10-litre bucket.
- Flow rate = volume ÷ time × 60.
Examples
Repeat the test two or three times and use the average. For a combi boiler, compare the result to the boiler's rated hot water flow. If your mains delivers 14 L/min and the boiler is rated at 16 L/min, you won't get the full rated flow. The Boiler Size Calculator uses this figure.
What Is a Good Water Flow Rate for a House?
| Situation | USA | UK |
|---|---|---|
| Minimum for comfortable single-bathroom use | 6 GPM | 12 L/min |
| Typical family home | 8–12 GPM | 15–25 L/min |
| Large home with multiple simultaneous showers | 12–20 GPM | 25–40 L/min |
| Small combi boiler (24–28 kW) needs | — | 10–12 L/min |
| Large combi boiler (35–40 kW) needs | — | 14–17 L/min |
| Well pump, typical 3–4 bedroom house | 8–12 GPM | — |
| Lawn irrigation zone | 6–15 GPM per zone | 10–40 L/min per zone |
In the USA, well contractors often rate the pump at 1 GPM per fixture in the house as a rough minimum. In the UK, water companies only have to supply a fairly modest minimum, so homes at the end of long mains or up hills can struggle. If your flow is low, the fix might be upgrading the incoming supply rather than the internal pipework.
Why Your Flow Rate Might Be Low
- Undersized or long supply pipe, especially old ½-inch or 15mm mains.
- Scaled galvanized steel or lead pipe with a much smaller effective bore.
- Partially closed stop valve at the meter, under the sink, or in the street.
- Pressure reducing valve set too low or failing.
- Clogged aerators, showerheads or filters.
- Shared supply with neighbours, common in older UK terraces.
- Elevation: upstairs bathrooms in homes on hills.
Measure first with the bucket test, then use the pressure drop tab to see whether the pipe size explains the result.
Frequently Asked Questions
How do you calculate flow rate in a pipe?
Multiply the pipe's internal cross-sectional area by the water velocity (Q = A × v). In US units, GPM = 2.448 × velocity in ft/s × internal diameter in inches squared. If you know the pressure drop instead of the velocity, use the Hazen-Williams formula.
How many GPM can a ¾-inch pipe carry?
At a comfortable 5 ft/s, ¾-inch Type L copper carries about 7.5 GPM. At the usual 8 ft/s maximum for cold water, it carries about 12 GPM. Actual flow depends on pressure and length.
How many litres per minute through a 15mm pipe?
About 13 L/min at 1.5 m/s and 17 L/min at 2 m/s. On a 15-metre run with 0.5 bar available, expect roughly 16 L/min.
Does higher water pressure mean higher flow rate?
Yes, more available pressure pushes more water through the same pipe, but flow rises with roughly the square root of pressure. Doubling the pressure gives about 45% more flow, not double.
How do I measure my water flow rate at home?
Use the bucket test. Open the cold tap fully with nothing else running, time how long a bucket of known size takes to fill, and divide the volume by the seconds, then multiply by 60.
What flow rate do I need for a combi boiler?
Most combis need at least 10–12 L/min from the mains to run at their rated output, and larger 35–40 kW models need 14–17 L/min. Check your mains with a bucket test before choosing a combi.
How many GPM does a house need?
Most US homes need 6–12 GPM at peak, depending on the number of bathrooms. Add 1 GPM per fixture as a rough well pump guide, and more for irrigation.
What is the difference between US and imperial GPM?
A US gallon is 3.785 litres and an imperial gallon is 4.546 litres. One imperial GPM equals about 1.2 US GPM. Always check which gallon a product sheet uses.
Low flow or pressure? Get a plumber to check your supply
A plumber can test flow and pressure at the meter and quote for a new main if you need one. Get local quotes before you start.