Free Plumbing & HVAC Tool
Flow Rate Calculator
Convert between GPM and pipe velocity in either direction, measure real flow with a bucket test, and see whether your velocity sits inside the design range — plus friction loss per 100 feet for the material you are actually running.
Built by SubcontractorHub — the software plumbing contractors use to quote, finance, and run every job.
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Calculate Flow Rate, Velocity & Friction Loss
Pick a pipe size, then work in whichever direction you have data for. Every mode returns flow, velocity, and friction loss together, so you can size on all three at once.
Most designs target 4 to 5 ft/s.
Affects friction loss only, not velocity.
Flow rate
10.8 GPM
Velocity
4 ft/s
Friction loss
6.36 ft/100 ft
2.75 psi per 100 ft
Also equals
647 GPH
40.8 L/min
4 ft/s — In the recommended range
This is the range most designers target. Quiet operation, low friction loss, and minimal erosion risk on every common piping material.
Velocity and flow are derived from the inner diameter you enter; friction loss uses the Hazen-Williams equation with the C value for the selected material and does not include losses through fittings, valves, or elevation change. All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.
Schedule 40 flow at common design velocities
| Nominal size | Inner diameter | GPM at 4 ft/s | GPM at 8 ft/s |
|---|---|---|---|
| 1/2" | 0.622" | 3.8 | 7.6 |
| 3/4" | 0.824" | 6.6 | 13.3 |
| 1" | 1.049" | 10.8 | 21.6 |
| 1-1/4" | 1.38" | 18.6 | 37.3 |
| 1-1/2" | 1.61" | 25.4 | 50.8 |
| 2" | 2.067" | 41.8 | 83.7 |
| 2-1/2" | 2.469" | 59.7 | 119.4 |
| 3" | 3.068" | 92.2 | 184.3 |
| 4" | 4.026" | 158.7 | 317.4 |
| 6" | 6.065" | 360.2 | 720.4 |
4 ft/s is a common design target; 8 ft/s is the usual practical ceiling. Copper hot-water lines are typically held to 5 ft/s.
From Sizing Numbers to a Signed Proposal
SubcontractorHub is the platform plumbing contractors use to turn sizing and cost numbers like these into a branded proposal, financing the homeowner can accept on the spot, and a scheduled job — without re-entering anything. This tool is free to use; the platform is here if you want a closer look.
Field crews rarely lose money on the arithmetic — they lose it on the trip back to the supply house. When the sizing decision, the material list, and the change order all live in the same job record, the office sees a spec change the day it happens instead of the week the invoice lands. That is the gap plumbing contractor software is meant to close, and it is the same reason HVAC contractor software puts load and flow figures next to the proposal rather than in a separate spreadsheet.
How Flow Rate Is Calculated
Flow rate and velocity are two views of the same thing, linked by the pipe's cross-sectional area. Multiply velocity by area and you get volume per unit time:
GPM = velocity (ft/s) × area (ft²) × 448.831
Area = π × (inner diameter in inches ÷ 24)². The 448.831 factor converts cubic feet per second to gallons per minute.
The practical consequence is that flow scales with the square of the diameter. Going from a 1-inch to a 2-inch pipe does not double capacity at a given velocity — it nearly quadruples it. That is why one size up is often a cheaper fix than a bigger pump.
Why velocity is the number that matters
A pipe has no fixed capacity — it passes whatever the available pressure pushes through it. What tells you whether that flow is acceptable is velocity. Too slow and sediment and air settle out instead of being carried along. Too fast and you get flow noise, water hammer, and erosion at the fittings, where the direction change concentrates the wear. Most designs land at 4 to 5 ft/s, with 8 ft/s as the usual ceiling and 5 ft/s for copper hot water.
Friction loss rises faster than flow
The Hazen-Williams equation puts friction loss proportional to flow raised to the 1.852 power. Double the flow through a given pipe and the loss goes up by roughly a factor of 3.6. This is the single most common sizing surprise on long runs: the velocity looks acceptable, but the fixture at the end of a 300-foot run has lost more pressure than the design allowed for. Multiply the per-100-foot figure by your developed length — including an allowance for fittings — before you commit.
The bucket test is still the fastest field check
When you need actual flow rather than design flow, a known-volume container and a stopwatch beat any estimate. Open the fixture fully, time the fill, and divide gallons by seconds before multiplying by 60. Run it two or three times and average, and confirm nothing else is drawing on the same branch. Use the Measure mode above to skip the arithmetic.
Frequently Asked Questions
- How do you calculate flow rate in GPM?
- Flow rate in gallons per minute equals velocity multiplied by the pipe's cross-sectional area. In practical units: GPM = velocity (ft/s) × area (ft²) × 448.831. The 448.831 factor converts cubic feet per second into gallons per minute. If you already know the flow and want the velocity instead, divide: velocity = GPM ÷ (area × 448.831). This calculator does both directions, and also lets you measure flow directly with a bucket and a stopwatch.
- What is a good water velocity for plumbing pipe?
- Most designers target 4 to 5 ft/s. Below about 2 ft/s, sediment and air can settle out rather than being carried along. Above about 8 ft/s you get audible flow noise, water hammer risk, and erosion at fittings and elbows. Copper hot-water lines are the strictest case: 5 ft/s is the usual working ceiling because erosion-corrosion accelerates sharply above it. PVC and PEX tolerate the upper end better than copper does.
- How many GPM can a 1-inch pipe handle?
- A 1-inch Schedule 40 pipe has an inner diameter of 1.049 inches. At a 4 ft/s design velocity it carries about 10.8 GPM; at the 8 ft/s practical ceiling it carries about 21.6 GPM. The honest answer is that a pipe does not have one fixed capacity — it carries whatever the pressure pushes through it, and the velocity is what tells you whether that flow is acceptable. Size on velocity and friction loss, not on a single capacity number.
- How do you measure flow rate without a flow meter?
- Use the bucket test. Take a container of known volume — a 5-gallon bucket is standard — open the fixture fully, and time how long it takes to fill. Flow rate in GPM equals gallons divided by seconds, multiplied by 60. A 5-gallon bucket that fills in 40 seconds is 5 ÷ 40 × 60 = 7.5 GPM. Run it two or three times and average, and make sure no other fixture is drawing at the same time. The Measure mode in this calculator does the arithmetic for you.
- What is friction loss and why does it matter?
- Friction loss is the pressure a system gives up to move water against the pipe wall. It is reported in feet of head per 100 feet of pipe, and it rises much faster than flow does — roughly with flow to the 1.85 power. Doubling the flow through a given pipe nearly quadruples the loss. It matters because it determines whether the fixture at the far end of the run has usable pressure, and whether a pump is correctly sized. This calculator uses the Hazen-Williams equation, the standard method for water in pressurized pipe.
- Does pipe material change the flow rate?
- Material does not change the relationship between velocity and flow — that is pure geometry. What it changes is friction loss, through the Hazen-Williams C value: about 150 for new PVC, CPVC and PEX, 130 for copper, 120 for galvanized steel, and near 100 for aged cast iron. A lower C means a rougher wall and more pressure lost per 100 feet. Aged pipe can lose a third or more of its original C value, which is why retrofit work often needs a larger size than the original install.
- How do I convert GPM to feet per second?
- Divide the flow by the pipe area and the conversion factor: velocity (ft/s) = GPM ÷ (area in ft² × 448.831). The area of a round pipe is π × (inner diameter in inches ÷ 24)². For a 2-inch Schedule 40 pipe carrying 40 GPM: area is 0.0233 ft², so velocity is 40 ÷ (0.0233 × 448.831) = about 3.8 ft/s. Switch this calculator to Flow to velocity mode to run the conversion directly.
- Why does flow rate matter for HVAC contractors?
- On hydronic and chilled-water systems, flow rate is what actually delivers capacity. A coil rated for a given number of BTUs assumes a design GPM through it — short the flow and you short the heat transfer, no matter how well the equipment is sized. Flow also drives pump selection, balancing valve settings, and the delta-T you will read at commissioning. Getting it wrong shows up as a comfort complaint that no amount of thermostat adjustment fixes.
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Book a DemoFlow and velocity figures are derived from the inner diameter entered and assume full, steady, single-phase flow of water at ordinary temperatures. Friction loss uses the Hazen-Williams equation and excludes fittings, valves, elevation change, and the effect of aged or scaled pipe, any of which can dominate the result on a real system. All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.