ToolwickHow to Use This Pipe Sizing & Flow Rate Calculator
This pipe size calculator has three modes. Flow Rate turns a pipe inside diameter and water velocity into gallons per minute (GPM) and friction loss. Find Pipe Size takes the flow you need and a maximum velocity, then recommends the next size up from a Schedule 40, copper Type L or PEX list. Pressure Drop gives the friction loss for a known pipe and flow. Set the material, water temperature, fittings allowance, vertical rise and optional supply pressure first, enter your numbers, then press the button or Enter. Results show velocity, Reynolds number, pressure loss and the pressure left at the outlet.
Why This Matters
Pipe size decides whether a fixture delivers a strong stream or a trickle. A 1-inch Schedule 40 line carrying 20 GPM runs at about 7.4 ft/s and loses roughly 8.8 PSI every 100 feet. That is noisy, hard on fittings, and enough to starve the last shower on the run. Moving the same flow to 1¼-inch pipe drops velocity to about 4.3 ft/s and friction loss to roughly 2.4 PSI per 100 feet.
Plumbers use this check when a long run feeds an outbuilding, when adding a bathroom to an existing branch, or when replacing galvanized pipe. HVAC contractors use it for hydronic loops, where oversized pipe wastes money and undersized pipe forces a bigger pump. Irrigation installers use it to find how many zones one supply line can feed. Homeowners can sanity-check a contractor's pipe choice in a minute.
Oversizing has costs too: more material, more labour, and slow hot-water delivery because more cold water sits in the line. The goal is the smallest standard size that keeps velocity and pressure loss within sensible limits.
How It's Calculated
The tool uses the Darcy-Weisbach equation, the standard method for friction loss in full pipes, with the Swamee-Jain approximation of the Colebrook friction factor.
- Area:
A = π × D² / 4 - Flow:
Q (ft³/s) = A × V, thenGPM = ft³/s × 448.831 - Reynolds number:
Re = V × D / ν(D in ft; ν is water's kinematic viscosity at your temperature, for example 1.217×10⁻⁵ ft²/s at 60 °F) - Friction factor:
f = 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re⁰·⁹)]², or64/Rewhen Re is below 2,300 - Head loss:
h = f × (L/D) × V² / (2g), g = 32.174 ft/s² - Pressure:
ΔP (PSI) = h × ρ / 144, with ρ the water density (about 62.37 lb/ft³ at 60 °F) - System check:
total loss = friction × (1 + fittings %) + rise × ρ / 144, andoutlet pressure = supply − total loss
Between Re 2,300 and 4,000 the flow is transitional, so the higher of the laminar and turbulent friction factors is used. For sizing, the result is rounded up to the next listed size, never down. If you enter a supply pressure, the recommended size must also leave at least 15 PSI at the outlet. Inside diameters come from ASME B36.10M (Schedule 40), ASTM B88 (copper Type L) and typical SDR-9 PEX dimensions, which vary a little by manufacturer. The 30% fittings default is a rule of thumb, not a count of your actual fittings.
Worked Example
You need 20 GPM through 100 ft of Schedule 40 PVC at 60 °F, keeping velocity to 5 ft/s or less. Choose Find Pipe Size, pick Schedule 40, and enter those numbers with the default 30% fittings allowance.
- Flow: 20 ÷ 448.831 = 0.04456 ft³/s.
- Minimum area: 0.04456 ÷ 5 = 0.008913 ft², so the minimum inside diameter is 1.28 in.
- Next size up: 1¼-inch Schedule 40, ID 1.380 in. The 1-inch size would run at about 7.4 ft/s, so it is rejected.
- Actual velocity: 4.29 ft/s. Reynolds number is about 40,500 (turbulent) and the friction factor is about 0.0219.
- Head loss: 0.0219 × (100 ÷ 0.115) × (4.29² ÷ 64.35) ≈ 5.46 ft of water.
- Pressure drop: 5.46 × 62.37 ÷ 144 ≈ 2.36 PSI. The 30% fittings allowance adds 0.71 PSI.
- With the outlet 10 ft higher, elevation adds 10 × 62.37 ÷ 144 ≈ 4.33 PSI. Total loss is about 7.40 PSI, so a 60 PSI supply leaves about 52.6 PSI.
Switch the pipe type to copper Type L and the answer changes: 1¼-inch Type L has an ID of only 1.265 in, so velocity is 5.1 ft/s and the tool recommends 1½-inch instead.
Tips & Common Mistakes
- Use the real inside diameter. Nominal size is a label. ¾-inch copper Type L has an ID of 0.785 in, ¾-inch Schedule 40 is 0.824 in, and ¾-inch PEX is about 0.68 in. Pick the right pipe type, or type in your product's true ID.
- Check fittings and valves. Elbows, tees and valves can add 30–50% to the friction of a long run. Raise the allowance if your layout has many of them.
- Enter the vertical rise. Every 2.31 ft of height costs about 1 PSI whatever the pipe size, which matters for second-storey and hillside runs.
- Keep hot lines slower. Hot lines are often held to about 3–5 ft/s to reduce noise and erosion in copper.
- Round up, never down. If a result falls between sizes, take the larger one. Never size down to save money on a line you cannot easily replace, and remember that code sizing for a whole house uses fixture units, not one flow figure.
Frequently Asked Questions
What velocity should I target for water pipe?
For general water supply, 2–5 ft/s is a comfortable design range. Many guides allow up to about 8 ft/s for cold lines and keep hot lines and copper lower. These are rules of thumb, so check your local code.
What is the difference between nominal pipe size and inside diameter?
Nominal size is a trade name, not a measurement. A 1¼-inch Schedule 40 pipe has an inside diameter of 1.380 inches, while 1¼-inch copper Type L is 1.265 inches. Flow calculations need the actual ID, which depends on material and wall thickness.
Does this calculator work for gas lines or other fluids?
No. It models liquid water only. Gas sizing involves compressibility, specific gravity and different pressure equations, and oils or glycol mixes have different viscosity. Use a dedicated gas or fluid-specific tool for those systems.
Why does water temperature matter?
Warm water is thinner, so its viscosity is lower and friction loss changes slightly. At 140 °F the kinematic viscosity is about a third of its value at 40 °F. The effect on pressure drop is modest but real, which is why the temperature selector is included.
How do I account for fittings, valves and elevation?
Enter a fittings allowance (30–50% is typical for a run with several elbows and valves) and the vertical rise in the settings box. The tool adds friction, the fittings share and about 1 PSI per 2.31 ft of rise into one total. A detailed layout still needs a full equivalent-length count.
How accurate is the Swamee-Jain approximation?
It stays within roughly 1–2% of the full Colebrook-White equation for Reynolds numbers from 5,000 to 100 million and typical roughness ratios. That is well inside the uncertainty of real pipe roughness, so it suits planning work.