Calculator
Pipe Sizing Calculator
Why Pipe Sizing Matters for Plumbing Contractors
Wrong pipe size is one of the most common callback triggers in residential plumbing. Undersized pipes cause low pressure, noisy walls, and premature erosion. Oversized pipes waste material and can cause stagnation in hot water lines. This pipe sizing calculator uses the Hazen-Williams formula to estimate pressure drop and flow velocity for copper, PEX, PVC, and galvanised pipe. Enter the flow rate directly or estimate it from the fixtures on the run. The comparison table shows all standard sizes at once, so you can pick the right one without running the numbers multiple times. Use the results alongside TackonFSM’s estimating tools to build accurate material lists and quotes.
Plumbing Pipe Sizing Calculator
Calculate pressure drop, flow velocity & recommended pipe size for water supply lines
Flow Rate
Fixture Count (Peak Simultaneous Use)
🚿 Shower2.5 GPM
0
🚰 Kitchen Sink2.2 GPM
0
🧼 Bathroom Faucet1.5 GPM
0
🚽 Toilet (tank)3.0 GPM
0
🍽️ Dishwasher1.5 GPM
0
🧺 Washing Machine2.0 GPM
0
🌿 Hose Bib / Outdoor5.0 GPM
0
🛁 Bathtub4.0 GPM
0
Estimated Peak Flow: — GPM
(Diversity factor applied — not all fixtures run simultaneously at peak)
Pipe Parameters
Supply Pressure (Optional)
Results
Flow Rate—
Pipe Size—
Pipe Material—
Flow Velocity—
Friction Loss (per 100 ft)—
Pipe Friction Loss—
Fittings Loss—
Total Pressure Drop—
Pipe Size Comparison
| Pipe Size | Velocity (ft/s) | Pressure Drop (PSI) | Status |
|---|
Green = recommended range (velocity 2–8 ft/s, pressure drop < 5 PSI)
Quick Reference: Pipe Size by GPM (Copper, 0.08 friction rate)
| GPM | Min Pipe Size | Velocity (ft/s) | Common Application |
|---|---|---|---|
| 1–3 | ½" | 2–5 | Single fixture branch |
| 3–6 | ¾" | 3–5 | 2–3 fixture branch |
| 6–12 | 1" | 3–5 | Small home main, multi-fixture |
| 12–22 | 1¼" | 3–6 | Typical residential main |
| 22–40 | 1½" | 4–7 | Large home main |
| 40–75 | 2" | 4–8 | Multi-unit / commercial |
Calculation basis: Hazen-Williams formula for friction loss. Fitting losses estimated using equivalent length method (each fitting ≈ 2.5 ft equivalent per inch of pipe diameter). Velocity calculated as V = 0.4085 × GPM / D². This is an estimate for planning purposes — always verify with local plumbing codes and a licensed plumber for final pipe sizing.
Frequently Asked Questions
Start with the peak flow rate in gallons per minute (GPM) that the pipe needs to deliver. Then select a pipe size that keeps velocity between 4 and 8 feet per second — low enough to avoid noise and erosion but high enough to deliver adequate flow. Check that the total pressure drop across the run stays under 5 PSI for branch lines or under your available pressure budget. The Hazen-Williams formula is the standard method for calculating friction loss in water supply pipes. This calculator handles the math automatically.
For 10 GPM in copper pipe, a 1-inch line gives you about 4.1 ft/s velocity — well within the safe range. A ¾-inch pipe would push velocity to 7.2 ft/s, which is acceptable for short runs but may cause noise on longer ones. For PEX, size up by one size because the inner diameter is slightly smaller than copper. Use the comparison table in this calculator to see pressure drop across all sizes at once.
Most plumbing codes limit water velocity in residential supply lines to 8 feet per second. Above 8 ft/s, you risk pipe erosion (especially in copper), water hammer, and noticeable noise. For quiet installations like bedrooms and bathrooms, contractors often target 5 ft/s or lower. The Uniform Plumbing Code and International Plumbing Code both recommend keeping velocity below 8 ft/s for supply piping.
The Hazen-Williams formula calculates friction loss in water pipes based on flow rate, pipe diameter, and a roughness coefficient (C-value) that varies by material. Copper has a C-value of 150 (very smooth), while old galvanized pipe might be 100 or lower. The formula is: P = (4.52 × Q^1.85) / (C^1.85 × D^4.87), where P is friction loss in PSI per foot, Q is flow in GPM, C is the roughness coefficient, and D is the inside diameter in inches. It's the standard method for sizing domestic water supply pipes.
Not always, but PEX typically has a slightly smaller inside diameter than the same nominal size in copper. For example, ¾" PEX has an ID of about 0.671" compared to 0.785" for ¾" Type L copper. That smaller ID increases velocity and friction loss at the same GPM. For high-flow applications, some plumbers upsize PEX by one nominal size to compensate. PEX also has a slightly lower Hazen-Williams C-value (around 140) than copper (150), which adds a small amount of additional friction.
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