Pipe Size Calculator
Proper pipe sizing for water supply lines balances two competing limits: minimum velocity (2 ft/sec to prevent sediment settling in hot water lines) and maximum velocity (8 ft/sec for general supply, 4 ft/sec for hot water recirculation to prevent erosion and water hammer noise). This calculator converts your peak flow rate in GPM to the minimum pipe internal diameter needed, then rounds up to the nearest standard nominal pipe size. Copper Type L is the standard for residential supply lines; PEX is increasingly common for its flexibility, freeze resistance, and easier installation; CPVC handles hot water but is more brittle than copper. Estimate peak flow by summing simultaneous fixtures: a shower uses 2.5 GPM, a lavatory 1 GPM, a toilet 1.6 GPM. For whole-house main lines, 3/4" handles 5–7 simultaneous fixtures; 1" handles 8–12.
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Top Pick3/4" Copper Type L Pipe — Home Depot~$12/10 ft* Affiliate links — we may earn a commission at no cost to you.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
What pipe size do I need for a residential main water line?
Most residential main lines use 3/4" or 1" pipe. At a peak demand of 10 GPM, a 3/4" copper pipe carries water at ~7 ft/sec — within the 8 ft/sec limit. For homes with high fixture counts (5+ bathrooms), use 1" main lines.
Why does pipe velocity matter?
Velocity above 8 ft/sec causes water hammer noise, erosion at fittings, and premature pipe failure. Below 2 ft/sec, sediment can accumulate in hot water lines. The sweet spot is 3–6 ft/sec for supply lines.
What is the difference between copper, PEX, and CPVC?
Copper is the traditional choice — durable, long-lasting (50+ years), and compatible with all fittings. PEX is flexible plastic tubing that resists freezing, requires no soldering, and is faster to install — increasingly the professional's choice for remodels. CPVC is rigid plastic rated for hot water — cheaper than copper but more brittle and prone to cracking in freezing conditions. Never use PVC (not CPVC) for hot water supply lines.
How do I calculate water flow rate (GPM) for my home?
Estimate peak simultaneous demand by adding up the fixtures likely to run at the same time during peak morning use. A shower = 2.5 GPM, lavatory = 1 GPM, kitchen faucet = 2 GPM, dishwasher = 1.5 GPM, toilet = 1.6 GPM. For a 3-bathroom house with a kitchen, assume 8–12 GPM peak demand. The IPC also provides fixture unit calculations for more precise sizing.