Optimal Pipe Sizing for Tankless Water Heater Flow: The Engineer’s Guide to GPM, Pressure Drop, and Friction Loss

You bought a 9.5 GPM tankless unit, the biggest your money could get. The shower still turns lukewarm when someone flushes the toilet. The heater isn’t the problem. The pipe is.

Most people size a tankless water heater by counting faucets and picking a GPM rating. That approach ignores the delivery side entirely. Your heater can produce 9.5 gallons per minute, but if the pipe feeding it or distributing hot water can only carry 6 GPM without a massive pressure drop, you’ll never see the rated flow at the showerhead. The pipe is the bottleneck.

This guide covers the engineering behind pipe sizing: pressure drop, friction loss, material differences, and code-compliant fixture unit methods. You’ll walk away with a repeatable two-step process for sizing pipe that actually delivers the flow you paid for.

Before diving into the math, one tool worth having in your truck: the HYDRO MASTER Tankless Water Heater Service Valve Kit. It includes full-port 3/4-inch isolation valves and a pressure relief valve, which means you can isolate the unit for maintenance without restricting flow. The full-port ball valves matter more than most people realize — a standard valve can add the equivalent of several feet of pipe in friction loss. This kit keeps the flow path clean.

Why Pipe Size Matters More Than Heater GPM

Every tankless heater has a published maximum flow rate, usually measured at a specific temperature rise. A typical unit might deliver 8 GPM at a 35°F rise, but only 4 GPM at a 77°F rise. That’s the demand side.

The delivery side is about what the pipe can physically move. Water flowing through a pipe loses pressure to friction against the pipe walls. The smaller the pipe, the higher the velocity, and the faster pressure drops. At some point, the pressure available at the fixture falls below what the shower valve needs to maintain temperature. Cold showers follow.

You can’t solve a pressure drop problem with a bigger heater. The heater only adds heat, not pressure. If your incoming water pressure is 50 PSI and the pipe drops 15 PSI before the shower, you’re left with 35 PSI. The heater’s GPM rating becomes irrelevant.

The Two-Step Sizing Method: Demand vs. Delivery

Proper sizing requires two separate calculations. First, determine the peak flow rate your home demands. Second, determine whether the pipe can deliver that flow at an acceptable pressure drop.

Step 1: Calculate Your Peak Flow Rate (GPM)

Start with a fixture count. List every hot water fixture in the house: showers, faucets, tubs, washing machines, dishwashers. Assign each a flow rate based on the fixture’s aerator or valve rating. A standard showerhead flows 2.5 GPM, a bathroom faucet 1.5 GPM, a kitchen faucet 2.2 GPM, a washing machine 2.0 GPM.

Add them all up. That’s the theoretical maximum if every fixture ran simultaneously. Most homes never hit that number. The plumbing codes use a probability-based method (more on that later), but for a quick estimate, assume 70-80% of fixtures run at once during peak use. A family of four might have two showers running, a faucet, and the washing machine — that’s roughly 8 GPM.

Don’t guess. Use a tankless water heater sizing guide with actual fixture flow rates to get a realistic number.

Step 2: Calculate Your Pressure Drop (Friction Loss)

Friction loss depends on three things: pipe diameter, pipe length, and flow rate. Every pipe material has a roughness coefficient (C factor) that affects friction. Copper and PEX have different C values, and PEX has a smaller internal diameter than copper of the same nominal size.

The Hazen-Williams equation is the standard method for calculating pressure loss in water systems:

Pressure drop (PSI per 100 ft) = 4.52 × Q^1.85 / (C^1.85 × d^4.87)

Where Q is flow rate in GPM, C is the roughness coefficient (150 for PEX, 140 for copper, 150 for CPVC), and d is the internal diameter in inches.

Here’s the practical takeaway: a 3/4-inch pipe at 6 GPM loses about 4 PSI per 100 feet. At 8 GPM, that jumps to 7 PSI. A 1-inch pipe at the same 8 GPM loses only 1.5 PSI per 100 feet. Doubling the diameter cuts pressure drop by roughly 90%.

Pipe Diameter and Material: The Friction Factor

Most residential tankless installations use 3/4-inch pipe for the main hot water line. That works for short runs under 50 feet. But if your heater sits in the basement and your master bath is on the second floor, 3/4-inch might not cut it.

Here’s where material matters. PEX has a smaller internal diameter than copper of the same nominal size. A 3/4-inch copper pipe has an ID of 0.785 inches. A 3/4-inch PEX pipe has an ID of 0.681 inches — about 13% smaller. That smaller ID translates to higher velocity and more friction loss at the same flow rate.

In practice, this means a 3/4-inch PEX run can carry about 20% less flow than copper before hitting the same pressure drop. If you’re replacing copper with PEX, don’t assume the same size will perform the same.

PEX vs. Copper vs. CPVC: Internal Diameter Differences

Nominal 3/4-inch pipe:

• Copper (Type L): ID = 0.785 inches
• PEX (ASTM F876): ID = 0.681 inches
• CPVC (Schedule 80): ID = 0.625 inches

CPVC is the worst offender. Its thick walls reduce the internal diameter significantly. A 3/4-inch CPVC line at 6 GPM loses almost twice as much pressure as copper.

When in doubt, go up one size for PEX or CPVC. A 1-inch PEX line has roughly the same pressure drop as a 3/4-inch copper line at the same flow rate.

The Critical Pipe Sizing Table: Flow Rate vs. Length vs. Diameter

Use this table to size your main hot water line. Values are pressure drop in PSI for the entire pipe length, assuming a C factor of 150 and water at 120°F. These are for straight pipe runs; add 10-20% for fittings and valves.

Pipe Size	Length	4 GPM	6 GPM	8 GPM	10 GPM
3/4″ Copper	50 ft	1.2	2.6	4.4	7.0
3/4″ Copper	100 ft	2.4	5.2	8.8	14.0
3/4″ PEX	50 ft	1.8	3.8	6.5	10.2
3/4″ PEX	100 ft	3.6	7.6	13.0	20.4
1″ Copper	50 ft	0.4	0.9	1.5	2.4
1″ Copper	100 ft	0.8	1.8	3.0	4.8
1″ PEX	50 ft	0.6	1.3	2.2	3.5
1″ PEX	100 ft	1.2	2.6	4.4	7.0
1-1/4″ PEX	100 ft	0.4	0.9	1.5	2.4

Here’s the rule of thumb: if your total pipe length from heater to furthest fixture exceeds 60 feet, use 1-inch pipe for the main trunk. Branch lines to individual fixtures can stay 3/4-inch or even 1/2-inch if the run is short.

Also factor in the pressure drop across the tankless unit itself. Most manufacturers specify a pressure drop of 3-8 PSI at rated flow. That’s not negligible. Add it to your pipe friction loss when calculating total system pressure drop.

How to Size the Recirculation Loop (If You Have One)

Recirculation loops are becoming standard in larger homes. They keep hot water waiting at the tap, but they add a second pipe that needs sizing.

The return line carries cooled water back to the heater. Its size depends on the pump flow rate, not the fixture demand. A typical recirculation pump moves 2-4 GPM. For that flow, a 1/2-inch or 3/4-inch return line works fine. Going larger than 3/4-inch on the return line wastes material without any benefit.

But the supply line feeding the recirculation loop must handle the combined flow of the fixtures and the recirculation flow. If your peak demand is 8 GPM and the pump moves 3 GPM, the supply line needs to carry 11 GPM. Size the supply line for that combined number.

One more thing: the recirculation pump adds head loss. Check the pump curve and add that to your total pressure drop calculation. A pump that can’t overcome the system pressure will stall, and you’ll get cold water at the tap.

Code-Compliant Sizing: The Fixture Unit Method

Plumbing codes don’t use the simple “count faucets” method. The International Plumbing Code (IPC) and Uniform Plumbing Code (UPC) use a fixture unit system that assigns weighted values to each fixture based on the probability of simultaneous use.

For example, a shower is worth 2 fixture units, a bathroom faucet is 1.0, a kitchen faucet is 1.5, a washing machine is 2.0. You add up the total fixture units, then use a conversion chart to find the corresponding flow rate in GPM. For a typical 3-bedroom home, that’s usually 6-8 GPM.

The fixture unit method is more conservative than counting fixtures directly because it accounts for the fact that not every fixture runs at once. It’s also the method building inspectors will use when checking your work.

If you’re pulling a permit, use the code method. If you’re doing a DIY install and don’t need a permit, the simplified method works, but you risk undersizing if you have a large family or a soaking tub that draws 8 GPM by itself.

When to Upsize: Long Runs, High Pressure, and Future Expansion

There are three situations where you should go up a pipe size beyond what the table suggests.

First, long runs over 100 feet. At 100 feet, a 3/4-inch copper line at 8 GPM loses 8.8 PSI. Add the heater’s 5 PSI drop and a few fittings, and you might lose 15 PSI total. If your incoming pressure is 50 PSI, you’re down to 35 PSI at the fixture. That’s borderline for a shower valve. Upsizing to 1 inch cuts that loss to 3 PSI.

Second, high incoming pressure (above 60 PSI). You might think higher pressure lets you get away with smaller pipe. It doesn’t. Higher pressure increases the flow rate through fixtures, which increases the friction loss. A shower valve set to 2.5 GPM at 50 PSI will flow 3.0 GPM at 70 PSI, and the pipe has to handle the extra flow.

Third, future expansion. If you’re planning to add a bathroom, a hot tub, or an outdoor shower, run the larger pipe now. The cost difference between 3/4-inch and 1-inch PEX is pennies per foot. Retrofitting later means cutting open walls.

Common Sizing Mistakes That Cause Cold Showers

Mistake #1: Using the heater’s rated GPM as the pipe sizing basis. The heater may be rated for 8 GPM, but your home only demands 6 GPM. Pipe for the demand, not the heater’s maximum.

Mistake #2: Ignoring the pressure drop across the heater. That 5-8 PSI drop is real. Add it to your pipe loss.

Mistake #3: Using the same pipe size for the entire run. A long trunk line should be larger than the short branch lines. Run 1-inch from the heater to the bathroom group, then reduce to 3/4-inch for the final run to the shower.

Mistake #4: Forgetting about the cold water line. The cold line feeding the heater also needs to be sized. If the cold line is undersized, the heater won’t get enough water to heat, and you’ll get temperature fluctuations.

Mistake #5: Assuming PEX and copper have the same internal diameter. They don’t. Check the manufacturer’s spec sheet for the actual ID before calculating friction loss.

Final Checklist Before You Buy Pipe

• Calculate peak GPM using the fixture unit method or a realistic simultaneous-use estimate.
• Measure the total pipe length from heater to furthest fixture, including fittings and valves.
• Choose pipe material and verify the internal diameter.
• Use the friction loss table or Hazen-Williams equation to find pressure drop.
• Add the tankless unit’s pressure drop (from the manufacturer’s data).
• Ensure total pressure drop stays under 20% of incoming pressure.
• If you have a recirculation loop, size the supply line for combined fixture + pump flow.
• Go up one size if you’re near a boundary or planning future additions.

Don’t skip the last one. The cost difference between 3/4-inch and 1-inch PEX is about $0.30 per foot. The cost of tearing out drywall to replace it later is $500 minimum.

One more thing: check the water softener for tankless if your supply is hard. Scale buildup inside the pipe narrows the effective diameter over time, which increases friction loss. A softener protects both the heater and your pipe sizing assumptions.

Frequently Asked Questions

What size pipe do I need for a tankless water heater?

Most residential tankless installations use 3/4-inch pipe for the main line if the run is under 60 feet. For longer runs or high flow rates above 8 GPM, use 1-inch. Check the pressure drop table above for your specific flow and length.

Can I use PEX for a tankless water heater?

Yes, but you must account for PEX’s smaller internal diameter. A 3/4-inch PEX line has about 13% less cross-sectional area than copper. If the run is long or flow is high, go up one size.

How do I calculate pressure drop in my water lines?

Use the Hazen-Williams equation or a friction loss chart. You need the flow rate in GPM, the pipe’s internal diameter, the pipe length, and the roughness coefficient (C factor). For most residential systems, keep total pressure drop under 10 PSI.

Does the tankless unit itself cause a pressure drop?

Yes. Every tankless heater has an internal heat exchanger that restricts flow. Manufacturer spec sheets list the pressure drop at various flow rates, typically 3-8 PSI. That’s in addition to pipe friction loss.

Is a 3/4-inch pipe enough for two showers running simultaneously?

Two showers at 2.5 GPM each is 5 GPM. A 3/4-inch pipe can handle that if the run is under 50 feet. Beyond that, the pressure drop climbs. If the run exceeds 75 feet, step up to 1-inch.

If you have a well water system, check out our guide on tankless water heaters for well water — the pressure and flow characteristics differ from municipal supply, and that affects pipe sizing too.

And for high-demand households, a high-flow propane tankless might be the right call, but only if the pipe can deliver the gallons.