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How to Size a Tankless Water Heater: Flow, Temp Rise, and Math
-
By
Michael Haines
- Aug 4, 2026
The two numbers that decide whether your shower stays hot in January, and how to run them yourself.
Sizing a tankless water heater comes down to two numbers: peak simultaneous flow rate in gallons per minute (GPM) and required temperature rise in degrees Fahrenheit. Add up the GPM of every fixture you expect to run at the same time, then subtract your local groundwater temperature from your target hot water temperature. Match that GPM-at-that-rise to a unit's published flow curve. Undersizing shows up as lukewarm showers in winter; oversizing wastes capacity you paid for.
- Two numbers do the sizing: peak GPM and temperature rise (ΔT). Neither works alone.
- Winter groundwater is the design case. A unit sized for July will disappoint in January.
- Manufacturer max-GPM ratings are usually stated at a low ΔT. Read the whole flow curve, not the headline number.
- Gas units carry high ΔT at usable flow. Electric units run into an amperage wall fast in cold climates.
- For long horizontal pipe runs or split-zone floor plans, two smaller units can beat one large one.
What follows is the math a plumber runs in their head before quoting a unit. Once you have seen it once, spec sheets stop looking like Greek. If you would rather see the wallet side first, our companion piece on tankless water heater installation cost breaks down the equipment-versus-labor split.
A quick note before we dive in: AC Direct ships the equipment, you (or your installer) does the plumbing and venting. A quality unit sized correctly and installed by a licensed plumber will outlast the argument you had picking it out. Our tankless water heater sizing calculator page carries the models that pair with the math below. For efficiency benchmarks and certified models, the ENERGY STAR water heaters directory is the neutral reference.
What are GPM and temperature rise?
GPM (gallons per minute) is how much hot water a unit can push out. Temperature rise, written as ΔT, is how many degrees Fahrenheit the unit has to raise incoming water to hit your target. A tankless heater's capacity is a curve between the two: as ΔT climbs, deliverable GPM falls. One number without the other is meaningless.
The physics behind this is a single line of arithmetic. Heating one gallon of water by one degree Fahrenheit takes roughly 8.34 BTU (water weighs 8.34 pounds per gallon, and a BTU is the energy to raise one pound one degree). Multiply by 60 minutes per hour and you land at the working formula plumbers use:
BTU/hr ≈ 500 × GPM × ΔT
A 199,000 BTU gas unit at 100% efficiency, asked for an 80°F rise, can move about 199,000 ÷ (500 × 80) = 4.98 GPM. Ask the same unit for a 50°F rise instead and it delivers about 7.96 GPM. Same hardware. Different weather.
That inverse relationship is the entire game. Every reputable manufacturer publishes a curve derived from that equation. When someone tells you a tankless unit is "8 GPM," always ask: at what rise?
How do I estimate peak simultaneous flow?
Add the GPM of every hot water fixture you realistically expect to run at the same time. Not every fixture in the house, just the ones that overlap in a worst-case morning. Standard reference values: shower 2.5 GPM, kitchen sink 1.5 GPM, bathroom faucet 1.0 GPM, dishwasher around 2.0 GPM, clothes washer 3 to 5 GPM. Total those, and you have your peak demand.
Here is the trap most homeowners fall into: picking a scenario that has never actually happened in their house. If nobody runs the dishwasher during morning showers, do not size for that. Sit down and honestly walk through the peak 15 minutes of a Tuesday: who is up, what are they running, what could reasonably overlap.
The reference numbers
Standard residential fixture flow rates, aligned with values published in the ENERGY STAR water heaters guidance:
| Fixture | Typical Hot Water Draw (GPM) |
|---|---|
| Shower (standard) | 2.5 |
| Shower (low-flow) | 1.5 to 1.75 |
| Kitchen sink | 1.5 |
| Bathroom faucet (lavatory) | 1.0 |
| Dishwasher | 2.0 |
| Clothes washer | 3.0 to 5.0 |
| Tub filler | 4.0 |
Meet Jenna. She has a two-bath ranch in Cleveland, two teenagers, and a dishwasher that only ever runs after dinner. Her honest peak is two showers back-to-back on school mornings, with the kitchen faucet occasionally involved. That's the scenario she sizes for: 2.5 + 2.5 + 1.5 = 6.5 GPM. The washing machine at 10 p.m. doesn't fight anyone for hot water, so it doesn't get added to peak.
Common household scenarios
| Household | Realistic Peak Scenario | Peak Flow |
|---|---|---|
| 1 to 2 people | 1 shower + 1 bathroom faucet | 3.5 GPM |
| 1 to 2 people | 1 shower + kitchen sink | 4.0 GPM |
| 3 to 4 people | 2 showers | 5.0 GPM |
| 3 to 4 people | 1 shower + dishwasher + kitchen sink | 6.0 GPM |
| 5+ people | 2 showers + dishwasher + bathroom faucet | 8.0 GPM |
| 5+ people | 2 showers + clothes washer + kitchen sink | 10.5 GPM |
A small safety cushion of 5% on top of your calculated peak is normal industry practice. Not double, not "just to be safe" doubled again. Five percent.
What is your incoming water temperature?
Your incoming groundwater temperature ranges from roughly 35°F in the far north to 75°F in tropical regions, and it varies by season. Sizing must use your coldest expected inlet temp, because that produces the highest ΔT, which is the hardest condition your unit will ever face.
Here is a starting map you can adjust with a five-minute call to your water utility, who will happily tell you their winter delivery temperature:
| Region | Winter Groundwater (°F) | ΔT to Target 120°F |
|---|---|---|
| Alaska, northern Rockies | 35 to 42 | 78 to 85 |
| Upper Midwest, New England | 40 to 45 | 75 to 80 |
| Mid-Atlantic, Ohio Valley | 45 to 55 | 65 to 75 |
| Southeast, Southwest | 60 to 70 | 50 to 60 |
| Gulf Coast, Southern California | 65 to 72 | 48 to 55 |
| Hawaii, Puerto Rico | 75 | 45 |
Most households set output around 120°F, which mixes down at the shower valve to a comfortable 105°F or so. Higher target temperatures increase your ΔT and drop your available GPM, which is why setting a tank to 140°F "for extra hot water" is not a free upgrade.
Worked example: two-bath home in a cold climate
A two-bathroom home in Ohio, winter groundwater around 40°F, target output 120°F, peak demand of two showers plus the kitchen sink. Required ΔT is 80°F. Peak flow is 2.5 + 2.5 + 1.5 = 6.5 GPM. Plugging into the formula: 500 × 6.5 × 80 = 260,000 BTU/hr of usable heat needed. That exceeds a single standard 199,000 BTU residential gas unit.
Here is the part most quotes leave out. That 260,000 BTU figure assumes 100% efficiency, which no real appliance hits. A condensing gas tankless running at 96% needs closer to 270,000 BTU/hr of input to satisfy that load. Jenna has three honest paths from here:
- Accept slightly reduced peak. A 199,000 BTU condensing gas unit will typically deliver roughly 4.5 to 5 GPM at 80°F rise. That covers two simultaneous showers. The kitchen sink may run cool if it joins in.
- Install two units in parallel. Two 160,000 BTU units together handle the full 6.5 GPM at 80°F ΔT with headroom to spare, and give redundancy.
- Retrain the fixture peak. Swap standard showerheads for 1.75 GPM low-flow models. Peak drops from 6.5 to 5.0 GPM, and a single 199k unit handles it clean.
The third option is what most homeowners actually pick, because it is cheap and effective. A modern low-flow showerhead is nearly indistinguishable from a standard one, and it makes the sizing math cooperate.
Worked example: three-bath home in a warm climate
A three-bathroom home outside Tampa, summer groundwater around 70°F, target 120°F, peak demand of one shower plus dishwasher plus a lavatory faucet. Required ΔT is 50°F. Peak flow is 2.5 + 2.0 + 1.0 = 5.5 GPM. Formula: 500 × 5.5 × 50 = 137,500 BTU/hr. Well within the reach of a mid-size residential gas tankless.
A 180,000 BTU condensing gas unit here isn't just adequate. It's genuinely overqualified. The same appliance that struggles in Ohio hums along in Tampa because the required rise is so much smaller. This is why regional sizing charts published without groundwater context are worse than useless: they encourage Floridians to overbuy and Michiganders to underbuy.
How do I read a manufacturer flow chart?
Every reputable tankless spec sheet publishes a flow-versus-rise curve. Temperature rise runs along one axis, GPM along the other. Find your calculated ΔT, trace up to the curve, then across to GPM. That's the max flow the unit produces at your conditions. Compare it against your peak demand. If the curve number equals or beats your demand, the unit fits.
The published headline GPM is almost always stated at a low temperature rise, because that produces the biggest, most marketable number. A unit advertised as "8.0 GPM" might land closer to 4.5 GPM at a 77°F rise. Neither number is a lie. They just describe the same appliance under different conditions.
Look for AHRI-certified performance data, listed in the AHRI product directory. Those figures are lab-verified under standardized conditions, not marketing extrapolations. If a spec sheet quotes only a single max GPM without a curve or a temperature rise column, treat it with suspicion and go find one that shows its work.
What's different about sizing gas versus electric tankless?
Gas tankless units run 150,000 to 199,000 BTU/hr for residential models and hold high flow even at demanding temperature rises. Electric whole-home units run 18 to 36 kW and are constrained by household electrical service. A 36 kW electric unit, the top of the residential range, delivers roughly 2.7 GPM at a 77°F rise. Gas wins in cold climates. It isn't close.
The reason is straightforward. A 36 kW electric heater draws roughly 150 amps at 240V. That's more than an entire typical 100-amp house panel. Even homes with 200-amp service usually cannot dedicate 150 amps to hot water without a service upgrade. Meanwhile, a 199,000 BTU gas unit pulls maybe 3 amps of ignition and fan power, and its heat comes from a burner that doesn't care about your panel.
Gas units also require venting and combustion air, which adds installation complexity. Electric units skip venting entirely. That trade is real, and it's why electric point-of-use heaters (small, single-fixture) remain popular even in cold climates.
When should you consider two smaller units instead of one large?
Two smaller units make sense in three situations: very long horizontal pipe runs where a single central unit means long wait times at distant fixtures; split-zone homes with clearly separate wings; and peak demands that exceed a single residential unit's capacity but don't justify commercial equipment. In each case, parallel or point-of-use arrangements beat one oversized central heater.
A long-run example: a rambler with the water heater in a garage on one end and the master bath 70 feet of pipe away. A central 199k unit sized to serve everything still makes that master shower wait 30+ seconds for hot water. Drop a small 11 kW electric point-of-use heater under the master vanity, keep the main gas unit for the rest of the house, and the wait time collapses. Same total capacity, better delivered experience.
Two whole-home gas units plumbed in parallel is the answer for high-demand homes in cold climates. Two 160,000 BTU units share the peak, either takes the load if the other faults, and neither is being pushed to its cliff. It costs more up front. It works for decades.
The one thing this arrangement does not do is halve your gas line requirements. Sizing the gas supply to feed two units simultaneously matters, and a licensed plumber running a gas load calculation is the person to talk to. If you are still weighing the whole tankless-versus-tank question, the heat pump water heater installation route is worth reading before you commit.
Paying for the unit and the plumber who installs it
A tankless water heater is two purchases in the same project: the equipment, which AC Direct ships to your door, and the installation labor, which you pay directly to the licensed plumber you hire. Good hvac financing covers both.
Three partners we work with, each doing a different job:
- Hearth is the one most people use for a whole-project bundle. It finances the equipment and the plumber's labor together, offers 0% APR promotions for qualified buyers, and approves applicants with credit scores from about 550. If your question is "how do I pay my installer," Hearth answers it.
- Breeze is a no-credit-check option. Applying doesn't affect your credit score, which is useful if you're mid-mortgage or otherwise sensitive to hard inquiries.
- Affirm is available at checkout with 0% APR offers for qualified buyers, best for people who want to finance only the equipment side.
Between these three, hvac financing options exist across the credit spectrum. Terms, rates, and score thresholds shift, so the financing page is the current source of truth for hvac system financing details.
Frequently Asked Questions
What size tankless water heater do I need for a family of four?
A family of four in a moderate climate typically needs a unit that delivers 6 to 8 GPM at a 60 to 70°F temperature rise. That usually means a 180,000 to 199,000 BTU condensing gas tankless. Cold-climate families may need to add a low-flow showerhead or accept slightly reduced peak simultaneous demand.
Can one tankless water heater run two showers at once?
Yes, if it is sized for that peak. Two standard showers pull 5.0 GPM combined. A gas tankless delivering 5 GPM at your winter ΔT will handle it. In cold climates that generally means a 180k BTU or larger unit. Electric whole-home units rarely hit two simultaneous showers at winter temperatures.
Why does my tankless water heater struggle in winter?
Winter groundwater is often 20 to 30°F colder than summer, which raises your required temperature rise significantly. A unit that comfortably handled peak flow in July can fall short in January because deliverable GPM drops as ΔT climbs. This is the most common sizing mistake, and the flow curve on the spec sheet predicts it.
How many BTU tankless water heater do I need?
Multiply your peak GPM by your temperature rise, then multiply by 500 to get the BTU/hr of heat you need to deliver. Divide by the unit's thermal efficiency (0.95 for condensing gas, 0.82 for non-condensing) to get the input BTU rating to shop for. Most residential homes land between 150,000 and 199,000 BTU/hr.
Is 199,000 BTU enough for a whole house?
In warm and moderate climates, yes, comfortably. In cold climates with a required 75 to 80°F rise, a 199,000 BTU condensing gas unit delivers roughly 4.5 to 5 GPM, which covers two showers but may leave the kitchen sink cool if it joins in. Consider two units in parallel or low-flow fixtures if that scenario worries you.
Can I use an electric tankless water heater for a whole house?
In warm climates with modest peak demand, yes. In cold climates, electric whole-house tankless units struggle. Even a 36 kW top-of-line residential unit delivers only about 2.7 GPM at a 77°F rise, enough for one shower. Cold-climate homes almost always require gas for whole-house tankless service unless a point-of-use strategy is used.
Once you have your peak GPM and your winter ΔT, the shopping is the easy part. AC Direct carries the full residential lineup of condensing gas, non-condensing gas, and electric tankless models with published flow curves and AHRI-certified data.
The right tankless water heater is the one whose flow curve meets your peak on a February morning, not the one with the biggest number on the box.
