Sizing a tankless water heater correctly is the single most important decision on any tankless job. Get it right and your client has endless hot water. Get it wrong and you are back on the phone explaining why the shower goes cold when someone starts a load of laundry. This guide covers exactly how to size any tankless water heater — gas or electric, residential or commercial — with specific guidance for every brand we carry at PlumbersCrib.com.
The two numbers that matter: GPM and temperature rise
Unlike a tank water heater — which is sized by storage capacity in gallons — a tankless unit is sized by two things:
- Flow rate (GPM) — how many gallons per minute of hot water the home needs at peak demand, based on how many fixtures run simultaneously
- Temperature rise (°F) — how many degrees the unit needs to raise the incoming cold water to reach the desired delivery temperature, typically 120°F
Every tankless unit's GPM rating is published at a specific temperature rise. The higher the required rise, the lower the effective GPM — which is why a unit that works fine in Florida may be undersized for the same home in Minnesota. Always size for your actual ground water temperature, not the number on the box.
Step 1: calculate your peak hot water demand
Add up the flow rates of every fixture you expect to run at the same time. This is your peak GPM demand.
| Fixture | Typical flow rate (GPM) |
|---|---|
| Shower | 2.0 – 2.5 |
| Kitchen faucet | 1.5 – 2.0 |
| Bathroom faucet | 0.5 – 1.0 |
| Dishwasher | 1.0 – 1.5 |
| Washing machine | 2.0 – 2.5 |
Example: one shower (2.5 GPM) + one kitchen sink (1.5 GPM) + one washing machine (2.0 GPM) = 6.0 GPM peak demand. That is the minimum GPM the unit needs to deliver at your region's temperature rise.
Step 2: calculate your temperature rise
Ground water temperature varies significantly by region. In cold northern states (Minnesota, Wisconsin, Michigan, New England), incoming water can be as cold as 37–45°F in winter. In warm southern states (Florida, Texas, Arizona), incoming water may be 65–75°F year-round. The colder your region, the higher the temperature rise required — and the larger the unit you need.
To calculate temperature rise: subtract your region's ground water temperature from your target delivery temperature.
Cold climate example: 120°F target − 45°F incoming = 75°F rise required
Warm climate example: 120°F target − 70°F incoming = 50°F rise required
Once you have your temperature rise, check the unit's performance chart at that specific rise. Every major brand publishes GPM output at multiple temperature rise intervals — always use the chart, not just the headline GPM figure.
Key rule: A unit's headline GPM is always published at a 35°F rise — the industry standard for comparison. In a cold northern climate where you need a 70–80°F rise, the same unit may deliver 30–40% less flow. Always size for your actual temperature rise, not the 35°F benchmark. For example, the Rheem IKONIC S11i delivers 11.2 GPM at 35°F rise but only 5.6 GPM at a 70°F rise — the difference between a comfortable large home installation and a system that cannot keep up.
Step 3: match your demand to a unit
Once you know your peak GPM demand and temperature rise, use this as a starting framework. These are general guidelines — always verify against the specific unit's performance chart before finalizing the spec.
| Home type / usage | Warm climate (~50°F rise) | Cold climate (~70°F+ rise) |
|---|---|---|
| Small home, 1–2 fixtures at once | 6–7 GPM gas | 8–9 GPM gas |
| Medium home, 2–3 fixtures at once | 8–9 GPM gas | 10–11 GPM gas |
| Large home, 3–4 fixtures at once | 10–11 GPM gas | 11+ GPM gas or cascaded units |
| Single fixture / point-of-use | Electric tankless | Electric tankless (check rise carefully) |
| Commercial / high demand | Commercial-grade system sized to peak load — see Intellihot below | |
Brand-specific sizing guidance
PlumbersCrib.com carries five tankless brands covering residential, point-of-use, and commercial applications. Here is how each one fits into a sizing strategy.
Rheem gas tankless
Rheem's condensing IKONIC series is the most widely specified residential gas tankless line we carry, covering 8.4 to 11.2 GPM at 35°F rise across six residential models. The SR variants add a built-in recirculation pump and EcoNet Wi-Fi. All IKONIC models carry a 0.96 UEF, dual stainless steel heat exchangers, and a 15-year heat exchanger warranty — the best in the category. For most single-family whole-home jobs, the IKONIC is the right starting point. Browse the full Rheem tankless lineup.
American Water Heaters gas tankless
American Water Heaters' Adapt X3 series is a strong alternative to Rheem for whole-home residential jobs, particularly on hard water sites. The X3 models include patented Scale Prevention Technology built into the unit — eliminating the need for annual descaling and extending unit life up to three times compared to unprotected models. The Adapt X3 delivers 10.5 GPM at 0.95 UEF, uses a commercial-grade copper primary heat exchanger and stainless steel secondary heat exchanger, carries a 15-year heat exchanger warranty, and is available in both indoor and outdoor configurations. Cascades up to 12 units for larger applications. Browse the full American Water Heaters lineup.
Takagi gas tankless
Takagi gas tankless units are a proven workhorse for residential and light commercial jobs, available in condensing and non-condensing configurations from 140,000 to 199,000 BTU. Takagi units use commercial-grade copper alloy heat exchangers and are certified up to 10,100 ft altitude on indoor models. Note that Takagi was discontinued as of December 31, 2025 — we are selling down remaining inventory. If you are speccing a new job, Rheem or American Water Heaters is the better long-term choice for ongoing parts and support. Browse available Takagi inventory while supplies last.
Stiebel Eltron electric tankless
Stiebel Eltron electric tankless units are German-engineered premium systems with up to 99% efficiency and Advanced Flow Control technology that maintains consistent temperatures under heavy use. Electric tankless units are best suited for point-of-use applications — single sinks, bars, utility sinks, and supplemental hot water — or whole-home use in warm climates where temperature rise requirements are moderate. In cold northern climates where a 70°F+ rise is required, electric tankless is generally not recommended for whole-home use due to the electrical capacity required and the limited GPM output at high rise. Browse the full Stiebel Eltron lineup.
Intellihot commercial tankless
Intellihot systems are purpose-built for commercial and high-demand applications where residential capacity is not sufficient. Intellihot units feature 316L stainless steel heat exchangers, 96–98% thermal efficiency, and masterless cascading up to 10 units — eliminating single-point failure without a master controller. All units are made in the USA and include telliCare connectivity for real-time remote monitoring. Intellihot's Neuron series (iN199, iN401, iN501) is designed as a drop-in replacement for tank-type systems, fitting in the same footprint with similar connection locations to make commercial retrofits significantly simpler. Ideal for restaurants, hotels, gyms, spas, multifamily buildings, and any facility requiring continuous hot water with built-in redundancy. Browse the full Intellihot lineup.
Quick reference: brand and size by application
| Application | Recommended size | Best brands |
|---|---|---|
| Small home, 1–2 fixtures, warm climate | 6–7 GPM gas | Rheem, American Water Heaters |
| Small home, 1–2 fixtures, cold climate | 8–9 GPM gas | Rheem, American Water Heaters |
| Medium home, 2–3 fixtures, warm climate | 8–9 GPM gas | Rheem, American Water Heaters |
| Medium home, 2–3 fixtures, cold climate | 10–11 GPM gas | Rheem IKONIC S10i/S11i, American MTM-199X3 |
| Large home, 3–4 fixtures, any climate | 11+ GPM gas | Rheem IKONIC S11i, American cascaded |
| Point-of-use / single fixture | Electric tankless sized to fixture | Stiebel Eltron, Rheem electric |
| Commercial / high-demand facility | Commercial system sized to peak load — see Intellihot | |
Signs a tankless water heater is undersized
If a client reports any of these after installation, undersizing is the most likely cause:
- Hot water cuts out mid-shower when a second fixture opens
- Water temperature fluctuates during heavy use periods
- The unit runs at maximum output continuously
- Unable to run two or more fixtures simultaneously without temperature drop
These symptoms almost always mean the unit's GPM capacity at the actual temperature rise is insufficient for the home's peak demand. The fix is sizing up — not adjusting the thermostat.
Frequently asked questions
What GPM do I need for a 3-bedroom home?
In a warm climate, an 8–9 GPM gas unit is typically sufficient. In a cold northern climate where temperature rise requirements are higher, size up to 10–11 GPM. When in doubt, size up — the cost difference between adjacent models is small compared to the cost of a callback.
Can I use an electric tankless for whole-home hot water in a cold climate?
Generally not recommended. Electric tankless units have limited output at high temperature rise, and a whole-home system in a cold climate would require significant electrical capacity — typically multiple 240V circuits at 40–60 amps each. For most cold-climate whole-home applications, a gas condensing tankless is the correct choice. Electric tankless is best for point-of-use or warm-climate supplemental applications.
What happens if I install a tankless unit that is too small?
The unit will run at maximum output trying to meet demand and either deliver water below the target temperature or trigger a flow restriction that cuts output. Over time, running at continuous maximum output accelerates wear on the heat exchanger and burner assembly. Always size with headroom, not just to the minimum requirement.
Do I need to account for hard water when sizing?
Hard water does not directly affect GPM sizing, but it does affect long-term performance. Scale buildup inside the heat exchanger reduces efficiency over time, which effectively shrinks the unit's output. If the installation site has hard water, spec a unit with built-in scale prevention — the American Water Heaters Adapt X3 series includes patented X3 Scale Prevention Technology built in — or confirm a water softener or filter is in place upstream of the unit.
Can tankless water heaters be cascaded for larger demand?
Yes — most brands support cascading multiple units for higher simultaneous demand. American Water Heaters Adapt units cascade up to 12 units. Intellihot's masterless cascading connects up to 10 units without a single point of failure. Rheem IKONIC commercial models (CS11i, CSR11i) cascade up to 32 units. For large residential or commercial projects where a single unit is not enough, cascading is a standard solution.
Shop tankless water heaters at PlumbersCrib.com
- Rheem tankless water heaters — IKONIC condensing series, gas and electric
- American Water Heaters — Adapt X3 condensing gas series with built-in scale prevention
- Stiebel Eltron electric tankless — point-of-use and warm-climate whole-home
- Intellihot commercial tankless — high-demand and commercial applications
- Takagi tankless — remaining inventory available while supplies last
Not sure which unit is right for your job? Contact our team or call 877-881-5501 — we will help you size the right unit for your climate, fixture count, and application. Free shipping on all orders over $199.