How to size tankless water heaters correctly is all about math and realistic assumptions, not guesswork. If you undersize the unit, showers go lukewarm or the flow drops to a trickle when multiple fixtures run. Oversize it, and you may pay more for equipment and infrastructure than you need.
This guide walks through the process step by step so you can confidently determine what size tankless unit you actually need (and then confirm it with manufacturer tools and a professional).
Understand How Tankless Sizing Really Works
A tankless water heater is sized by how much hot water it can produce at a given temperature rise, not by storage gallons.
Three key concepts:
- Flow rate (GPM – gallons per minute)
How many gallons of hot water your fixtures demand at the same time. - Temperature rise (ΔT)
How many degrees you need to heat the incoming (groundwater) water to reach your desired hot water temperature.ΔT=Tout−TinΔT=Tout−Tin - Heater capacity (BTU/h or kW)
The gas input (for gas units) or electrical power (for electric units) required to deliver that flow and temperature rise.
Manufacturers publish charts or calculators that tell you, for example, “this model can deliver 5.0 GPM at a 70°F temperature rise.”Rheem+1
Your job is to calculate:
Required GPM at peak use + Required temperature rise → Select a model that meets or exceeds both.
Step 1 – Define the Application
Before any math, be clear about what you’re sizing for:
- Whole-house tankless (most common)
- All showers, sinks, laundry, etc.
- Point-of-use (single fixture or small group)
- Example: one bathroom group, or a remote studio sink/shower.
- Residential vs. small commercial
- Commercial may have different diversity and fixture types.
Also consider:
- Number of bathrooms
- Typical household size (people)
- Any high-demand fixtures: large soaking tub, body spray shower, commercial dishwasher, etc.
This will drive what “peak simultaneous use” actually looks like.
Step 2 – List Your Hot-Water Fixtures and Their Flow Rates
Next, list all fixtures that use hot water, plus their typical flow rates. You can find exact flow on product spec sheets or measure with a bucket, but here are common ranges from tankless sizing resources:PlumbingSupply.com+1
Typical residential flow rates
- Standard shower: 1.5–2.5 GPM
- Water-saving shower: 1.2–1.8 GPM
- Bathroom sink faucet: 0.5–1.5 GPM
- Kitchen faucet: 1.5–2.2 GPM
- Clothes washer: 1.5–3.0 GPM
- Dishwasher: 1.0–2.0 GPM
- Large tub filler: 4.0–6.0+ GPM
- Body spray/“carwash” shower systems: often 4.0–8.0+ GPM total
Create a simple table like this for your project:
| Fixture | Flow (GPM) | Notes |
|---|---|---|
| Shower #1 | 2.0 | Primary bathroom |
| Shower #2 | 2.0 | Hall bath |
| Bathroom sink (same bathroom) | 0.7 | Brushed nickel low-flow faucet |
| Kitchen sink | 2.0 | High-arc faucet |
| Dishwasher | 1.5 | Spec sheet list |
| Clothes washer | 2.0 | Top-load |
Step 3 – Decide What Really Runs at the Same Time
You don’t size for every fixture running at once. You size for a realistic peak scenario.
Examples of reasonable peak scenarios:
- Family home (3–4 people)
- Two showers running + one bathroom sink intermittently
- Small home / couple
- One shower + kitchen sink or dishwasher
- Home with large soaking tub
- Either tub filling or one shower + one sink (not both scenarios at once unless that’s realistic for that client)
For each project, write down one or two “design scenarios” you actually want the system to support.
Example design scenario
Two showers running at the same time, plus a bathroom sink occasionally.
From the table above:
- Shower #1: 2.0 GPM
- Shower #2: 2.0 GPM
- Bathroom sink: 0.7 GPM
Total design flow = 2.0 + 2.0 + 0.7 = 4.7 GPM
(Round to 4.5–5.0 GPM for simplicity.)
Step 4 – Determine Incoming Water Temperature and Temperature Rise
Now estimate your incoming (groundwater) temperature. This varies by climate. Many sizing tools and guides use groundwater temperature maps and typical desired hot water temperature around 120°F.energy.gov+1
You can:
- Look up your region on a groundwater temperature map from a sizing tool.
- Use local data (well temp, etc.) if you have it.
- Use conservative values for winter conditions (coldest likely incoming temp).
Typical incoming temps:
- Cold northern climates: 35–50°F
- Mixed / temperate: 50–60°F
- Warm southern climates: 60–75°F
Most households are happy with 115–120°F at fixtures, even if the water heater is set slightly higher.
Example
- Incoming water: 50°F (cool climate)
- Desired outlet temp: 120°F
ΔT=120°F−50°F=70°FΔT=120°F−50°F=70°F
So this home needs about a 70°F temperature rise at their design flow rate.
Step 5 – Calculate Required Heating Capacity (BTU/h)
With flow (GPM) and temperature rise (ΔT), you can estimate the BTU/h required:BTU/h≈500×GPM×ΔTBTU/h≈500×GPM×ΔT
The factor 500 comes from water’s weight and the minutes-to-hours conversion.
Using the example:
- Flow = 4.7 GPM (round to 4.5)
- ΔT = 70°F
BTU/h≈500×4.5×70=157,500 BTU/h (approx.)BTU/h≈500×4.5×70=157,500 BTU/h (approx.)
Most residential gas tankless units are in the 150,000–199,000 BTU/h range, so in this example you’d likely be looking at the larger end of that range to comfortably meet 4.5–5.0 GPM at a 70°F rise.
Rule of thumb: For typical homes in cool climates, whole-house gas tankless systems often land in the 180k–199k BTU/h range. Electric whole-house tankless units may require very large electrical service to match this performance.
Step 6 – Match Your Numbers to Manufacturer Sizing Charts
Now that you know GPM and ΔT, you compare that to manufacturer data.
Manufacturers provide:
- Online sizing tools (you enter baths, people, climate, etc.).Rheem+3Navien+3Noritz+3
- PDF charts and spec sheets showing max flow at different temperature rises.Gateway ACPro+1
Look for a table or chart that says something like:
- 70°F rise → 4.5 GPM
- 60°F rise → 5.5 GPM
- 50°F rise → 7.0 GPM
Then compare your requirement:
Need ≥ 4.5 GPM at 70°F rise
Pick a model that meets or exceeds this capacity.
If a model only provides 3.5 GPM at a 70°F rise, it’s too small for our example home. Either:
- Move up to a larger model, or
- Decide that your realistic design scenario is less demanding (e.g., only one shower + a small sink at once).
Step 7 – Check Fuel Type and Site Constraints
Even if the math says a certain size will work, your building infrastructure has to support it.
Gas tankless units
Check:
- Gas line size and pressure – High-input gas models (e.g., 180k–199k BTU/h) often require upgraded gas piping and proper inlet pressure.
- Venting – Condensing models may allow PVC/CPVC/polypropylene venting and have higher efficiency; non-condensing may need metal venting.Navien+1
- Combustion air – Especially for indoor installations.
- Condensate drain – Required for condensing units; you need a place to route neutralized condensate.
Electric tankless units
Check:
- Available service amperage and voltage – Whole-house electric tankless units may require multiple 40–60A double-pole breakers and heavy conductors.Stiebel Eltron USA+2Stiebel Eltron USA+2
- Panel capacity – Many homes don’t have enough spare capacity without a service upgrade.
- Local code requirements for electric water heaters.
If the site can’t support a large enough unit, you may need:
- Multiple smaller point-of-use units, or
- A high-efficiency tank or heat pump water heater instead of whole-house tankless.
Step 8 – Decide Between One Large Unit or Multiple Units
If one heater can’t cover everything (or would be very expensive to support with gas/electric infrastructure), consider multiple units:
- Two smaller tankless units in parallel (“ganged”) to share load. Many manufacturers specifically allow this, and some calculators will flag when ganging is recommended.PlumbingSupply.com+1
- Separate zones:
- One unit for “main house” (showers, laundry)
- One smaller unit for “kitchen/guest wing” or remote structures
This can also offer redundancy and shorter hot-water runs (less wait time, less wasted water).
Step 9 – Adjust for Special Fixtures and Recirculation
High-demand fixtures
If the home includes:
- Large soaking tub,
- Multi-head shower or body sprays,
- Commercial-style kitchen equipment, or
- Multiple simultaneous uses in a big family,
You must account for those extra GPM in your design scenario, or accept that they can’t all run at once.
Often this pushes designers toward:
- Two tankless units, or
- Hybrid solutions (tankless feeding a small buffer tank, etc., in some designs).
Recirculation systems
Tankless units with recirculation:
- Don’t change the peak GPM the heater must produce,
- But they do impact control strategy, pump sizing, and standby losses.
Many manufacturers offer built-in recirc pumps or recirc-ready models, with specific piping diagrams and control options that should be followed.Navien+1
Step 10 – Confirm Everything with Manufacturer Tools and a Pro
Once you’ve worked through the sizing yourself, sanity-check your decision using:
- Manufacturer online sizing tools (Rinnai, Navien, Noritz, Rheem, Stiebel Eltron, Bosch, etc.).PlumbingSupply.com+6Rinnai+6Navien+6
- Official spec sheets and temperature rise charts for the exact model(s) you’re considering.Navien+4Gateway ACPro+4Navien+4
Important fine print: many of these tools explicitly state that they’re guides only, and that the contractor or engineer is responsible for the final selection and ensuring code compliance.naviensizing.com+2Gateway ACPro+2
Work with a qualified installer or engineer to:
- Verify your load calculations
- Confirm gas/electric capacity and venting
- Ensure code compliance and proper safety/clearance provisions
Common Sizing Mistakes to Avoid
1. Using number of bathrooms only
Bathroom count is a rough screening tool, but real sizing needs actual fixture flow and realistic concurrency.
2. Ignoring cold-climate conditions
If you size using average or summer groundwater temperatures, you may be under-sized in winter.
3. Not checking gas or electric infrastructure
Discovering after purchase that your gas line or electrical panel is too small is an expensive surprise.
4. Forgetting about special fixtures
Body sprays, big tubs, and commercial appliances are GPM hogs—don’t ignore them.
5. Assuming any “199k BTU” unit is automatically enough
Different models can have different actual delivered GPM at your ΔT. Always check the flow vs. temperature rise table.
Quick Sizing Checklist
Use this as your “don’t guess” checklist:
- List all hot-water fixtures and their flow rates (use spec sheets or typical values).
- Define a realistic peak use scenario (what’s actually on at once).
- Add up GPM for that scenario.
- Determine incoming groundwater temperature and desired outlet temperature; compute ΔT.
- Use BTU/h ≈ 500 × GPM × ΔT to estimate needed capacity.
- Compare your GPM & ΔT to manufacturer flow-vs-rise charts or calculators.
- Verify gas line, venting, and electrical capacity can support the chosen unit.
- Decide whether you need one whole-house unit or multiple / point-of-use units.
- Factor in high-demand fixtures and recirculation if present.
- Confirm selection with manufacturer tools and a qualified installer.
Follow that process and you’ll be sizing tankless systems deliberately, not guessing.
