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GPM Explained for Tankless

Quick answer: GPM (gallons per minute) is how much hot water a tankless can deliver at once at a given temperature rise. Add fixtures you run together: two 2.0 GPM showers plus a 1.0 GPM faucet need about 5 GPM. Whole-home gas units commonly deliver 5–9 GPM at moderate rise — less in winter.

Context: the number that sets comfort

With a tank, the question is "how many gallons are stored?" With tankless, the question is "how many gallons per minute can it heat right now?" GPM is that answer. Every tankless rating is really a pair: a GPM at a specific temperature rise. A unit advertised as "9.5 GPM" usually reaches that only at a small rise in warm weather.

Terms

Peak demand: total GPM of fixtures used at the same time.
Rated GPM: manufacturer's maximum flow, usually at a low rise.
GPM at rise: the realistic flow at your winter temperature rise.
Minimum activation flow: the smallest flow that starts the unit.

How GPM and rise trade off

For gas: GPM ≈ BTU/h × efficiency ÷ (rise × 500). For electric: GPM ≈ kW × 6.83 ÷ rise.

Unit	40°F rise	60°F rise	80°F rise
199,000 BTU/h gas (0.95)	9.5 GPM	6.3 GPM	4.7 GPM
150,000 BTU/h gas (0.82)	6.2 GPM	4.1 GPM	3.1 GPM
27 kW electric	4.6 GPM	3.1 GPM	2.3 GPM
18 kW electric	3.1 GPM	2.0 GPM	1.5 GPM

Process: finding your GPM

Walk through a busy morning or evening and list what runs at once.

2. Use the flow rates in the reference table below or the ratings on your fixtures.

3. Total them in the GPM calculator.

4. Find your winter rise in the temperature rise calculator.

5. Choose a unit whose GPM at that rise exceeds your total with a small margin, using the size calculator.

Worked example

Figures below are illustrative examples (2026-10-29), not quotes or guarantees.

Assumptions: household of three; peak = one 2.0 GPM shower + one 1.8 GPM low-flow shower + kitchen tap 1.0 GPM = 4.8 GPM. Winter rise 75°F.

Gas: needs 4.8 × 75 × 500 ÷ 0.95 ≈ 189,000 BTU/h — a 199,000 BTU/h unit fits.
Electric: needs 4.8 × 75 ÷ 6.83 ≈ 53 kW — beyond typical residential electric units.

If the household swapped the 2.0 GPM showerhead to 1.5 GPM, peak falls to 4.3 GPM and the gas requirement to about 170,000 BTU/h, opening up smaller models.

Mistakes to avoid

Using the headline GPM rating without checking the rise behind it.
Counting every fixture in the house instead of the simultaneous ones.
Forgetting tub fills, which can draw 3–4 GPM on their own.

Measuring a fixture yourself

Hold a bucket marked in gallons under a fully open hot tap or shower for 15 seconds, then multiply by four. Simple, free and more accurate than guessing for older fixtures.

FAQ

Is more GPM always better?

Only up to your real peak demand; extra capacity adds cost and gas or electrical load.

What GPM do most homes need?

Many need 4–7 GPM in winter; large homes with three showers may need 8+ or two units.

Why is my flow lower in winter?

Colder inlet water raises the rise, so the same unit heats fewer gallons per minute.

Does GPM affect water pressure?

Tankless units add a little pressure drop; very high flows through a small unit can reduce pressure.

Safety and code reminders

Gas, venting, combustion air, condensate disposal and high-amperage electrical work all carry real safety risks, including carbon monoxide, fire, scalding and shock. They are also governed by local code and permits that differ between towns and provinces or states. Treat every step on this page as preparation for a professional visit, not as instructions to do the work yourself. Keep the manufacturer's manual for your exact model, set water temperature with scald risk in mind (120°F is a common household target), and stop and call a licensed professional whenever something is unclear.

Quick reference: typical fixture flow rates

These example flow rates help you translate everyday use into the GPM numbers used throughout this site. Check the rating stamped on your own fixtures, as older fixtures often run higher.

Fixture	Example hot-water flow
Low-flow showerhead	1.5–1.8 GPM
Standard showerhead	2.0–2.5 GPM
Bathroom faucet	0.5–1.2 GPM
Kitchen faucet	1.0–1.8 GPM
Dishwasher	0.8–1.5 GPM
Washing machine (warm)	1.0–2.0 GPM
Bathtub fill	2.0–4.0 GPM

Add only the fixtures that genuinely run at the same time. Running the dishwasher or laundry outside shower hours is the simplest way to stay within a unit's capacity.

Questions to ask a licensed installer

Bring these questions to every quote so you can compare them fairly. Good installers welcome them, and the answers often reveal hidden costs before work starts.

What peak GPM and temperature rise did you size this unit for, and what inlet water temperature did you assume for winter?
Is my existing gas line, meter or electrical panel large enough, and if not, what exactly needs upgrading and what does that line item cost?
What venting route will you use, how long is it, and does it meet the manufacturer's instructions and local code?
Does the quote include a permit, isolation (service) valves for future flushing, a condensate drain if needed, and removal of the old heater?
What are the labour warranty and the manufacturer's heat-exchanger and parts warranty terms, and do they require registration or yearly professional service?
How long will the install take, and will there be a period without hot water?

How to read the numbers on this site

Every figure on TanklessMath is a planning estimate. Ranges reflect typical North American examples as of 2026-10-29 and will differ with your region, home, fuel prices and installer. Equipment ratings vary by model, so always check the specification sheet for the exact unit you are considering. When our example and a licensed installer disagree, trust the installer who has seen your home.

Next steps

Write down your peak fixtures, coldest incoming water temperature, fuel type and possible locations, then ask at least two licensed installers to confirm sizing, venting, gas or electrical capacity, permits and local code. Compare written quotes line by line so the unit, labour, materials and upgrades are separated. Questions about this site: hello@charlandandco.com.