Tankless Water Heater Sizing Calculator

Frequently Asked Questions

How do I size a tankless water heater for my house?

List peak simultaneous fixtures, sum GPM, and pick a unit rated at that GPM for the winter ΔT. Two showers (2 × 2.0 = 4 GPM) + kitchen (1.5 GPM) = 5.5 GPM at ΔT 70 °F.

How does temperature rise affect output?

A unit rated 6 GPM at ΔT 35 °F may deliver only 3 GPM at ΔT 70 °F - output is roughly inversely proportional to ΔT. Always use the GPM at your local winter inlet temperature.

What inlet temperature should I assume?

Southern U.S.: 60–70 °F. Mid-Atlantic / Midwest: 45–55 °F. Northern tier / mountain: 35–45 °F. NREL's ground temperature map gives accurate seasonal lows.

Gas vs electric tankless - which fits?

Gas tankless can hit 150,000–199,000 BTU/h (8+ GPM whole house). Electric tankless tops out around 27 kW = ~5.5 GPM @ ΔT 35 °F and needs 100–150 A of dedicated service.

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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

Unlike a storage water heater that buffers BTUs in 40–80 gallons of pre-heated water, a tankless heats water on demand as it flows through the unit. Its capacity is therefore quoted as GPM at a given temperature rise (ΔT), not in gallons of storage. The same 199,000 BTU/h gas tankless that comfortably delivers 9.4 gpm at a 45 °F rise (Florida) drops to 5.0 gpm at an 80 °F rise (Minnesota in January). Sizing therefore depends on the simultaneous fixtures running in the worst-case scenario and on the coldest expected inlet water temperature, not on the climate annual average.

Formula / Method

Temperature rise ΔT = output target − inlet temperature. Heat duty in BTU/h to raise water by ΔT at flow Q (gpm) is BTU/h = Q × 60 × 8.33 × ΔT, where 60 converts gpm to gph and 8.33 is water's pounds per gallon (specific heat = 1). To pick a unit, sum the simultaneous fixture flows you actually need: a low-flow showerhead is about 2.0 gpm, a kitchen faucet 1.5 gpm, a tub fill 4–5 gpm, a clothes washer 2.0 gpm during fill. The tankless must deliver that total GPM at the design ΔT. For electric tankless, divide BTU/h by 3,412 to get kW - then verify the panel and service entrance can handle 80–160 A of dedicated load.

Worked Example

A northern household with 50 °F winter inlet wants 120 °F output (ΔT = 70 °F) and needs to run 2 showers and 1 kitchen faucet simultaneously: 2×2.0 + 1.5 = 5.5 gpm. Required heat duty = 5.5 × 60 × 8.33 × 70 ≈ 192,500 BTU/h. A 199,000 BTU gas tankless covers it with a few percent margin. The same fixtures with 65 °F inlet (southern climate) drop to 5.5 × 60 × 8.33 × 55 ≈ 151,200 BTU/h - a 160k–180k unit suffices. For an electric replacement at the northern conditions, 192,500 BTU/h ÷ 3,412 ≈ 56 kW - far beyond what a residential panel can deliver; gas is the practical choice.

Common Mistakes

Sizing on annual-average inlet temperature instead of coldest winter inlet - the unit stalls on the day you need it most.

Choosing electric tankless without checking panel ampacity - 27 kW pulls 112 A on 240 V across three or four 40-A circuits.

Assuming "endless hot water" means "unlimited flow" - the unit modulates its output GPM down as ΔT climbs.

Forgetting the minimum activation flow (typically 0.5–0.75 gpm) - very small trickles at a faucet won't fire the burner.

Skipping the dedicated gas line upsize - a 199k BTU tankless needs 3/4 in or larger gas line over typical runs.

Tips & FAQ

Gas or electric? Gas dominates whole-home residential because of the high BTU output per dollar; electric works for point-of-use (single bath, vacation cabin) and where natural gas is unavailable. What is the coldest inlet water in my area? Northern US: 35–45 °F. Mid-Atlantic and Midwest: 45–55 °F. South and Southwest: 55–70 °F. Use your area's coldest realistic value, not the annual average. What output temperature should I set? 120 °F is the safe scald limit and standard for sinks and showers; 140 °F for dishwashers without booster heaters (use a mixing valve at fixtures). Hard water: tankless heat exchangers scale quickly above 7 gpg hardness - install a softener or commit to annual descale with white vinegar. Recirculation: tankless units take 5–10 seconds to fire; add a dedicated recirc loop with a smart pump or use the "internal recirc" feature on premium models to cut wait time. Venting: condensing tankless vents through 2 or 3-in PVC; non-condensing requires Category III stainless. Manifold two units for very large simultaneous loads or to add capacity to an existing install. This is a planning estimate - follow the manufacturer's install manual and local plumbing/gas codes.

Tankless Water Heater Sizing Calculator

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