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Tankless Water Heater Sizing Calculator

Size an instantaneous water heater from simultaneous flow and the temperature rise your incoming water actually needs.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-27 · v1.0.0

Market
Imperial · sales tax

Heater output required

26.79 kW

High confidence

Exact thermodynamics for the stated flow and rise. Real units are rated at a specific rise, so compare against the manufacturer's flow-at-rise table rather than a headline kW.

Temperature rise
32 K
BTU/h
91412.65 BTU/h
Flow in US GPM
3.17 GPM
Flow in gallons per hour
190.2 GPH

At the values currently entered, the heater output required works out to 26.8 kW. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Add the equipment this sizes

This result is a specification — 26.79 kW — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Q = ṁ · c · ΔT. For water: kW = L/min × ΔT × 0.0698 (from 4.186 kJ/kg·K)
  • Typical fixture flows: shower 7-11 L/min, basin 4-6, kitchen sink 6-9 — WaterSense and UK Water Regulations fittings data

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Still deciding? Tankless vs Tank Water Heater — the factors that actually differ, with no invented prices.

Frequently asked questions

Why does the winter mains temperature matter so much?
Because it sets the rise, and the rise is directly proportional to the power needed. Going from 15 °C summer mains to 4 °C winter mains raises the required output by about 40% for the same shower. A unit sized in July disappoints in January, and that is the single most common complaint about tankless heaters.
Will it run out of hot water?
No — it runs out of flow instead. A tankless unit heats indefinitely, but only as much water per minute as its rating allows at your rise. Open a second shower and either both drop in temperature or the unit limits flow. That is the trade against a cylinder, which has a fixed quantity but no flow limit.
Can I run one on a normal electrical supply?
Rarely, at whole-house size. Twenty-seven kilowatts is well over 100 A at 230 V, which most domestic supplies cannot provide. Electric tankless units are practical for a single outlet; whole-house instantaneous heating is usually gas for exactly this reason.