Craft Quantities
Plan & Takeoffs

Plumbing & HVAC

Tankless Water Heater Sizing Calculator

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

  • Answers as you type
  • Every formula cited
  • Calculated in your browser
SettingsSettings for this calculationUS
Market
Imperial · sales tax
Total flow of the fixtures used at once.

Not the sum of every fixture in the house — the realistic worst case, in the unit shown. Two showers at 9 L/min each is 18 L/min (about 4.8 gal/min); a shower and a basin is about 14 L/min (3.7 gal/min). This is the single decision that sizes the unit.

Cold mains temperature in winter.

Use the winter figure, which is what sizes the unit. Mains water can be 15 °C in summer and 4 °C in February, and that difference alone changes the required output by nearly half.

Temperature at the outlet.

42 °C is a comfortable shower. Storing or delivering above 60 °C risks scalding and usually requires a thermostatic mixing valve; below 60 °C in a stored system raises legionella concerns, which is why tankless and stored systems have different rules.

Heater output required

25.35 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
86,508.62 BTU/h
Flow in US GPM
3 GPM
Flow in gallons per hour
180 GPH
Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • 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

Inputs used

Simultaneous flow
3 gal/min
Incoming water temperature
50 °F
Delivered temperature
107.6 °F

Intermediate steps

Temperature rise
32 K
BTU/h
86,508.62 BTU/h
Flow in US GPM
3 GPM
Flow in gallons per hour
180 GPH
Final result25.35 kW

Confidence note: 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.

What this calculation does not cover

  • Gas units need adequate supply pipe and flue; an undersized gas run limits output regardless of the appliance rating.
  • Electric units of this size often need a dedicated high-current supply that many domestic boards cannot provide.
  • Assumes no simultaneous demand beyond what you entered. A tankless unit does not run out of hot water, but it does run out of flow.

Add the equipment this sizes

This result is a specification — 25.35 kW — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-08-27 · in the site-wide review of 2026-09-06 · v1.0.0

Regulatory standards & verification citations2
  1. Q = ṁ · c · ΔT. For water: kW = L/min × ΔT × 0.0698 (from 4.186 kJ/kg·K)
  2. Typical fixture flows: shower 7-11 L/min, basin 4-6, kitchen sink 6-9 — WaterSense and UK Water Regulations fittings data
Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

  • Choosing a Cylinder a Heat Pump Can Actually Heatuses this calculator

    On a changeover the cylinder is judged on the square metres of its coil, not its litres — the same coil sees roughly a quarter of the driving temperature.

  • Matching a Shower to the Water Supply You Haveuses this calculator

    A shower valve carries a minimum working pressure on its box, and a loft-fed house rarely has it — so four measurements decide the product, not the showroom.

  • Sizing Domestic Hot Wateruses this calculator

    Domestic hot water systems are sized by the worst sixty minutes a building sees, not by the gallons it burns over a whole day.

Called something else where you work? Immersion heater · Combination boiler — the term in each market, how close the equivalence really is, and the standard that governs it.

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

Already gone wrong? My shower runs cold, or keeps changing temperature

How to calculate tankless water heater sizing in 4 steps

  1. Simultaneous flowTotal flow of the fixtures used at once.
  2. Incoming water temperatureCold mains temperature in winter.
  3. Delivered temperatureTemperature at the outlet.
  4. Heater output requiredThe tool computes the heater output required from those figures and shows the formula, its sources, and a confidence rating alongside it.

Heater output required by simultaneous flow

Page defaults, not your figures above.

Simultaneous flowHeater output required (kW)
2 gal/min16.9
3 gal/min25.4
4 gal/min33.8
5 gal/min42.3
6 gal/min50.7

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-three kilowatts is exactly 100 A at 230 V, and a whole-house flow at a winter rise sizes above that whichever units you enter it in — more current than most domestic supplies can provide. Electric tankless units are practical for a single outlet; whole-house instantaneous heating is usually gas for exactly this reason.
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 — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.