How to use the water heater size calculator
Hot-water shortages show up at peak hour — usually morning showers overlapping with laundry or a dishwasher. This tool estimates the first-hour rating a storage tank needs, or the peak flow a tankless heater must support, from household size, simultaneous fixtures and temperature rise. Figures are labelled estimates for early design, not a stamped equipment schedule.
- Choose units and heater type — tank (FHR) or tankless (peak gpm / L/min).
- Pick household size from the bedroom / people bands that DOE-style guides use.
- Enter inlet and desired temperatures so the tool can compute temperature rise and tankless BTU or kW.
- Count fixtures that can run together at the peak hour, then read the recommended tank size and minimum FHR, or the peak flow and fuel input for tankless.
Water heater sizing formulas
Temperature rise ΔT = Thot − Tcold
People-based FHR (approx.): 1–2 people → 45–55 gal; 2–3 → 55–65; 3–4 → 65–75; 4–5 → 75–85; 5–6 → 85–100+ gal
Fixture peak draw ≈ showers×gpm + sinks×gpm + dishwasher + washer volume/hour + baths×volume
Tank FHR needed ≈ max(people table, fixture peak draw in that hour)
Suggested tank gallons ≈ FHR × 0.7 to 0.85 (typical usable fraction of rated FHR)
Tankless peak gpm = sum of simultaneous fixture flows
Gas input ≈ gpm × 500 × ΔT BTU/h · Electric ≈ L/min × ΔT × 0.07 kW (approx.)
Default fixture flows used here: shower 2.0 gpm (7.6 L/min), lav/sink 1.5 gpm (5.7 L/min), dishwasher ~1.5 gpm equivalent during a short draw, clothes washer ~2.0 gpm equivalent, bathtub ~15–20 gallons (55–75 L) of hot water per fill credited to the peak hour. Recovery on a tank is approximated so a stated tank size can be checked against the needed FHR.
| Household | Typical tank | Min FHR (approx.) |
|---|---|---|
| 1–2 people | 30–40 gal | 45–55 gal |
| 2–3 people | 40–50 gal | 55–65 gal |
| 3–4 people | 50–60 gal | 65–75 gal |
| 4–5 people | 60–75 gal | 75–85 gal |
| 5–6+ people | 75–80+ gal | 85–100+ gal |
Worked examples
A three-bedroom house with one shower peak
Three to four people, inlet 50°F, setpoint 120°F (ΔT = 70°F). One shower, one lav and a dishwasher overlapping need about 2.0 + 1.5 + 1.5 ≈ 5 gpm of hot water if they all run hard, but for a tank the people table already points to roughly 65–75 gal FHR and a 50–60 gal tank. A 50 gal Energy Star unit with a solid recovery rate often lands near 65–70 gal FHR — enough for this peak if baths are staggered.
Tankless for two showers in a cold climate
Two showers at 2.0 gpm plus one lav at 1.5 gpm is 5.5 gpm peak. With 40°F inlet water and 120°F outlet, ΔT = 80°F. Required gas input ≈ 5.5 × 500 × 80 = 220,000 BTU/h. Many residential tankless units top out near 180,000–199,000 BTU/h, so you either reduce simultaneous fixtures, raise inlet temperature with a tempering strategy, or step up to a larger / commercial unit — the calculator flags that limit.
Metric flat with a compact tank
A two-person flat with 10°C inlet and 49°C setpoint (ΔT = 39°C), one shower and one sink. The people band suggests about 170–210 L of FHR and a 120–150 L tank. Peak flow is roughly 7.6 + 5.7 ≈ 13 L/min, which a mid-size tankless can also cover if electrical or gas supply allows.
Who this calculator helps
- Architects and housing designers picking plant space and fuel type early in schematic design.
- Plumbers and mechanical contractors cross-checking FHR or tankless gpm before ordering.
- Homeowners and renovators replacing an undersized heater after morning shortages.
- Energy raters comparing tank standby loss versus tankless peak capacity in retrofit packages.
Practical tips
- Count fixtures that can truly run together; a locked bathroom door often means one shower, not three.
- Set the tank thermostat to about 120°F (49°C) for scald and standby control unless a higher setting feeds a mixing valve.
- Check plant ventilation and combustion air with the ventilation calculator, and envelope loads with the heat loss calculator.
- Fixture counts for code minimums live on the plumbing fixture calculator — that is a different problem from peak hot-water flow.
- Gas tankless units need adequate meter and pipe capacity; electric tankless units often need a large dedicated service.
Showing a client a renovated bath layout around the new heater? Mock it up on a photo with Room Redesign.
