Water Heater Size Calculator

Frequently Asked Questions

How do I size a storage tank water heater?

Match the First Hour Rating (FHR) to peak-hour demand. Estimate gallons used in your busiest hour: shower 15 gal, dishwasher 6 gal, clothes washer 7 gal - a family of 4 morning peak ≈ 70–80 gal FHR, often a 50 gal tank with high recovery.

How do I size a tankless heater by GPM?

Tankless capacity = required GPM at the design temperature rise. Two showers (2 × 2.0 = 4 GPM) at 70 °F rise (40 °F inlet, 110 °F output) needs a unit rated for 4 GPM @ ΔT 70 °F.

What is the energy equation for ΔT?

BTU/h = GPM × 500 × ΔT (°F). 4 GPM × 500 × 70 = 140,000 BTU/h input (gas) or ≈ 41 kW (electric) - which is why electric whole-house tankless usually needs 150–200 A service.

Does cold inlet temperature matter that much?

Yes. Inlet water is 70 °F in Florida summer and 38 °F in northern winter. A tankless rated 5 GPM at ΔT 50 °F may deliver only 3.5 GPM at ΔT 70 °F - always size for winter, not the spec sheet headline.

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How This Calculator Works

A storage water heater is sized for peak hour demand - the worst-case 60-minute window of the day - not annual usage. The metric the EnergyGuide label publishes for storage tanks is the first-hour rating (FHR), the gallons of hot water a fully heated tank can deliver in the first hour of heavy use, combining the stored hot water plus what the burner or element can heat during that hour. Tankless heaters are sized differently: they have no storage, so the spec that matters is the GPM at a given temperature rise (ΔT). A 199,000-BTU gas tankless will produce roughly 5 gpm at a 70 °F rise; a 27 kW electric unit produces a similar flow but pulls 112 A at 240 V.

Formula / Method

Peak-hour gallons = sum of typical hot-water demand for fixtures used in the busiest hour: about 12 gal per shower, 20 gal per tub fill, 6 gal per dishwasher cycle, 7 gal per warm-wash laundry load, plus 2 gal per person for handwashing and incidentals. Required FHR should equal or exceed peak hour. From FHR, recommended tank capacity is roughly FHR × 0.65 for a gas tank (faster recovery) and FHR × 0.80 for electric (slower recovery). For tankless, the heat duty required is BTU/h = GPM × 60 × 8.33 × ΔT, where ΔT = target − inlet temperature. Convert to kW by dividing BTU/h by 3,412.

Worked Example

A family of 4 with 2 morning showers, 1 dishwasher cycle, and 1 laundry load in the same hour: 24 + 6 + 7 + 8 = 45 gallons of hot water in the peak hour. A 40–50 gal gas tank with an FHR around 60–70 covers it comfortably; an electric tank should be 50 gal at minimum because its slow recovery means more of the demand must come from stored water. For the tankless scenario: feeding 2 fixtures (5 gpm combined) with 55 °F well water heated to 120 °F is a 65 °F ΔT, giving 5 × 60 × 8.33 × 65 ≈ 162,000 BTU/h - a high-end residential gas tankless (180,000–199,000 BTU) handles it.

Common Mistakes

Sizing on total tank volume instead of FHR - a 40 gal gas tank with a strong burner can outperform a 50 gal electric on peak-hour delivery.

Forgetting to subtract cold mix - only about 60–70% of shower flow is hot water; the rest is cold mixed at the valve.

Specifying a tankless on summer water temperature; size to the coldest winter inlet (often 35–45 °F in northern climates) or the unit will stall at peak.

Picking an electric tankless without verifying the panel can deliver 120–160 A on dedicated circuits.

Ignoring recirculation losses - a constantly running hot-water loop can add 15–25% to daily standby load.

Tips & FAQ

Tank or tankless? Tankless wins on space, hot-water-runs-forever, and standby loss; tanks win on simultaneous high-flow demand, simpler retrofits, and lower install cost. Heat-pump (hybrid) tanks? 3–4× more efficient than resistance electric and qualify for the federal 25C tax credit; they need an unconditioned mechanical room and produce cool air as a byproduct. Gas vent type: atmospheric vent (B-vent) is cheapest but needs a chimney; power vent and condensing vent through PVC sidewall - check setbacks. Recirculation: a small recirc pump on a timer or demand button cuts hot-water wait to near-zero at distant fixtures. Anode rods: inspect every 3–5 years; a depleted anode lets the tank rust through years sooner. Set point: 120 °F is the safe scald limit; 140 °F kills Legionella but needs a mixing valve. Expansion tank: required by code when a check valve or PRV is installed; protects the T&P relief from nuisance discharges. This is a planning estimate - permitted work should follow the local plumbing and mechanical codes.

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