Tankless BTU rule used here?
BTU/hr ≈ GPM × °F rise × 500.
Choose tank or tankless mode.
Tank mode estimates peak-hour hot water (showers × 20 gal + occupants × 3 gal + dishwasher/washer allowances) and recommends a common tank size (40–80 gal) based on that demand. Size by first-hour rating (FHR) when shopping—not the tank label alone.
Tankless mode sums simultaneous showers (2.5 GPM), sinks (1.5 GPM), and optional dishwasher/washer flow, then estimates BTU/hr as peak GPM × °F rise × 500.
Concise answers for common searches — definitions, steps, and comparisons.
BTU/hr ≈ GPM × °F rise × 500.
20 gallons per peak shower in the peak-hour estimate.
Tank sizing uses a peak-hour gallon estimate mapped to standard tank sizes. Tankless sizing uses fixture GPM totals and BTU/hr ≈ GPM × ΔT(°F) × 500.
Formula
Tank: peakHourGal = showers×20 + occupants×3 + (dishwasher?6:0) + (washer?7:0); tank size from peak-hour bands (≤40→40, ≤55→50, ≤70→65, ≤85→80, else 80 gal). Tankless: peakGpm = showers×2.5 + sinks×1.5 + optional appliances; btu = round(peakGpm × tempRiseF × 500).Select tank mode for storage heaters or tankless mode for on-demand GPM/BTU planning.
Tank: occupants, peak showers, dishwasher/washer. Tankless: simultaneous showers/sinks, optional appliances, and temperature rise °F.
Tank mode shows recommended gallons and peak-hour demand. Tankless shows peak GPM with estimated BTU/hr.
Input
tank mode · occupants 4 · showers 2 · dishwasher + washerOutput
65 gal tank · ~65 gal peak-hour2×20 + 4×3 + 6 + 7 = 65; band maps to 65 gal.
Input
tankless · 2 showers · 1 sink · rise 70°FOutput
6.5 GPM · ~227,500 BTU/hr2×2.5 + 1×1.5 = 6.5 GPM; 6.5×70×500 = 227,500.
Common real-world scenarios where this tool saves time.
4 occupants, 2 peak showers, dishwasher and washer → about 65 gal peak-hour demand → 65 gal recommended tank.
2 showers + 1 sink at 70°F rise → about 6.5 GPM and roughly 227,500 BTU/hr class capacity.
Lower occupant/shower counts reduce peak-hour gallons and may drop into a 40–50 gal recommendation band.
Step-by-step chains that connect related tools for common tasks.
Size a tankless unit here, then refine peak flow on the Hot Water Flow Calculator if needed.
| Peak-hour demand | Recommended tank |
|---|---|
| ≤ 40 gal | 40 gal |
| ≤ 55 gal | 50 gal |
| ≤ 70 gal | 65 gal |
| ≤ 85 gal | 80 gal |
Use tank mode for storage heaters and tankless mode for on-demand units—do not mix the outputs.
Use this estimate to shortlist sizes, then verify FHR or tankless GPM charts from the manufacturer.
Compare first-hour rating (FHR) to peak-hour demand—the calculator’s peak-hour gallons are the planning target.
Colder inlet water needs more BTU/hr for the same GPM. Enter a realistic °F rise.
Tankless mode should reflect what actually runs together, not the whole house fixture inventory.
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It builds a peak-hour gallon estimate from occupants, showers, and appliances, then maps that demand to a common tank size (40–80 gal).
FHR is how much hot water a tank can deliver in an hour. Shop by FHR against peak-hour demand—not tank volume alone.
It sums simultaneous fixture GPM, then estimates BTU/hr as GPM × temperature rise (°F) × 500.
Rise is desired hot temperature minus incoming cold water. Many homes use roughly 50–80°F; the tool accepts about 30–100°F.
No. It estimates capacity (gallons or GPM/BTU). Fuel type and venting still require product selection and a plumber.
No. It is a residential planning estimate. Confirm with local code and a licensed plumber before purchase.
Water heater inputs remain local — EverydayTools does not collect water-heater calculator entries.
Estimates only — not professional plumbing advice. Verify with a licensed plumber and local code before installation.
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Reviewed on 2026-06-28.
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