Count your household's busiest hot-water hour and get the First Hour Rating to shop for — then check any tankless unit's honest GPM at your winter temperature rise, with the electric service impact worked out too.
What Your Results Mean
Two takeoffs run side by side because a tank and a tankless answer the same question differently. The tank panel counts your busiest hour in gallons of hot water and converts it into the First Hour Rating to shop for — the yellow EnergyGuide number that combines what the tank stores with what its burner or elements can reheat on the fly. The tankless panel ignores storage entirely and does pure arithmetic: how many degrees of rise your winter water demands, and how many gallons per minute the unit can push through that rise.
The worksheet defaults are the industry's standard per-use figures — a 10-minute shower at a 2.0 GPM head is 15 gallons of hot water because roughly 75 percent of what flows is hot, mixed down at the shower valve. Every count is editable because households are not averages: the family with a 2.5 GPM head in every bathroom uses a quarter more hot water per shower than the worksheet assumes, and the family with a soaking tub lives by that tub's spec sheet. Change a count and both panels update instantly.
The two tank-class rows are where fuel matters. A "50-gallon" tank is a different appliance depending on what heats it: gas burners reheat fast enough to carry an FHR of 70–85 in that size, while a pair of 4,500-watt elements manages only 55–62 — which is why the same peak hour that a 50-gallon gas tank clears with room to spare pushes an electric household into the 65–80 gallon class. The smallest-that-clears row shows the honest minimum; the shop-for row adds the 20 percent of headroom that lets a dishwasher or a laundry load join the morning without a strategy meeting.
On the tankless side, the verdict row compares the unit's deliverable output against the flow you actually run at once — two showers plus a faucet is 4.5 GPM of hot water, a number far below the headline rating on the box. Headline GPM is quoted at a 35-degree rise, which roughly describes Hawaii year-round and Texas tap water in August. Size instead for the coldest groundwater your address ever sees; the region field fills typical winter values, and the arithmetic shrinks the unit's output to match. The service impact row is the electric-tankless honesty check: whole-house units draw 28–36 kW, which is 120–150 amps at 240 volts — often more than the whole panel has to spare.
Reading the Lines
- Peak hour demand: the gallons of hot water (not total flow) your household draws in its single busiest 60 minutes. Count what overlaps — the shower running while the dishwasher finishes — not what the whole day uses.
- Minimum FHR target: the peak rounded up to the next 5 gallons per hour. A tank must deliver this in its first hour starting fully heated; the EnergyGuide label states the tested value.
- Recommended FHR: 20 percent of headroom on top of the minimum. Not a code rule — a working margin, so the washing machine joining the hour is a non-event.
- Temperature rise: your setpoint minus winter groundwater. The 500 in the equation is water's thermal constant in English units — 8.33 pounds per gallon times 60 minutes.
- Verdict: output minus your simultaneous peak. A positive number is flow to spare; a negative one is the shower going lukewarm in February, computed in advance.
- Service impact: amps at 240 volts for electric tankless. Above about 100 amps, this stops being a water-heater question and becomes an Article 220 service load question.
What the Calculator Does Not Do
It sizes the appliance, not the installation. Gas tankless units may need a bigger gas line than the old tank had — that's a pipe-sizing question your plumber or gas utility answers. It does not price operating costs: a UEF number on the EnergyGuide label (conventional gas tanks around 0.60–0.70, condensing gas 0.80–0.90, gas tankless 0.90–0.97, heat-pump tanks 2.0–4.0) is the efficiency story, and it deserves its own comparison. It also cannot see your local code: storage temperature rules, TPR valve and expansion-tank requirements, venting, and seismic strapping are regulated locally and belong to the licensed plumber who signs the permit. And heat-pump buyers should remember those tanks chill the room they stand in by a few degrees — free cooling in July, a real cost in January.
Pro Tip
When you're replacing a water heater — especially swapping fuel types — shop by FHR, never by tank gallons. The gallons on the box are the marketing number; the First Hour Rating on the yellow label is the engineering one, and it's the only figure that stays comparable across gas, electric, and heat-pump tanks. The sizing guide shows the full method by hand.
The Math Behind the Calculator
The tank side is addition; the tankless side is one division:
peak = showers(15) + older_showers(19) + baths(20) + tub_fills × tub_volume
peak = peak + dishwashers(4) + hot_washes(10) + faucet_gallons
fhr_min = round_up_5(peak) fhr_rec = ceil(fhr_min × 1.20)
rise = setpoint − winter_groundwater
gpm_gas = (btu_per_hour × efficiency) ÷ (rise × 500)
gpm_electric = (kw × 3412 × 0.99) ÷ (rise × 500)
amps = kw × 1000 ÷ 240
The per-use gallons are the Department of Energy worksheet values the sizing guide publishes, with the hot fraction already mixed in at a 120°F storage setting. The tank-class table below is what the calculator searches — typical First Hour Ratings by size and fuel, read off real EnergyGuide labels:
For the tankless side, winter groundwater is the input people get wrong most. The table below is the regional reality — and the rise column is what the flow equation actually divides by:
Electric tankless math converts kilowatts at 3,412 BTU per kWh with resistance heating at essentially 99 percent efficiency at the element — the losses that matter are in your panel, not the heater. Gas math uses the nameplate input times the unit's stated efficiency, which is why the efficiency field exists: a 0.95 condensing unit and a 0.82 non-condensing unit with identical BTU figures deliver 15 percent different flows.
Assumptions & Limitations
Every number above rests on these constants. If your house differs, change the matching input — or read the linked guide for the full reasoning.
- Showers: 10 minutes at 2.0 GPM heads = 15 gal hot, 2.5 GPM heads = 19 gal (75% hot fraction at a 120°F setting)
- Baths: 20 gal hot for a standard tub (the published range is 20–26); soaking tubs sized by their own fill volume
- Dishwasher: 4 gal per modern cycle (3–5 range; many machines heat their own water — check the spec)
- Clothes washer: 10 gal per hot cycle (7–15, HE front-loaders at the low end)
- The 20% headroom is a recommendation, not a code rule — the worksheet itself follows the DOE consumer method
- Tank FHR bands are typical EnergyGuide values per class — the label on the actual unit wins
- Gas GPM: nameplate input × stated efficiency (0.95 condensing default; use 0.80–0.85 for non-condensing)
- Electric tankless: 3,412 BTU per kWh at 99% element efficiency; amps = kW × 1000 ÷ 240 V
- Metric mode converts usage, temperatures and flow (L, °C, L/min); FHR and tank classes stay in gallons — the EnergyGuide labels use them
- Fuel choices, gas-line sizing, TPR and expansion-tank requirements, and permits belong to a licensed plumber and your local code
Worked Example: The 7 a.m. Family of Four
The same household walked through in the water heater sizing guide, so every number below is checkable against it. Leave the defaults as they load: three 10-minute showers at 2.0 GPM heads, one dishwasher cycle, and 3 gallons of faucet and kitchen use in the busy hour:
- Peak hour: 3 × 15 + 1 × 4 + 3 = 52 gallons of hot water.
- Minimum FHR, rounded up: 55 gal/hr — "a tank with an FHR of 55 or more," in the guide's words.
- Recommended FHR with headroom: 66 gal/hr.
- Gas: a 50-gal gas tank (FHR 70–85) clears it with room for the washing machine; electric: a 50-gal tank (FHR 55–62) clears it with nothing to spare, so the calculator sends you to the 65-gal class (66–75) — exactly the guide's advice.
Now flip the tank fuel select to electric and watch the same 52 gallons become a different appliance — that's the fuel swap most replacement quotes get wrong. Then add one hot laundry load (washer count 1) and the peak climbs to 62: the electric pick jumps a class, and even the gas recommendation tightens.
The tankless panel with its defaults is the guide's other worked example: a 199,000 BTU/h condensing unit at 95 percent efficiency, middle-of-the-country groundwater at 50°F, so the rise is 70°F. The math: 199,000 × 0.95 ÷ (70 × 500) ≈ 5.4 GPM — roughly two simultaneous showers plus a faucet, which is exactly the 4.5 GPM the demand side totals, with 0.9 to spare.
Two more moves worth making. Switch the region to northern tier (41°F groundwater): the rise grows to 79°F and the same unit drops to about 4.8 GPM — still clears, barely. That's the whole trap of tankless shopping in one keystroke: the headline number never moves, the water does. Then flip tankless fuel to electric at the default 28 kW: 2.7 GPM at the same 70°F rise — it falls 1.8 GPM short of two showers and a faucet, and it wants roughly 117 amps of service to do it. A 36 kW unit manages 3.5 GPM but draws a flat 150 amps. Before anyone falls in love with that plan, run the panel through the service load calculator.
Run your own busy hour before you believe any of it — the morning you actually remember, not the polite version. And when the numbers point at a class, read the FHR on the EnergyGuide label of the specific unit, because the label is the tested number and the class is only the neighborhood it lives in.