Nothing in a house exposes a sizing mistake faster than hot water. A tank that's one size too small announces itself on the third consecutive shower of a January morning, exactly when nobody wants a lesson in engineering. And the classic fix — buy the biggest tank that fits — wastes energy every single day for fifteen years while a smaller one would have carried the same morning without complaint.
The honest way to size a water heater is the same whether the box is a tank or a wall-hung heat exchanger: figure out how much hot water your household actually uses in its busiest hour, then buy the equipment whose tested rating covers that hour. This guide shows the method — First Hour Rating for tanks, gallons-per-minute at temperature rise for tankless — with a worked worksheet and the by-household shorthand at the end.
Size to your busiest hour, not your household headcount. Count the showers, laundry and dish loads of that hour in gallons of hot water: a typical family of four lands near 50–60 gallons, which means a tank with a First Hour Rating of 60–70 (a 50-gallon gas tank, an 80-gallon electric) or a tankless rated 4.5–5 GPM at your winter temperature rise. Two-person households are comfortably covered at FHR 45–55 or 3–3.5 GPM.
The Question Both Types Answer Differently
A tank heater and a tankless heater solve the same problem from opposite ends. A tank is a battery: it stores an hour's worth of heated water and spends it down, then recharges at whatever rate its burner or elements can manage. A tankless is a generator: it heats water the instant it flows, and its limit is not storage but the rate at which it can add heat to moving water.
That's why the two types carry two different ratings, and why comparing "gallons" to "GPM" directly is meaningless. The number that matters on a tank is the First Hour Rating — how much usable hot water it delivers in the first hour starting from fully heated, combining what's stored with what the burner reheats on the fly. The number that matters on a tankless is its flow rating at a stated temperature rise — gallons per minute it can heat by that many degrees. Both numbers live on the EnergyGuide label, which is measured under federal test conditions, which is exactly why it's worth reading.
Sizing a Tank: The First Hour Rating
Tank gallons are the marketing number; FHR is the engineering one. A 50-gallon tank does not deliver 50 gallons of fully hot water in an hour — it starts losing temperature as soon as the first cold gallon enters, and electric tanks in particular reheat slowly. A 50-gallon gas tank typically carries an FHR of 70–85 because a 40,000 BTU/h burner keeps reheating during the draw; a 50-gallon electric with a pair of 4,500-watt elements usually manages an FHR of 55–62, because 9,000 watts is barely a third of that gas burner's heating power.
The practical consequence trips up homeowners constantly: the same "50-gallon" tank is a different appliance depending on fuel. If you're replacing gas with electric or vice versa, the gallons on the label are not interchangeable — the FHR is.
The Peak-Hour Worksheet
Here's the counting method, adapted from the Department of Energy's consumer worksheet. Walk through your household's busiest hot-water hour (for most families, the morning rush or the evening wind-down) and tally the hot water each use consumes. Hot-water figures below assume fixtures mixing 120 °F stored water down to comfortable temperatures — the hot fraction of a shower is roughly 75 percent of what flows.
| Use | Typical hot water | Notes |
|---|---|---|
| Shower, 10 min | 15 gal | 2.0 GPM head, ~75% hot fraction |
| Shower, 10 min (older 2.5 GPM head) | 19 gal | The head alone changes the math |
| Bath, standard tub | 20–26 gal | Mixed down to ~104°F |
| Deep soaking tub | 45+ gal | Sized by tub volume; these break small tanks |
| Dishwasher, modern cycle | 3–5 gal | Often heats its own water; check the spec |
| Clothes washer, hot cycle | 7–15 gal | HE front-loaders at the low end |
| Shaving / hand-washing at faucet | 1–2 gal | Minor unless habits are generous |
A worked example: a family of four whose busiest hour is 7:00–8:00 a.m. — three 10-minute showers (45 gal), one dishwasher cycle started as everyone leaves (4 gal), and shaving plus kitchen faucet use (3 gal) — has a peak hour demand of 52 gallons. The right tank is one with an FHR of 55 or more, not one whose capacity is 55 gallons: a 50-gallon gas tank at FHR 75 clears it with room for the washing machine; a 50-gallon electric at FHR 58 clears it with nothing to spare, which is why careful installers move electric households up to the 65–80 gallon class instead.
Sizing Tankless: GPM at Temperature Rise
A tankless doesn't store anything, so its deliverable output is pure arithmetic: how many degrees can it raise water, how fast. The governing equation is simple enough to do on paper —
GPM = (BTU/h input × efficiency) ÷ (temperature rise × 500)
The 500 is water's thermal constant in English units (8.33 lb/gal × 60 min, adjusting for a few engineering realities). A 199,000 BTU/h condensing unit at 95 percent efficiency, asked for a 70 °F rise, delivers about 199,000 × 0.95 ÷ (70 × 500) ≈ 5.4 GPM. That's roughly two simultaneous showers plus a faucet — in winter, in a cold-groundwater state. The same unit at a 45 °F rise in the Gulf South delivers 8.4 GPM.
Which is the entire trap of tankless shopping: the headline GPM is always quoted at a small temperature rise (often 35 °F), which no northern winter ever produces. Temperature rise is the difference between your desired output temperature (120 °F is the standard storage setting) and your incoming groundwater at its coldest:
| Region | Winter groundwater | Required rise to 120°F |
|---|---|---|
| Northern tier (MN, ND, ME) | 37–45°F | 75–83°F |
| Middle of the country | 47–52°F | 68–73°F |
| Sun Belt (TX, FL, AZ) | 57–70°F | 50–63°F |
Size the unit for the winter number. Groundwater temperature follows a seasonal curve, and the coldest week of February is precisely when the household runs two showers and the dishwasher at once.
One whole-house electric tankless caution before the comparison: matching a gas unit's output with electric resistance takes 28–36 kW, which is 120–150 amps at 240 volts — often more than the service has to spare after everything else. If that sentence means nothing to you yet, read our NEC Article 220 service load guide before falling in love with an electric tankless. Point-of-use electric units (one bathroom, one kitchen) draw a sane 7–13 kW and are genuinely excellent at that scale.
Tank vs Tankless, Compared Honestly
| Factor | Tank | Tankless |
|---|---|---|
| First cost | $900–2,000 installed (gas) | $2,500–4,500 installed (gas), plus possible gas-line upsizing |
| Endless supply | No — the hour is the limit | Yes, up to the GPM ceiling |
| Simultaneous-use limit | Handled by FHR sizing | Flow drops as rise grows; winter is the test |
| Standby loss | Yes — a tank cools all day | Essentially none between draws |
| Power outage | Stored hot water keeps flowing | Nothing — controls and ignition need power |
| Maintenance | Flush sediment yearly | Descale annually in hard water (non-negotiable) |
| Cold-water sandwich | No | A brief cool slug between draws; newer units minimize it |
| Space | Closet floor, ~16–20 in diameter | Wall-hung, ~1–2 sq ft |
Both are defensible choices. Households with predictable one-hour peaks and modest budgets are well served by a right-sized tank; households with sprawling schedules, big soaking tubs, or space constraints justify the tankless premium. The failure mode is identical on both sides of the table — buying the rating that matches the marketing instead of the rating that matches the worksheet.
Choosing by Household Size: The Shorthand
With the method established, the shorthand table. These are typical guidance, the kind that's right about 80 percent of the time — the worksheet above is the authority when your home has a fixture or schedule that isn't typical (deep tubs, five teens, a home daycare).
| People | Tank FHR target | Typical tank choice | Tankless target (at winter rise) |
|---|---|---|---|
| 1–2 | 40–50 gal/hr | 40 gal gas / 50 gal electric | 2.5–3 GPM |
| 2–3 | 50–60 | 50 gal gas / 65 gal electric | 3.5–4 GPM |
| 3–4 | 60–70 | 50 gal gas / 80 gal electric | 4.5–5 GPM |
| 5–6 | 75–90 | 75 gal gas / two tanks or one 100+ gal electric | 5.5+ GPM, or unit + small buffer tank |
The Label That Predicts the Bill
Once the size is right, efficiency is the second number on the EnergyGuide label that actually matters. The modern metric is UEF (Uniform Energy Factor) — the successor to EF — and it captures standby loss, cycling behavior, and recovery efficiency in a single number per test protocol. Conventional gas tanks land around 0.60–0.70 UEF, high-efficiency condensing gas tanks 0.80–0.90, gas tankless 0.90–0.97, heat-pump tanks 2.0–4.0 (they move heat rather than making it). Heat-pump tanks are the efficiency play of the decade for basements and garages — with two honest caveats: they heat slowly, so FHR sizing matters even more, and they chill the room they stand in by a few degrees, which is free air conditioning in July and a real cost in January.
Frequently Asked Questions
What size water heater does a family of four need?
Count the peak hour, not the headcount: three 10-minute showers plus a dishwasher load comes to roughly 50 gallons of hot water, so a First Hour Rating of 60–70 covers it. A 50-gallon gas tank usually delivers that; a 50-gallon electric typically doesn't and wants the 65–80 gallon class. For tankless, the same family generally needs 4.5–5 GPM at their winter temperature rise.
Why does my tank water heater run out so fast?
Three usual suspects: the tank is smaller than the household's real peak hour, the First Hour Rating is lower than the advertised gallons (especially on electric tanks), or one fixture quietly consumes more than expected — a 2.5 GPM shower head uses 25 percent more hot water than a 2.0. Sediment buildup also shrinks effective capacity over the years; draining the tank can restore some of it.
Can a tankless water heater fill a large soaking tub?
If the math holds, yes — a filling tub is a flow question, not a capacity one. A tub faucet at 4–5 GPM needs a unit rated for that flow at your winter temperature rise, which in cold groundwater means a large gas unit, possibly paired with a small buffer tank. The check is always GPM at your temperature rise, never the headline rating at a 35-degree rise.
Do tankless water heaters work in cold climates?
They do, but groundwater temperature shrinks the output: the same unit that delivers 7 GPM at a 45-degree rise may deliver 4.5 GPM at a 75-degree rise in a northern winter. Size for winter water, not the annual average, and consider a small buffer tank if the household stacks fixtures in December.
Are electric tankless water heaters a good whole-house choice?
Rarely. A whole-house electric tankless that matches a gas unit's output draws 28–36 kW — 120–150 amps at 240 volts, often more than the entire service has to spare. They shine at point-of-use scale: one bathroom, one kitchen, one distant guest suite. Run the service-load math first; our NEC Article 220 guide walks through it.
General guidance for planning. Fuel choices, gas-line sizing, TPR valve and expansion-tank requirements, and permits are the domain of a licensed plumber and your local code — storage temperature, venting and seismic strapping are regulated locally.
Where to Read Next
- NEC Article 220 service load, explained — whether the panel can actually feed the equipment you're planning, which is the question electric tankless and heat-pump buyers meet first.
- The Home Maintenance category — the seasonal rhythm that catches measurement problems before they become emergencies.
- Kitchen & bathroom planning — the two rooms that set your peak-hour schedule in the first place.
The bottom line: count your busiest hour in gallons of hot water, then buy the rating that covers it — First Hour Rating for a tank, GPM at your winter temperature rise for a tankless. Tank gallons and headline flow numbers are marketing; the yellow EnergyGuide label is the engineering. Size once, correctly, and the morning rush stops being a gamble.