What Size Water Heater Do I Need? (Free Calculator)

"What size water heater do I need?" is the question every homeowner faces the day their old tank starts leaking or the showers turn cold halfway through. Buy too small and you'll run out of hot water every busy morning; buy too big and you'll pay more up front and waste energy keeping water hot you never use. The good news is that sizing a water heater isn't guesswork — there's a clear method, and the calculator below runs it for your specific household, fuel type, and climate in seconds. The rest of this guide explains exactly where the numbers come from, the difference between tank and tankless sizing, why your fuel type matters, and how to make the call between a 40- and 50-gallon tank.

How the calculator sizes your water heater

The calculator answers two questions at once: what tank size you'd need (in gallons, anchored to first-hour rating) and what tankless unit you'd need (in GPM, anchored to temperature rise). It's built on the U.S. Department of Energy's own water-heater sizing method, which sizes tanks by peak-hour demand rather than by a crude per-person rule.

Tank sizing: it's really about first-hour rating

The single most important number on a tank water heater isn't its capacity — it's its first-hour rating (FHR): the gallons of hot water it can deliver in one hour starting from a full tank. FHR combines the water already stored with whatever the burner or element can reheat during that hour, so it captures both size and recovery rate.

That's why tank size alone is misleading. A 40-gallon gas heater with a strong burner can post a higher first-hour rating than a 50-gallon electric model that reheats slowly. The DOE's guidance is blunt: choose a heater whose first-hour rating meets or exceeds your peak-hour demand. You'll find it on the federal EnergyGuide label, listed as "Capacity (first hour rating)" in the top-left corner.

Peak-hour demand: the number that drives everything

Your peak-hour demand is the most hot water you use in any single hour — almost always a weekday morning. The DOE's worksheet assigns average gallons to each use, and the calculator above uses the same figures:

In the DOE's own example, a household running three showers, one shave, one round of hand dishwashing, and a load of laundry in the same hour totals about 66 gallons of peak demand — so they'd need a tank with a first-hour rating of 66 gallons or more. Add up your own busy hour the same way, and you have your target.

A worked example: a family of four

Say four people all shower on a weekday morning, and the dishwasher runs after breakfast in the same hour:

• 4 showers × 20 gallons = 80 gallons
• 1 dishwasher cycle = 7 gallons
• Total peak-hour demand = ≈ 87 gallons

No single standard tank stores 87 gallons, and it doesn't need to — the burner reheats water during that hour. A 50-gallon gas tank with a strong burner typically posts a first-hour rating around 80–90 gallons, so it meets this demand. A 50-gallon electric tank, recovering at half the speed, posts an FHR closer to 60–70 gallons — short of the target — which is why the calculator nudges electric households up to a 65- or 80-gallon tank. Same family, same demand, different tank, purely because of fuel.

Tankless sizing: GPM and temperature rise

A tankless (or "demand") water heater has no tank to draw from — it heats water as it flows through, so it's sized completely differently. Two numbers decide it:

1. Flow rate (GPM). Add up the gallons-per-minute of every hot-water fixture you'd run at the same time. The DOE uses about 2.5 GPM for a shower and 0.75 GPM for a faucet. Run two showers plus a kitchen faucet and you need roughly 5.75 GPM.
2. Temperature rise. Subtract your incoming water temperature from your target. Assume incoming water of about 50°F unless you know otherwise, and a target of 120°F — that's a 70°F rise. In the warm south, incoming water is closer to 70°F (an easy 50°F rise); in the cold north it can be 40°F or lower (a demanding 80°F rise).

Here's the catch: a tankless unit's GPM rating depends on the rise. The DOE notes that a 70°F rise is typically possible at about 5 GPM through a gas unit but only 2 GPM through an electric one. Push the rise higher — colder climate, hotter target — and the deliverable flow drops. That's why a tankless unit that's perfect in Florida may struggle in Minnesota, and why most whole-house tankless installs are gas.

Tank size by household: a quick-reference chart

If you want a starting point before fine-tuning with the calculator, here's how household size maps to tank capacity and a realistic first-hour-rating target:

These are starting ranges, not gospel — a household that staggers its showers can drop a size, while one where everyone leaves at 7 a.m. should size up. The first-hour-rating column is what you actually shop on.

Tankless GPM by fixture

For tankless sizing, build your number from the fixtures you'd run simultaneously:

A family of four that might run two showers and the kitchen sink at once needs a unit rated for roughly 6–7 GPM at their temperature rise — comfortably in gas-tankless territory, but more than a single electric unit can usually deliver in a cold climate.

40 vs 50 gallon: how to decide

This is the most common real-world sizing question, so it deserves a direct answer. For one or two people with a single bathroom, a 40-gallon tank is plenty. For three or more people, or any home with two or more bathrooms, choose 50 gallons. The price difference at purchase is small — often under a couple hundred dollars — while the comfort difference during back-to-back morning showers is large.

But don't stop at gallons. A 50-gallon electric tank and a 40-gallon gas tank can have nearly identical first-hour ratings because gas recovers about twice as fast. If you're on gas, you can often size down a notch and still keep up; if you're on electric, lean toward the larger tank or a high-recovery model. When in doubt, compare the first-hour-rating numbers on the two EnergyGuide labels — that comparison settles the question better than the gallon figure ever will.

Why your fuel type changes the size

This is the variable most online calculators ignore, and it's the reason two homes with identical households end up with different tanks. Recovery rate — how many gallons a heater can reheat per hour — depends almost entirely on the heat source:

• Gas heaters recover fast. A 40-gallon gas tank can post a first-hour rating in the 60–70-gallon range because the burner refills it quickly during that busy hour.
• Electric heaters recover at roughly half the rate of gas. A 50-gallon electric tank often delivers a first-hour rating closer to its raw capacity, so you generally need to size up one tier to match the same household a gas unit would handle.
• Heat-pump (hybrid) heaters are electric and recover slowly in pure heat-pump mode, though most switch to backup electric elements under heavy demand. They're superbly efficient — often cutting water-heating energy use by half or more — but for sizing, treat them like electric and lean larger.

That's exactly why the calculator above asks for your heat source: pick electric or heat-pump and it bumps the recommended tank up a size so you still hit your first-hour-rating target. If you're switching fuel types during a replacement, size for the new fuel, not the old one.

Tank vs. tankless vs. heat-pump: which type fits

Sizing and type are two different decisions, and the right type depends on your priorities more than your household size. Here's how the three main options compare:

If endless hot water during long showers is the goal and you have adequate gas supply, tankless wins — just size it for your coldest-month temperature rise, not the average. If the lowest possible energy bill matters most and you have a basement or garage with room and mild ambient temperatures, a heat-pump tank is hard to beat. For most straightforward replacements on a budget, a right-sized, high-recovery tank is still the pragmatic choice.

What the right size costs

Sizing decisions have a price tag, so here are typical installed cost ranges for 2026 (equipment plus standard professional installation — your local quotes will vary with permits, venting, and code upgrades):

Going one tank size larger is one of the cheapest upgrades in the table — usually a few hundred dollars — and it's far cheaper than replacing an undersized heater two years early. If you're weighing a repair against a full replacement, run the numbers in our repair-or-replace cost calculator before you commit.

Keep your new heater at full capacity

Buying the right size is only half the job — an oversized heater clogged with sediment delivers like an undersized one. A few habits protect the capacity you paid for:

A yearly sediment flush is the highest-impact task on that list — sediment insulates the burner, slows recovery, and steals the very first-hour rating you sized for. Our step-by-step guide to flushing a water heater walks through it, and checking the anode rod on schedule is what decides whether your tank lasts 8 years or 15.

Common sizing mistakes to avoid

Sources & method

• U.S. Department of Energy, Energy Saver — Sizing a New Water Heater, the source for first-hour rating, the peak-hour-demand worksheet, per-use gallon figures, and tankless GPM and temperature-rise guidance.
• Tank capacity ranges map household size to first-hour rating using the DOE peak-hour method (about 20 gallons per shower plus kitchen and laundry overlap).
• Installed cost ranges are 2026 planning estimates that combine equipment and typical professional installation; local quotes vary with permits, venting, fuel type, and code upgrades.

These figures are planning estimates, not a quote. The numbers on the unit's EnergyGuide label — first-hour rating for a tank, GPM-at-your-temperature-rise for a tankless — are what you buy on. Confirm your final sizing, fuel type, and venting with a licensed plumber.