What Size Water Heater Do I Need?

Almost everyone sizes a water heater by tank capacity, because that is the number on the front of the box and the number the old unit had. It is a poor guide, and it is the reason households replace a 50-gallon heater with another 50-gallon heater and still run out of hot water.

The problem is that a tank does not deliver its capacity. As hot water leaves the top, cold water enters the bottom, and the two mix - so a 50-gallon tank does not give you 50 gallons of hot water, it gives you some smaller amount of usable hot water plus whatever the burner or elements can reheat while you are drawing. That second part is recovery, and it varies enormously: a gas burner puts far more energy into the tank per hour than a pair of electric resistance elements, which is why a 40-gallon gas unit often serves a household that struggles on a 50-gallon electric.

The industry's answer to this is a single combined figure. ENERGY STAR defines first hour rating as an estimate of the maximum volume of hot water in gallons that a storage water heater can supply within an hour, noting that it begins with a fully heated unit and is measured at a 125 degree Fahrenheit outlet temperature. It bundles usable capacity and recovery into one number, which is exactly what you want when the question is whether three consecutive showers will work.

For a sense of where certified equipment sits, ENERGY STAR's own criteria require a first hour rating of at least 51 gallons per hour for qualifying gas-fired storage water heaters and at least 45 gallons per hour for qualifying electric heat pump water heaters. Those are certification thresholds rather than recommendations for your house - but they show that the figure exists, is published per model, and is comparable across fuel types in a way tank size is not.

The right size is the one whose first hour rating covers your busiest hour of hot water use - not your daily total, and not an average. Most households have one obvious peak: weekday mornings, or the stretch after dinner. Think about that hour specifically and count what actually happens in it.

1. Pick your heaviest hour. For most families it is a weekday morning; for some it is evening when showers, dishes and laundry overlap. Choose the realistic worst case, not the quiet average.
2. Count the showers in that hour, and be honest about length. Showers are the dominant draw in nearly every household, and a long shower is a large fraction of a tank on its own.
3. Add the other fixtures that genuinely run in that same hour - a bath, the dishwasher, a clothes washer on warm, hand and face washing, shaving, food preparation.
4. Note anything unusual about your fixtures. A soaker tub, a large or multi-head shower, or a high-flow shower head changes the arithmetic substantially, and a big tub is the single most common reason a conventionally sized heater disappoints.
5. Think about simultaneity rather than sequence. Two showers at once is a different demand from two showers back to back, and the back-to-back case is where recovery rate rescues you.
6. Add headroom for how the household will change. If teenagers are coming, or you are adding a bathroom, size for that rather than for today.
7. Then compare against the first hour rating on the EnergyGuide label of the units you are considering - not the gallon capacity. Ask your plumber for the figure if the spec sheet is not to hand.

Sizing has a cost dimension, so it is worth seeing what the published figures look like across capacities before deciding to go larger. We are a free referral service, not a plumbing company - we do not set prices and we never publish a price as our own. These are published national figures from an independent cost-data publisher, named and linked with the date it was updated.

If you are considering tankless, the sizing question changes completely. There is no tank, so there is no capacity and no first hour rating - there is only flow rate, measured in gallons per minute, and the temperature rise the unit can hold at that flow.

Those two things trade against each other, which is the key insight. Any demand heater can deliver a high flow at a small temperature rise or a low flow at a large one. ENERGY STAR expresses its criteria accordingly: for gas-fired instantaneous water heaters it requires a maximum flow of at least 2.8 gallons per minute over a 67 degree Fahrenheit rise, and it defines the gallons-per-minute figure as the hot water flow an instantaneous heater can supply while holding a nominal temperature rise.

So sizing tankless means adding up the flow of the fixtures you expect to run at the same time, and then checking the unit delivers that flow at the temperature rise your incoming water actually requires. Incoming water temperature is seasonal - a unit that handles two simultaneous showers comfortably in August may not in a cold January, because the rise required is larger. That seasonal shortfall is the most common complaint about an undersized tankless installation, and it is entirely a sizing error rather than a product fault.

Two practical consequences. First, be realistic about simultaneity - a tankless unit sized for one shower is fine until someone starts the dishwasher. Second, a larger gas-fired tankless unit frequently needs a bigger gas line and a new vent, which is cost that sits outside the appliance price; ENERGY STAR also notes an eligibility size limit for instantaneous units of no more than one gallon of water per 4,000 Btu per hour of input, with input under 200,000 Btu per hour for gas-fired units.

Peak hour demand tells you the size you want. Several practical things decide the size you can actually have, and it is better to find them out before you buy.

• Physical space. A larger tank is taller and wider, and a great many water heaters in this market live in a closet, a garage alcove or an attic platform sized for the original unit. Measure, including the clearance above for the flue and the pan beneath.
• The pan and the drain. A replacement has to sit in a pan with a drain line, and that is inspected. An attic installation makes this non-negotiable, because a leak there comes through the ceiling below.
• Gas capacity, if gas. A bigger burner needs a line sized for the bigger load, and the existing line may not be. This is the most common hidden cost in going up a size.
• Venting. A larger gas unit may not work on the existing flue, and where a natural-draft vent will not serve, power venting is an added cost.
• Electrical capacity, if electric. A larger or higher-recovery electric unit needs an adequate circuit, and panel space.
• A heat pump water heater needs air volume and condensate drainage, and is taller than a conventional tank of the same capacity. Good in a garage, often impractical in a small interior closet.
• An expansion tank, which code-compliant installations in this area generally require on a closed system.
• Hard water. The North Texas Municipal Water District describes its treated water as moderately hard, and the City of Mesquite publishes an average of 93 to 134 milligrams per liter for its own supply, which it converts to roughly 5.44 to 7.84 grains per gallon. Scale settles as sediment in a tank and coats a tankless heat exchanger, so annual flushing matters here whatever size you choose.

There is a real temptation to oversize on the theory that it can only help. It is not free, and it is worth knowing what you are buying.

A larger tank costs more to buy and install - Fixr's published figures run from $550 to $2,100 for a 30-gallon unit to $1,200 to $3,600 for an 80-gallon - and it keeps a larger volume of water hot around the clock whether you use it or not. It takes more room, which may be room you do not have. And going up a size frequently triggers the infrastructure items above: a bigger gas line, a different vent, a new circuit. Those are usually a larger cost than the appliance difference.

The better approach is to size to a realistic peak hour with sensible headroom, and to use first hour rating to get there rather than gallons - because a higher-recovery unit of the same tank size often solves the problem without any of the knock-on costs. That is also why gas versus electric and tank versus tankless are worth settling alongside the sizing question rather than after it; our guides on gas versus electric water heaters and tankless versus tank cover those trade-offs.

Finally, if your current heater genuinely never runs short, you have your answer and the sizing exercise is already done - match the first hour rating of what you have and spend the attention on efficiency, warranty and a code-compliant installation instead.

Dallas Best Plumber is a free referral service, not a plumbing company. Tell us how many people are in the house, how many bathrooms, whether you have a large tub, where the heater lives and whether you currently run out of hot water, and we put your request in front of licensed, insured local plumbers who size and install water heaters, with a free, no-obligation quote. The recommendation and the price are theirs - we never set either.