Water Heater Size Calculator

How to Use This Water Heater Size Calculator

Start with the number of people in the house and the fuel type you plan to install, gas, electric, or heat pump. Then count the hot water events that happen in the single busiest hour of a normal day. The calculator treats each shower as about 20 gallons of stored hot water, each bath as 30, each laundry load as 20, each dishwasher load as 10, and each sink use as 4. Enter how many of each happen in that peak hour, and the calculator returns your peak hour demand, the smallest tank that covers it for your fuel type, the First-Hour Rating of that tank, and the tankless flow rate you would need if you skip the tank.

The demand numbers come from the same family of estimates that Energy Star and water heater manufacturers use on their sizing worksheets. They assume a standard 120°F tank temperature and standard fixtures. If you run the tank hotter, take long showers with rain heads, or soak in a deep tub, add an event or two before you shop. The calculator takes the larger of your counted demand and a simple people-based baseline, so a household that only enters its head count still gets a reasonable answer.

How Water Heater Sizing Actually Works

Two numbers matter. Peak hour demand is the total hot water your household draws in the single busiest hour of the day, usually the morning. First-Hour Rating is how much hot water a tank can deliver in that same hour, starting from a full, heated tank. The tank passes if its First-Hour Rating meets or beats your peak hour demand. Everything else in the brochure is noise.

A tank is not a bucket that holds exactly its rated gallons of hot water. As you draw hot water out of the top, cold water enters at the bottom and the burner or element starts reheating immediately. A 40-gallon gas tank delivers 70-plus gallons in the first hour because it recovers while it delivers. A 40-gallon electric tank delivers closer to 50 because electric elements heat more slowly. That is why the First-Hour Rating, not the gallon sticker, is the number to compare when you size a tank.

Tank Size vs First-Hour Rating: Why Gallons Mislead

The number printed on the side of a water heater is the tank capacity, the storage volume. The First-Hour Rating is a second number, usually printed on the Energy Guide label and in the spec sheet. For the same tank size, the FHR swings wildly by fuel type. A 40-gallon gas tank beats a 40-gallon electric tank by about 20 gallons of first-hour delivery. A heat pump tank of 50 gallons delivers like a larger electric tank because its compressor runs through the recovery.

So when someone tells you to "get a 50-gallon tank," the real question is what First-Hour Rating that model delivers. A 40-gallon gas tank with a 72-gallon FHR often beats a 50-gallon electric tank with a 62-gallon FHR. Compare the Energy Guide numbers for the exact models on your short list, not the gallon sizes. The calculator bakes this in by recommending the smallest tank whose FHR clears your demand for the fuel type you chose.

Quick Sizing Chart by Household Size

The table below gives typical peak hour demands and tank sizes for standard fixtures. If your family takes baths, stacks laundry in the evening, or runs a dishwasher during the morning rush, your demand lands at the high end of each row.

These rows assume showers are the main load and that the fixtures are standard. The HVAC BTU calculator on this site handles the other half of home comfort sizing, the heating and cooling load that decides the size of your furnace and air conditioner, while this calculator handles the domestic hot water side.

Tank vs Tankless vs Heat Pump

The three technologies size by different numbers, and picking the wrong frame is the first mistake buyers make. Here is the honest comparison.

• Storage tank ($500-1,200 installed): Stores 30 to 80 gallons, cheapest upfront, simple to replace, runs 12 to 15 years. The sizing number is the First-Hour Rating. The catch is standby loss, heat escapes through the tank walls even when nobody uses water.
• Tankless ($1,500-3,500 installed): Heats water on demand at 1.5 to 11 GPM, lasts 20-plus years, and eliminates standby loss. The sizing number is the flow rate in gallons per minute. It needs a bigger gas line or a dedicated 240V circuit, and if you open three fixtures at once, you can outdraw it.
• Heat pump, or hybrid ($2,000-3,500 installed): A 50 to 80 gallon tank plus a compressor that pulls heat out of the room air. Two to three times more efficient than a standard electric tank, with an FHR in the 65 to 100 range. It needs air volume and a condensate drain, which is why it belongs in a garage, basement, or utility room rather than a tight closet.

By 2026, several states require heat pump water heaters for new electric installations, including California, Washington, Oregon, Colorado, and New York. If you are replacing an electric tank in one of those states, budget for a heat pump model or check the current exemptions before you buy a standard electric unit.

Electric, Gas or Heat Pump: Choosing the Fuel

Gas recovers the fastest and delivers the highest First-Hour Rating per gallon, which is why 40-gallon gas tanks serve families that would need 50 or 66-gallon electric tanks. Gas needs a vent and a gas line, and it only makes sense where natural gas or propane is already plumbed in. Electric is the simplest install, no venting, no gas line, and it fits in closets and tight spaces, but the elements recover slowly and the running cost is higher. Heat pump models cut electric water heating cost by 60 to 70 percent on a 240V circuit, and they produce a small amount of condensate. In a cold garage the compressor works harder in winter, so most hybrids carry a backup electric element for the coldest days.

If you already have gas, a gas tank is usually the least disruptive replacement. If you have an electric tank, weigh a heat pump against a standard electric unit using the rebates in your area. Several utilities and states offer $500 to $2,000 in rebates on heat pump models, which often closes most of the upfront price gap and flips the math in the heat pump's favor.

Recovery Rate and Why It Matters

Recovery rate is how many gallons a tank can heat back to temperature in one hour. Gas tanks recover about 30 to 40 gallons per hour, electric tanks about 15 to 20, and heat pumps often less than 15 in winter before the backup elements engage. Recovery is already baked into the First-Hour Rating, but it matters on its own for back-to-back showers. Two long showers back to back can exhaust a slow-recovering electric tank even when the tank passed the peak-hour check, because the second shower draws water that never fully caught up.

If your household stacks showers or runs the laundry right after dinner, a faster-recovering gas or heat pump model covers you better than a bigger electric tank. The FHR number reflects this difference, which is why the calculator compares FHR to demand instead of matching gallons to people.

Tankless Sizing: Choosing a GPM Rating

Tankless water heaters are rated in gallons per minute, the flow of hot water the unit can heat while running. To size one, add the flow of the fixtures you might have open at the same time. A shower runs about 2.5 GPM, a bath faucet about 4, a laundry machine about 3, and a dishwasher or sink about 1.5 each. If two showers and a sink are on at once, you need about 6.5 GPM.

Common tankless sizes run from 3.5 to 11 GPM. Buy for your busiest simultaneous use, not the average, and always check the temperature rise the unit can handle. A unit rated 6.5 GPM at a 35°F rise may only deliver 4 GPM at a 70°F rise, which is the winter case in the northern half of the country. The GPM number in the calculator output assumes a moderate rise, so confirm the model's rise chart against your local ground water temperature.

Worked Example: A Family of Four

Take a family of four: two showers and a sink in the morning, one laundry load and a dishwasher in the evening. The busiest hour is the morning, two showers at 20 gallons plus one sink at 4 gallons, for a peak hour demand of 44 gallons. A 40-gallon gas tank with a 72-gallon FHR covers it easily. A 40-gallon electric tank with a 52-gallon FHR also covers it, with less margin. A heat pump 50-gallon tank with a 66-gallon FHR covers it too, and the calculator recommends the smallest tank that clears the demand for the fuel type you selected.

For a tankless unit, two showers and a sink at once means 2 times 2.5 plus 1.5, about 6.5 GPM. A 6.5 GPM tankless unit with a rise rating that matches the local ground water temperature would serve the family, as long as nobody opens the bath faucet at the same moment. If the bath is likely, the correct number becomes 10.5 GPM and a large tankless unit, which is exactly the trade this calculator shows before you buy.

5 Water Heater Sizing Mistakes That Leave You Cold

1. Sizing by tank gallons alone. The gallon label and the First-Hour Rating are different numbers. A 40-gallon gas tank can outperform a 50-gallon electric tank, and a family that sizes by gallons often buys the wrong fuel type. Compare the FHR on the Energy Guide label.
2. Ignoring the peak hour. Sizing by daily total use instead of the single busiest hour undersizes the tank. Daily averages hide the morning rush, and the morning rush is what decides whether your shower goes cold.
3. Forgetting recovery on electric. Electric tanks recover slowly, around 15 to 20 gallons per hour. Back-to-back showers can run a passing electric tank cold. If your household stacks showers, choose gas, heat pump, or a larger electric tank with a higher FHR.
4. Buying tankless without checking the rise. A tankless GPM rating means nothing until you factor the temperature rise in your climate. A unit that works in Houston can disappoint in Minneapolis, where the incoming water is 40 degrees colder in January.
5. Skipping the physical fit. A bigger tank needs clearances, a drain pan, and often a wider door route. A heat pump needs air volume and condensate drainage. Measure the space before you commit to a size, or the 80-gallon tank you just ordered will not fit the closet.