How to Choose the Right Size Water Heater
Updated February 20, 2026
Calculate the right water heater size so you never run out of hot water -- covers first-hour rating for tanks, GPM for tankless, quick sizing charts, and the mistakes that leave people taking cold showers.
Overview
Wrong-sizing a water heater is the most expensive mistake we see homeowners make. Too small and you are running out of hot water mid-shower when someone starts the dishwasher. Too large and you are paying to heat water nobody uses. For tank heaters, the number that matters is the first-hour rating (FHR) -- how many gallons it can deliver in the first hour. For tankless, it is flow rate in gallons per minute (GPM). Get these numbers right and you will never have a cold shower again.
What You'll Need
Step-by-Step Instructions
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Estimate Your Peak Hour Demand (Tank Water Heaters)
Figure out how much hot water your household burns through during the busiest hour of the day -- usually the morning rush when multiple people shower and the dishwasher kicks on. Add it up: a 10-minute shower uses about 20 gallons, a bath is 20 gallons, a dishwasher load is 6 gallons, a hot clothes washer load is 7 gallons, hand dishwashing is 4 gallons, and shaving or hand washing is about 2 gallons per person. Two people showering plus the dishwasher: 20 + 20 + 6 = 46 gallons. That is your peak hour demand.
Tip: Got low-flow showerheads and an efficient dishwasher? Drop the shower estimate to 15 gallons and the dishwasher to 4. That difference can actually change which size tank you need. -
Match Peak Hour Demand to First-Hour Rating (FHR)
Every tank water heater has a first-hour rating on its EnergyGuide label -- the number of gallons it can deliver starting with a full tank. Match your peak hour demand to the FHR. Quick reference: a 30-gallon tank delivers 40-50 gallons FHR, a 40-gallon delivers 55-65, a 50-gallon delivers 65-80, and a 75-gallon delivers 85-100+. The FHR is always higher than the tank size because the heater keeps heating incoming cold water while it is delivering hot. So if your peak demand is 46 gallons, a 40-gallon tank with a 60+ FHR handles it fine.
Tip: Do not just match tank size to household size. A family of 4 that staggers showers is completely different from a family of 4 where everyone showers in the same hour. Peak hour demand is the only number that matters. -
Size a Tankless Water Heater by Flow Rate (GPM)
Tankless heaters do not store water -- they heat it on the fly as it flows through. Two numbers matter: flow rate (GPM) and temperature rise. Add up the flow rates of everything you want to run at the same time: a shower is 2.0-2.5 GPM, a kitchen faucet is 1.5 GPM, a dishwasher is 1.0-1.5 GPM. Two showers plus the kitchen faucet: 2.5 + 2.5 + 1.5 = 6.5 GPM. Then figure your temperature rise -- subtract your incoming cold water temp (40-50 degrees in the north, 60-70 in the south) from your target output (120 degrees). Northern homes need a 70-80 degree rise, southern homes need 50-60.
Tip: Here is the catch most people miss: tankless units lose capacity as the temperature rise increases. A unit rated at 9 GPM with a 35-degree rise might only deliver 5 GPM with a 70-degree rise. Always check the GPM at your specific temperature rise, not the max GPM on the box. -
Quick Sizing Chart by Household Size
Quick reference if you do not want to do the math: 1-2 people, low usage: 30-40 gallon tank or 5-6 GPM tankless. 2-3 people, moderate usage: 40-50 gallon tank or 6-7 GPM tankless. 3-4 people, moderate to high: 50-60 gallon tank or 7-8 GPM tankless. 4-5 people, high usage: 60-75 gallon tank or 8-9 GPM tankless. 5+ people or heavy simultaneous usage: 75+ gallon tank, two tanks in parallel, or 9-11 GPM tankless. Got a big soaking tub or multiple showers running at once? Use the detailed calculation instead of this chart.
Tip: Between sizes? Go up. The energy difference between a 40 and a 50-gallon tank is $20-30 per year. The comfort difference when you run out of hot water mid-shower is massive. Slightly oversizing costs almost nothing. Undersizing costs you frustration every single day. -
Consider Energy Source and Recovery Rate
Gas heaters recover faster than electric at the same tank size. A 40-gallon gas tank can hit an FHR of 70+ gallons because the burner reheats quickly. A 40-gallon electric might only hit 55 gallons FHR because the elements are slower. Bottom line: if you are going electric, you generally need a bigger tank to get the same performance as gas. If switching from one to the other, do not assume the same tank size will work. Heat pump (hybrid electric) water heaters vary by mode -- heat pump mode is slower but way more efficient, electric-only mode is faster but uses more energy.
Tip: Natural gas available? Gas tanks typically have lower operating costs and faster recovery. But electric heat pump water heaters have the lowest operating costs of any type -- the trade-off is they need more space for airflow and recover slower in heat pump mode.
Pro Tips
- Before you buy a bigger tank, check this first: the number one reason people run out of hot water is sediment, not an undersized heater. A 50-gallon tank with 10 gallons of sediment in the bottom is effectively a 40-gallon tank. Flush it first (see our water heater flushing guide) and see if that fixes the problem.
- Was the old 40-gallon working fine for years and now you are running short? That is almost always sediment buildup or a failing lower heating element (electric), not a sizing issue. Diagnose before you spend $800+ on a bigger unit.
- Point-of-use tankless heaters ($100-200, installed under a single sink) are a smart move for fixtures far from the main heater. Instead of upsizing the whole system, add a small unit at the distant fixture. Often cheaper and more effective than running a recirculation line.
- Got a big soaking tub (50-80 gallons)? Size the water heater to fill it, not to your normal daily use. A 50-gallon tank cannot fill a 60-gallon tub with hot water -- you either need a bigger tank, a tankless unit, or warm baths.
- Compare the Uniform Energy Factor (UEF) alongside the FHR when shopping. UEF measures overall efficiency -- higher means lower operating costs per gallon of hot water. FHR tells you performance, UEF tells you cost. Look at both.
When to Call a Pro
You do not need a plumber to pick a size. But if you have unusual needs -- multiple bathrooms, big soaking tubs, commercial-grade kitchen fixtures -- a plumber can run a detailed load calculation for your specific home. They can also advise on whether a tankless, heat pump hybrid, or traditional tank makes the most sense for your setup.