What Size Water Heater Do I Need? Sizing Guide
If you're shopping for a new water heater, here's the short answer first: for most households, match the tank to your household size — 30–40 gallons for 1–2 people, 40–50 gallons for 3–4 people, and 50–80 gallons for 5 or more. A family of four with two bathrooms — the most common Lawrenceville household we see — usually lands on a 50-gallon tank, or a tankless unit rated around 7 gallons per minute (GPM).
But tank size alone isn't the whole story. Two 50-gallon tanks can deliver very different amounts of hot water, and the number that actually decides whether your morning showers stay hot is printed on the yellow EnergyGuide label: the First Hour Rating (FHR). Below is the quick chart, then the simple method to size yours properly — so you don't end up paying to heat water nobody uses, or rationing hot showers.
Quick Sizing Chart
| Household size | Tank size (general guide) | Tankless flow rate (general guide) |
|---|---|---|
| 1–2 people | 30–40 gallons | 5–7 GPM |
| 3–4 people | 40–50 gallons | 7–9 GPM |
| 5–6 people | 50–75 gallons | 9–11 GPM |
| 7+ people | 75–100 gallons, or tankless | 11+ GPM, or two units |
These are starting points, not rules. A couple that runs the dishwasher and laundry at the same time as their showers can need more than a family of four that staggers everything. That's why the pros size by your busiest hour, not just your headcount.
Why Tank Gallons Alone Can Mislead You
A storage water heater's gallon rating tells you how much water the tank holds. Its First Hour Rating tells you how much hot water it can actually deliver in one hour, starting with a full tank — the stored water plus what the burner or elements can reheat during that hour.
That's the number that matters on a busy morning. A gas tank and an electric tank can both say "50 gallons" on the box and have very different First Hour Ratings, because gas burners reheat water much faster than standard electric elements. This is also why a smaller gas tank can sometimes outperform a bigger electric one — and why you should compare FHRs, not just gallon ratings, when you're choosing between models.
You'll find the FHR on the yellow EnergyGuide label on the tank, or in the specifications section of the product page if you're shopping online.
Step 1: Figure Out Your Peak-Hour Demand
Your peak hour is the busiest hot-water hour in your home — usually the morning, when everyone showers and the kitchen and laundry are running. Add up what your household uses in that hour:
- Shower: about 10–20 gallons each (depends on length and showerhead flow)
- Bath: about 20–30 gallons
- Dishwasher load: about 10–15 gallons
- Washing machine load: about 20–30 gallons
- Kitchen sink use: about 5 gallons
Here's a worked example, similar to the one Rheem uses in its sizing guide: two morning showers (30 gallons) + one load of laundry (20 gallons) + kitchen use (5 gallons) = 55 gallons of peak-hour demand. That household needs a water heater with a First Hour Rating of at least 55 gallons. A 40-gallon tank with a strong FHR might clear that bar; a 50-gallon tank with a weak one might not — which is exactly why the FHR, not the tank size, is the number to check.
Run your own numbers with your real routine. If your household's peak hour is the evening — dinner, dishes, baths for the kids — use that hour instead. The method is the same.
Step 2: If You're Going Tankless, Size by Flow Rate (GPM)
Tankless heaters don't store water, so there's no tank size and no First Hour Rating. Instead, they're sized by flow rate in gallons per minute — how much hot water they can heat at once.
Add up the fixtures that might run at the same time during your peak hour. A standard showerhead uses roughly 2–2.5 GPM, a kitchen or bathroom faucet about 1–1.5 GPM, a dishwasher around 1–1.5 GPM, and a washing machine around 1.5–2 GPM. Two showers plus a running faucet, for example, needs roughly 6–7 GPM of capacity.
Two tankless gotchas to know before you buy:
- Temperature rise matters. A tankless unit's GPM rating assumes a certain temperature rise — how much it has to warm the incoming water. In winter, when incoming water is colder, the same unit delivers fewer gallons per minute. If your household's demand is close to a unit's rated GPM, size up rather than assuming the brochure number.
- Gas line and venting can limit you. Larger gas tankless units may need a bigger gas line than your home currently has. That's a real cost to factor in — our tank vs. tankless guide covers the trade-offs, and our replacement cost guide explains what drives the install price.
Gas vs. Electric: Recovery Speed Changes the Size
Fuel choice affects sizing because recovery speed differs. Gas burners reheat a tank faster than standard electric elements, so a gas unit's First Hour Rating is typically higher for the same tank size. Practical consequence: an electric household often needs to go one size up compared to the equivalent gas household to get the same peak-hour performance.
That's one reason the chart above is a range, not a single number. If your home is all-electric and your peak-hour demand is near the top of a range, pick the larger tank. If you're choosing between fuels as well as sizes, our gas vs. electric comparison walks through running costs and recovery for Georgia homes.
5 Factors That Push You Up a Size
Household size gets you in the ballpark. These five factors decide whether you need the top of the range — or the next size up:
- A big soaking tub. A standard tub takes 20–30 gallons; a deep soaking tub can take 50–80 gallons on its own. If baths are a regular thing, size for the tub, not just the showers.
- Simultaneous showers. Two or three bathrooms used at once is the classic reason a "big enough on paper" heater runs cold. If your household's peak hour includes overlapping showers, size for that overlap.
- High-flow fixtures. Older showerheads and body-spray systems use far more water than modern low-flow ones. If you have them, your per-shower gallons are higher than the chart assumes.
- Frequent guests or a growing family. A tank that's exactly right today is undersized the day a teenager moves back in or guests become regular. If growth is likely, buying the next size up now is cheaper than replacing early.
- Laundry and dishwasher timing. Households that run appliances during shower hours need more peak-hour capacity than households that run them later. Shifting appliance use out of the peak hour is also the cheapest "fix" for a borderline tank — more on that below.
Signs Your Current Water Heater Is the Wrong Size
Not sure whether sizing is your actual problem? These symptoms point to an undersized tank:
- The last shower of the morning is lukewarm — the tank can't keep up with back-to-back use.
- Hot water runs out during baths or laundry days but is fine the rest of the week — your peak-hour demand exceeds the FHR.
- You plan your day around the water heater — showering at odd hours or waiting between showers.
An oversized tank has quieter symptoms: higher energy bills (you're paying to keep water hot that nobody uses — and water heating is roughly a fifth of a typical home's energy use, per the U.S. Department of Energy) and a bigger unit taking up space. Oversizing is the less painful mistake, but it still costs you every month.
If your tank is only slightly undersized, a few fixes can buy time: stagger showers, run the dishwasher and laundry outside peak hours, or fit low-flow showerheads to cut per-shower gallons. If you're constantly running out, though, no habit change fixes a tank that's 20 gallons short — that's a sizing problem, and it ends with a replacement. Our lifespan guide can help you judge whether replacement is due anyway.
The Bottom Line for Lawrenceville Homes
Start with household size, then check your peak hour: pick a tank whose First Hour Rating covers your busiest hour, or a tankless unit whose GPM covers your simultaneous fixtures. When in doubt between two sizes, go bigger — the cost difference between a 40- and a 50-gallon tank is small compared to years of cold showers, and an oversized tank wastes far less than an undersized one frustrates.
Still unsure which size fits your home? Water Heater Pros sizes and installs water heaters across Lawrenceville and Gwinnett County every day — we'll check your household's actual peak demand, your gas or electrical setup, and the space you have, and give you a straight recommendation. Call us at (770) 796-7565 for a free quote, or start with our water heater replacement and installation pages to see what's involved. And once your new unit is in, our maintenance guide will help it last as long as possible.