Everything you need to launch your trade business.
Everything you need to launch your trade business.
Two quick tools in one. Get the right water heater size for your home, then size the supply pipe in copper or PEX. Real plumbing numbers, no guessing.
Most homes get this wrong in one of two directions: a tank too small that runs cold mid-shower, or one so big you pay to keep water hot you never use. The water heater size calculator above skips the guesswork. Tell it how many people live in the home and how many bathrooms you have, and it gives you the tank size and first-hour rating that actually fits.
The rule of thumb plumbers lean on: plan for about 12 to 15 gallons of hot water per person, then size the tank so its first-hour rating (FHR) covers your busiest hour. A family of four usually needs an FHR of 60 to 80 gallons, which lands on a 50-gallon gas tank or a 65 to 80-gallon electric one. Gas recovers faster (40 to 50 gallons an hour) than electric (20 to 25), so a smaller gas tank can keep up with the same house.
Tankless units are sized by flow, not gallons. Add up the gallons per minute of every hot fixture that might run at once (a shower is about 2 to 2.5 GPM, a sink about 1.5), and that is your target GPM. The catch is climate: the colder your incoming water, the harder the unit works, and the less flow it can deliver. In the warm South a single unit can push 7 to 8 GPM. Up North with 40-degree ground water, that same unit might only hold temperature at 3.5 GPM, so bigger homes often need a high-BTU unit or two in series. Switch the calculator to Tankless and set your climate to see your real number.
Under-size the supply pipe and every fixture drops pressure the second someone flushes. The pipe size calculator tab estimates your home’s peak demand from its fixtures, then recommends the trunk line size for your material. Most homes land on a 3/4″ or 1″ main that drops to 1/2″ branches at each fixture. PEX carries a touch less than copper for the same nominal size because its inside diameter is smaller, so when you are on the line between two sizes with PEX, go up. The tool already builds in the fact that not every fixture runs at once, so the number it gives you is real-world demand, not a worst-case pile-up.
Sizing jobs right is how you avoid callbacks and warranty headaches, and it is exactly the kind of judgment that separates a pro from a handyman. If you are setting up your own shop, get the rest of the launch dialed too: see how to start a plumbing business for the full playbook, and run your jobs through our free pricing calculator so every install and water-heater swap is priced to actually pay you.
The Plumbing Starter Kit hands you the pricing sheets, contracts, checklists, and templates to launch clean and look pro from day one. One download, no monthly fees.
Plan for a first-hour rating of 60 to 80 gallons. That usually means a 50-gallon gas tank or a 65 to 80-gallon electric one. Gas recovers hot water faster, so a smaller gas tank keeps up with the same household. Add a size if you have three or more bathrooms or a big soaking tub.
Add up the GPM of every hot fixture that might run at the same time (shower about 2 to 2.5 GPM, sink about 1.5), then check your climate. Colder incoming water lowers the flow a unit can deliver, so a home in the North needs more capacity than the same home in the South for the same number of fixtures.
It depends on total demand. At a safe velocity, 1/2″ copper carries about 3.2 GPM, 3/4″ about 6.5, and 1″ about 10.9. Most homes run a 3/4″ or 1″ trunk line and drop to 1/2″ branches at each fixture. PEX of the same nominal size carries a little less than copper.
Both are code-approved. PEX is cheaper, faster to run, and freeze-tolerant, but has a slightly smaller inside diameter, so it flows a bit less per size. Copper is rigid, long-lasting, and handles heat well. For flow, size PEX one notch generously and you will be fine.
