Tank gallons and first-hour rating, or tankless GPM and BTU, sized to your household — plus every install part and the cost.
Estimates are planning aids, not a professional quote. Double-check quantities and product coverage — and any local code or permit requirements — before you buy.
| Plumbing & Heating | $786–$1,064 |
| Total | $786–$1,064 |
Excludes tools, delivery, tax and permit fees. Labor not included — this is a DIY materials estimate.
These are affiliate links: we may earn a commission if you buy through them, at no extra cost to you. It never changes what the calculator recommends — the shopping list comes from your dimensions, not from what pays.
About this calculator
The Water Heater Calculator sizes the unit itself before pricing anything around it. Enter how many people live in the house and how many full bathrooms draw hot water at once, and it works out whether a storage tank or a tankless unit fits the demand — a tank's gallon size and first-hour rating, or a tankless unit's gallons-per-minute output at your groundwater's actual temperature rise, which is the number that decides whether a tankless model can really keep up.
Every water heater replacement carries the same overlooked parts: an expansion tank sized to the unit, a drain pan and pan line if it sits above living space, T&P discharge piping, flex connectors, dielectric unions between dissimilar metals, and either a gas vent run or a dedicated electric circuit. Choose gas or electric and the parts list, venting, and cost estimate all update together, so nothing gets left off the shopping list on install day.
Step by step
A step-by-step walkthrough to go with your plan above. Follow the product instructions and any local code or permit requirements as you work.
Close the cold supply valve, shut off the gas cock or breaker, and for gas units kill the pilot or control valve. Confirm everything is off before touching any connection.
Connect a hose to the drain valve and empty the tank to a floor drain or outside, opening a hot tap upstairs to break the vacuum. A tankless unit just needs its isolation valves closed and any residual water in the lines released.
Cut or unscrew the water, gas, and vent connections, then haul the old unit out — tanks are heavy and awkward on stairs, so plan the route and a second set of hands before you start.
Level a tank on its pan, or mount a tankless unit to the wall per its clearance and venting requirements. Confirm the pan and drain line are in place before the unit goes on top of them.
Install the dielectric unions, flex connectors, and expansion tank on the cold supply, then connect the T&P discharge piping down to within 6 inches of the floor or a drain.
Run and seal the gas vent to code, or land the 240V circuit on its own breaker for electric — this is the step worth hiring out if you're not confident with gas or panel work.
Open the cold supply, open a hot tap until the air stops sputtering out, then check every new connection for weeping or hissing before closing anything up.
Set the thermostat or control to 120°F, confirm the pilot or ignition cycles correctly, and run a full sink and shower cycle to check delivered temperature and flow before calling it done.
Equip the job
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Keep planning
Interior & Finishing
Answers
For a storage tank, size comes from first-hour rating, not raw gallons — how much hot water the tank can deliver in the first hour of heavy morning use, based on occupants and how many bathrooms draw at once. For a tankless unit, size comes from peak simultaneous flow in gallons per minute and the temperature rise your groundwater needs to reach a 120°F setpoint. This calculator runs both paths and recommends the equipment class that fits your household, not just a tank that's 'big enough' on paper.
A storage tank costs less upfront, is simpler to install, and tolerates a burst of simultaneous demand by drawing down stored hot water. A tankless unit costs more, needs its own vent or higher-amperage circuit, and never runs out — but its usable flow rate drops as the temperature rise climbs, so a unit that's plenty for a warm climate can fall short in a cold one. Households with steady, predictable use and a lower budget usually do better with a tank; households that want endless hot water and can afford the install usually go tankless.
A tankless unit only has a fixed amount of heating capacity, and it spends that capacity raising incoming water to your setpoint — so a bigger gap between groundwater temperature and 120°F means less flow the unit can deliver at full output. The same model that handles a full house in the South at a 50°F rise may only supply half the flow in the North at an 80°F rise. This calculator uses your selected climate to size the unit against the rise it will actually see, not a manufacturer's best-case rating.
Yes, on any closed system — and more supplies are closed than homeowners realize, since a check valve or pressure-reducing valve at the water meter closes the system even without a whole-house shutoff installed. Heated water expands, and with nowhere for that pressure to go it repeatedly trips the T&P relief valve, which shortens the tank's life and shows up as an intermittent drip. A small pre-charged expansion tank on the cold supply line absorbs that pressure spike and is inexpensive insurance most jurisdictions require by code anyway.
Any water heater installed above living space, in an attic, or over finished flooring is required by most codes to sit in a metal or plastic drain pan plumbed to a safe discharge point, because a slow tank failure otherwise drains straight through the ceiling below. The pan is sized to the tank's footprint and the pan line just needs a gravity path to a floor drain, exterior wall, or condensate line. It's one of the cheapest lines on this list and the one most often skipped on a rushed swap.