Calculator
Water Heater Size Calculator
A 50 gallon tank with a fast burner can outrun an 80 gallon tank with a slow one, because the number stamped on the outside of a water heater is stored volume, not delivery capacity. What actually decides whether a household runs out of hot water is the busiest single hour of a typical day — two showers back to back, a load of laundry, someone doing dishes — set against how much hot water the tank can supply plus reheat during that hour, not how many gallons it holds when everyone is asleep.
This calculator builds that peak hour fixture by fixture rather than reaching for a 'family of four' sizing table, then converts the total into both a tank size sized against a first-hour-style target and, separately, the flow rate and BTU input a tankless unit would need to keep up with the same fixtures running at once. Those are two different calculations built on different assumptions, not one number expressed two ways, which is why they get their own inputs below rather than sharing one.
Calculate your quantity
For the tankless option.
gpm
Peak-hour demand
61 gallons
Tank size
80 gallons
- Showers
- 40 gallons
- Baths
- 0 gallons
- Dishwasher
- 6 gallons
- Laundry
- 7 gallons
- Sinks
- 8 gallons
- Tankless flow needed
- 4 gpm
- Temperature rise
- 70 deg F
- Tankless input
- 139,944 BTU/hr
- Tank size and first-hour rating are different numbers. A 50 gallon tank with a fast recovery can beat an 80 gallon tank with a slow one — buy against the FHR on the label, not the nameplate volume.
Shopping summary
- • A 80 gallon tank with a first-hour rating of at least 61 gallons
- • Or a tankless unit delivering 4 gpm at a 70 deg F rise
This is an estimate — confirm structural work with a professional.
How this calculation works
Peak-hour demand adds up the gallons each fixture pulls in the household's busiest hour: showers at a chosen gallons-per-shower figure, baths, a dishwasher run, a laundry load, and sink use, each at a typical draw. This is not a full day's hot water use — it deliberately models the worst hour, the way a heat-loss calculation models the coldest design day rather than an average one, because that busiest hour is when a tank actually runs short.
Tank sizing divides that peak demand by 0.9 before rounding up to a stock size, because a tank never delivers its full nameplate volume as usable hot water in an hour — some capacity is lost to the cold water mixing in as hot draws out, and some is offset by whatever the burner or element manages to reheat during that same hour. The tankless side ignores tank volume entirely: it multiplies how many fixtures might genuinely run at the same moment by the flow each one demands, then sizes the BTU/hr input needed to raise that flow from your incoming water temperature to your target output temperature.
- Buy against first-hour rating (FHR) on the yellow EnergyGuide label, not the nameplate tank volume — FHR already accounts for recovery speed, which the raw gallon number does not.
- Incoming water temperature changes with season and region, and it only affects the tankless side of this page — tank sizing here is driven by fixture counts, not by how cold your groundwater runs in January.
- 'Simultaneous fixtures' for a tankless unit means how many might genuinely be open at the exact same moment in the worst realistic case, not every fixture the house owns added together.
- This page sizes flow and BTU input for a tankless unit; it does not size the gas line or venting a large BTU input requires, which is installer-scope work tied to the specific model chosen.
The formula
peakDemand = showers x showerGallons + baths x 20 + dishwasherRuns x 6 + laundryLoads x 7 + handWashes x 2; tankSize = nextStock(peakDemand / 0.9); tanklessBtu = (simultaneousFixtures x fixtureGpm) x 8.33 x tempRise x 60
- showers x showerGallons
- Number of showers in the peak hour times gallons drawn per shower — the single largest line item in most households and the one worth adjusting first if the tank sizing here feels off.
- baths x 20, dishwasherRuns x 6, laundryLoads x 7, handWashes x 2
- Typical gallon draws per use for the remaining fixtures, added at whatever count you expect to overlap with showers in the busiest hour, not a full day's total for each.
- peakDemand / 0.9
- The usable-fraction adjustment: a tank's real first-hour delivery runs below its stored volume because incoming cold water mixes with what remains as the tank draws down, so the target size is set a bit above raw peak demand.
- simultaneousFixtures x fixtureGpm
- How many fixtures you expect open at once, times the flow rate each draws — the input a tankless unit actually has to sustain continuously, which is a very different number from an hour's cumulative gallons.
- 8.33 x tempRise x 60
- 8.33 is the weight of a gallon of water in pounds; tempRise is output temperature minus incoming temperature; 60 converts a per-minute flow into an hourly BTU/hr input rating.
Where these numbers come from
- First-hour rating (FHR) as the buying figure
- Manufacturer-published: the yellow EnergyGuide label required on every residential water heater states FHR directly, derived from the unit's own recovery testing rather than its stored volume alone.
- 20 gal shower default, 20 gal bath, 6 gal dishwasher, 7 gal laundry load, 2 gal sink use
- Trade convention: typical per-use draws referenced across tank manufacturers' own sizing guides. Actual fixtures vary — a low-flow shower head or a modern dishwasher can both pull noticeably less than these defaults.
- 8.33 lb per gallon of water
- Physical constant, water's weight at typical residential temperatures, combined with its specific heat of 1 BTU per pound per degree F to convert a flow rate directly into a BTU/hr requirement.
- 0.9 usable-fraction factor for tank sizing
- Trade convention recognizing that a stored tank rarely delivers its full nameplate volume as usable hot water within one hour, due to cold-water mixing during draw-down.
Worked examples
A two-bath household on municipal water
| Showers in the peak hour | 2 |
|---|---|
| Gallons per shower | 20 |
| Baths | 0 |
| Dishwasher runs | 1 |
| Laundry loads | 1 |
| Sink uses | 4 |
| Simultaneous fixtures | 2 |
| Flow per fixture | 2 |
| Incoming water temperature | 50 |
| Output temperature | 120 |
| Peak-hour demand | 61 gallons |
|---|---|
| Tank size | 80 gallons |
| Showers | 40 gallons |
| Baths | 0 gallons |
| Dishwasher | 6 gallons |
| Laundry | 7 gallons |
| Sinks | 8 gallons |
| Tankless flow needed | 4 gpm |
| Temperature rise | 70 deg F |
| Tankless input | 139944 BTU/hr |
61 gallons of peak-hour demand rounds up to an 80 gallon tank once the 0.9 usable-fraction factor is applied — the jump from a 66 to an 80 gallon stock size, rather than landing neatly on 66, is exactly the kind of step this page exists to show before you are standing in a showroom guessing between two models.
On the tankless side, two fixtures running at 2 gpm each need 4 gpm at a 70 degree rise, which works out to roughly 140,000 BTU/hr input — worth comparing against a specific unit's rated input before assuming any tankless model on a shelf will do, because smaller residential units commonly rate well under that.
A larger household on well water in a cold winter
| Showers in the peak hour | 4 |
|---|---|
| Gallons per shower | 20 |
| Baths | 1 |
| Dishwasher runs | 1 |
| Laundry loads | 2 |
| Sink uses | 8 |
| Simultaneous fixtures | 3 |
| Flow per fixture | 2.5 |
| Incoming water temperature | 37 |
| Output temperature | 120 |
| Peak-hour demand | 136 gallons |
|---|---|
| Tank size | 100 gallons |
| Showers | 80 gallons |
| Baths | 20 gallons |
| Dishwasher | 6 gallons |
| Laundry | 14 gallons |
| Sinks | 16 gallons |
| Tankless flow needed | 7.5 gpm |
| Temperature rise | 83 deg F |
| Tankless input | 311126 BTU/hr |
Peak demand more than doubles to 136 gallons on fixture count alone, pushing the tank to the largest stock size on this page. That side of the answer is exactly what you would expect from more people and more fixtures — no surprise in the arithmetic.
The tankless side has a less obvious surprise in it. 7.5 gpm at a 120 degree target with 50 degree incoming water would need about 262,000 BTU/hr; the same fixtures at this well's actual winter incoming temperature of 37 degrees need roughly 311,000 — close to 49,000 BTU/hr more, from colder groundwater alone, on top of what the extra fixtures already added. Sizing a tankless unit from a summer flow test and ignoring winter incoming temperature is how a system that works in July runs short every January.
Common mistakes
- Sizing off a generic 'family of four needs a 50 gallon tank' table instead of actual habits — back-to-back showers before school, a soaking tub, or laundry that always runs during the morning rush can push real peak demand well past what a household-size rule assumes.
- Buying by the gallon number printed on the tank's own casing rather than the first-hour rating on the yellow EnergyGuide label — two tanks of the same stored volume can have meaningfully different FHR depending on burner or element size.
- Sizing a tankless unit's BTU requirement from a mild-season flow test and never checking incoming temperature in the coldest month it will actually run in, which can add tens of thousands of BTU/hr beyond what the fixture count alone suggests.
- Counting every fixture in the house as 'simultaneous' for the tankless calculation instead of the realistic worst case of what might genuinely be open at once — this inflates the required BTU/hr input well past what the household will ever actually draw.
- Choosing a tankless unit by flow and BTU input alone without confirming the gas line and venting it needs can actually be installed as sized — a unit rated for the load on paper can still be undeliverable on the gas meter or vent run already in the house.
- Ignoring recovery time entirely on the tank side. A fast-recovery gas tank and a slow-recovery electric tank of the identical first-hour rating behave very differently for a second peak hour close behind the first one, which this single-peak-hour calculation does not distinguish.
Shopping summary
Buy against the tank size or the tankless flow and BTU input this page produces, checking the specific model's own first-hour rating or BTU input rating against these numbers rather than assuming any unit at that stated capacity performs identically.
If you are weighing a heat pump water heater against a standard electric or gas tank, its higher efficiency does not change the peak-hour demand math here, but it does add a meaningful electrical load worth checking against the generator size calculator if backup power during an outage matters to you, and against the solar array calculator if you are sizing panels to offset it.
Installation and any gas line, venting, or electrical upgrade a new unit needs is real budget on top of the equipment cost itself — the material cost calculator is a reasonable place to total that alongside the unit price before committing to a model.
FAQ
Why does a 50 gallon tank sometimes outperform an 80 gallon one?
Because first-hour rating depends on recovery speed as much as stored volume. A 50 gallon gas tank with a large burner can reheat fast enough during a busy hour to deliver more usable hot water than an 80 gallon electric tank with a slow element, even though the electric tank holds far more water sitting still.
Where do I actually find the first-hour rating for a specific model?
That label figure is measured under one standardized DOE test — a fixed draw pattern and a fixed incoming water temperature — not your household's actual habits or your region's actual groundwater temperature. It's a close, reliable number for comparing models, but a genuinely colder incoming supply than the test condition can pull real delivered capacity a bit below what's printed, which is worth keeping in mind if your winter groundwater runs unusually cold.
Does colder winter incoming water change which tank size I need?
The reason it doesn't move the tank number is that a tank pre-heats before you need it, so a colder supply just means slower recovery afterward, not a bigger tank up front — the peak-hour target here is still gallons drawn, not gallons heated in real time. Where cold incoming water does matter on the tank side, even though this page doesn't model it directly, is a second busy hour close behind the first: a tank recovering from an unusually cold winter supply refills more slowly, so a household with back-to-back peak hours should treat this page's single-peak-hour size as a floor rather than a comfortable margin.
How many fixtures should I actually count as 'simultaneous' for a tankless unit?
Whatever might realistically run at the exact same moment in your household's worst case — a shower and the kitchen sink at once, say — not every fixture in the house added together. Counting the whole house's fixture inventory as simultaneous massively overstates the BTU/hr input a tankless unit needs to carry.
Does a heat pump water heater need a different number from this calculator?
The peak-hour demand and tank-size math is identical regardless of what heats the tank — the physics of how much hot water a household pulls in its busiest hour does not change. What differs is recovery behavior: a heat pump water heater typically recovers more slowly than a gas tank of the same first-hour rating, which matters more for a second busy hour close behind the first than for the single peak hour this page models.
Can this size a whole-house recirculating hot water system?
It sizes the tank or tankless unit's own capacity, which a recirculating loop still needs correctly sized underneath it. What it does not model is the loop's own standby heat loss, which adds continuous demand on top of fixture draws and is a separate calculation tied to pipe length and insulation rather than fixture count.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.