Find the bottleneck before replacing the fixture.

By DOE guidance, about 54 gallons of first hour rating, which is typically a 50 gallon gas tank or a 66 gallon electric one, since electric elements recover more slowly. The reason a 50 gallon tank can deliver 54 gallons in the first hour is that the burner is heating the whole time you are drawing. Always compare first hour ratings between models rather than nominal capacity — two 50 gallon heaters can differ by 20 gallons of first hour rating.

It is the number of gallons of hot water the heater can supply in one hour starting from a fully heated tank, and it appears on the yellow EnergyGuide label. It combines storage volume with recovery rate, which is what your morning actually looks like: everyone showering inside a 60 minute window. A small tank with a powerful burner can beat a bigger tank with a weak element, and buying on capacity alone hides that entirely.

Add up the fixtures you expect to run at once. A modern showerhead is 2.0 to 2.5 GPM, a kitchen faucet about 1.5, a bathroom faucet 1.0. Two showers at once is 4 to 5 GPM. The catch is temperature rise: a unit rated 5 GPM is rated at some specific rise, often 35 to 45 °F. If your groundwater arrives at 45 °F in February and you want 120 °F at the tap, that is a 75 °F rise and the same unit may only manage 3 GPM. Always check the manufacturer curve for your winter inlet temperature.

The common recommendation is 120 °F at the tap, which limits scald risk and slows mineral scaling. Legionella, however, grows in tanks held below about 122 °F. The standard resolution is to store at 140 °F and install a thermostatic mixing valve at the heater outlet so 120 °F is what actually reaches the fixtures. That is required in many jurisdictions on new installations, and it also increases the effective capacity of the tank, since you are diluting hotter water.

The metric rule this calculator ports uses 200 liters per person per day, or 52.8 gallons, times the days of reserve you want. The EPA puts actual average US household use closer to 82 gallons per person per day including irrigation, so for a US home with a yard the ported figure is conservative and you should either raise the per-person number or count only indoor use. One day of reserve covers a municipal outage; a well or cistern system usually wants two or three.

Diameter in inches equals the square root of 0.4085 times the flow in GPM divided by the velocity in feet per second. At 12 GPM and 5 ft/s that is about 0.99 inches, so a 1 inch line. Then round up to the next nominal size, which lowers velocity and pressure loss. Remember that nominal size is not bore: 3/4 inch copper type L is about 0.785 inches inside, and 3/4 inch PEX with insert fittings is meaningfully smaller than that at every joint.

Above about 8 feet per second in cold water and 5 in hot, you start getting audible flow noise, water hammer that is harder to arrest, and in copper the beginning of erosion-corrosion, where the moving water strips the protective oxide layer off the inside of the tube. Hot water accelerates that, which is why the hot limit is lower. Designing at 5 ft/s leaves headroom for the flow surges that a real system sees.

Water horsepower is GPM times total head in feet divided by 3,960. Divide by the pump efficiency for the shaft horsepower the motor has to deliver. At 12 GPM and 60 feet of head with 60% efficiency, that is 12 × 60 ÷ 3,960 ÷ 0.6, about 0.30 HP, so a 1/3 HP motor. The 3,960 constant is not arbitrary: it comes straight from the weight of a gallon of water and the definition of horsepower, and it is the exact imperial equivalent of the metric ρgQH formula.

Three things added together: the static lift from the water surface to the discharge point, the friction loss through the pipe, fittings and valves along the way, and any pressure you need at the outlet, converted at 2.31 feet of head per psi. On a long horizontal run, friction can easily exceed the vertical lift. Sizing on lift alone is the most common reason a new pump does not deliver its rated flow.

IPC minimums are: lavatory 1-1/4 inch trap, kitchen sink and bathtub 1-1/2 inch, shower and laundry standpipe 2 inch, floor drain 2 inch with a 3 inch drain line, and a water closet 3 inch. Branch drains serving several fixtures are sized on the total drainage fixture units, not on the biggest fixture. Note that some jurisdictions on the west coast follow the UPC instead, which sets a 2 inch minimum for kitchen sinks in some configurations.

A quarter inch per foot for lines up to 2-1/2 inches, and an eighth of an inch per foot for 3 inch and larger. Too little slope and the solids settle out; too much and the liquid runs away and leaves the solids behind, which fouls the line just as effectively. This is the piece of drainage design people most often get wrong by assuming steeper is always better.

The ones used here: 1 US gallon is 3.78541 liters, 1 liter per minute is 0.2642 GPM, 1 meter of head is 3.281 feet, 1.5 meters per second is 4.92 feet per second, and 50 mm of pipe is 1.97 inches. Note that a US gallon is not an imperial gallon — the imperial one is 4.546 liters, about 20% larger — so a British sizing table read as US gallons will oversize everything by a fifth.