The sink is the event
Household hot water sizing is a story about showers. Commercial kitchen sizing is a story about one fixture: the three-compartment sink, which takes something like eighty gallons of mixed water in a few minutes and then does it again two hours later. Nothing else in a small kitchen is in the same class. A hand sink is a cup. The dish machine, running forty racks in an hour, is a large but steady draw. The sink is a cliff.
Run the defaults. Three compartments of twenty-four by twenty-four by fourteen inches, filled to seventy percent, is 24.5 gallons each and 73 gallons a change. Filled to 110 degrees from a 55 degree inlet with 140 degree hot, the hot fraction is 55 over 85, about 65 percent, so each change draws about 48 gallons of hot water. Two changes in the peak hour is 95 gallons before the dish machine has washed anything.
Storage and recovery are two ways to buy the same thing
Any peak hour can be met by a large tank with a small burner or by a small tank with a large burner, and the trade is real. A hundred gallon tank at seventy percent usable delivers seventy gallons before it starts giving you lukewarm water; whatever the hour needs beyond that has to be made during the hour, and that is the recovery figure. The burner input required is that recovery in gallons times 8.34 pounds times the temperature rise, divided by efficiency.
This page runs it both ways, because the question arrives in both forms. Sometimes you know the tank and want the burner. More often you have a heater already, or a quote for one, and want to know how much storage has to sit next to it. The second calculation is the one that settles arguments with a supplier.
Size on the February inlet
The single most common sizing error is using an annual average or a summer inlet temperature. Cold inlet water does two things at once, and they compound. The rise gets bigger, so the same burner makes fewer gallons an hour. And the mixing fraction at the sink shifts, so filling to the same temperature takes a larger share of hot water. The sensitivity table on this page shows both moving together, which is why a system that is comfortable in August can leave the closing shift waiting for the sink in January.
What is not in these numbers
The booster heater on a high-temperature dish machine is separate equipment with separate energy use, and the gallons per rack here are the machine draw at building supply temperature. Rinse temperature requirements, wash water temperatures for manual warewashing, and the sanitiser concentration if you use chemical rather than heat, all come from your health department and the equipment listing, and they are verified with a thermometer or test strips rather than calculated.
The household version of this arithmetic is the water heater sizing calculator, which takes a heater and tests it against a peak hour; this page goes the other way and produces the heater. The water heater cost calculator handles tank against tankless over a year. The same sink dimensions drive the grease interceptor sizing calculator on the drain side, and the rack count comes from the dish machine throughput calculator.
Questions people ask
What size water heater does a small restaurant need?
Build the peak hour first, because that is the only figure a heater is really sized against. Count how many times the three-compartment sink gets dumped and refilled in the busiest hour, add the dish machine draw at gallons per rack, and add prep, hand sinks and mop. Then decide the split between storage and recovery — a big tank with a modest burner and a small tank with a large burner both work, and which is cheaper depends on the equipment available and on whether you have the floor space.
How many gallons does a three-compartment sink use?
Measure the compartments rather than guessing. Length times width times depth in inches, times how full you actually run them, divided by 231 gives gallons per compartment. A twenty-four by twenty-four by fourteen inch compartment at seventy percent is about 24.5 gallons, so three compartments is around 73 gallons a change. Only part of that is hot water — the rest is cold from the mixing tap, and the share depends on your fill temperature and inlet temperature.
Why does the water run out in winter but not in summer?
Two effects that compound. Colder inlet water means a larger temperature rise, and a burner delivering a fixed BTU per hour makes fewer gallons an hour across a larger rise. At the same time the sink needs a bigger share of hot water to reach the same fill temperature, so demand goes up while supply goes down. Sizing on a summer inlet temperature builds that failure in. Use the coldest inlet the building actually sees.
Does the dish machine booster count in this calculation?
No, and that is deliberate. The gallons per rack here are what the machine draws from your building hot water supply. A booster heater takes some of that water and lifts it further for the sanitising rinse, using its own energy on its own circuit or gas line. It has to be sized separately, usually by the machine manufacturer against your supply temperature. Leaving it out of the building heater calculation is correct, but do not leave it out of the electrical or gas load.
Should I use a tankless heater in a commercial kitchen?
The physics that makes tankless awkward here is that a sink fill is a short violent draw and a tankless unit is rated in gallons per minute at a given rise. Filling three compartments quickly asks for a flow rate that a single unit at a winter rise may not reach, which is why commercial tankless installations are usually banks of units with a small buffer tank. Work out the peak hour on this page, then take both the hourly figure and the fill rate you need to a plumbing contractor, because the fill rate is the constraint tankless struggles with.