Water Heater Sizing and First-Hour Rating

How to Use This Tool

Build your peak hour from the fixtures that actually run together, then compare it with the first-hour rating of the heater you are considering. Tank size on its own tells you almost nothing.

Only about 70 per cent of a tank is usable

As hot water leaves the top of the tank, cold water enters the bottom to replace it. The two mix, and long before the tank is empty the water coming out is too cool to use. The usable fraction is around 70 per cent, so a 50 gallon tank delivers about 35 gallons of genuinely hot water.

FHR = tank x 0.70 + recovery per hour

40 gal, gas      40 x 0.70 + 60 = 28 + 60 = 88
50 gal, electric 50 x 0.70 + 40 = 35 + 40 = 75
80 gal, slow     80 x 0.70 + 25 = 56 + 25 = 81

peak hour: 2 showers, hand wash, dishwasher
   2 x 8 min x 2.0 gpm = 32
   hand washing        =  2
   dishwasher          =  6
                       = 40 gal

The 40 gallon unit has the highest rating of the three

Recovery rate is why fuel matters more than size

A gas burner reheats far faster than a resistive element, so gas units are routinely specified two sizes smaller than electric ones for the same household. If you are switching fuel, carrying the old tank size across is a mistake in one direction or the other.

Peak hour, not daily total

A household might use 200 gallons a day and never run out, because it is spread across sixteen hours. The same household running two showers and a dishwasher between seven and eight in the morning can empty a heater that is comfortably sized on daily consumption. Size the busiest hour.

A long shower is the dominant term and the easiest to change: eight minutes at 2.0 gpm is 16 gallons, but the same shower at 2.5 gpm is 20, and twelve minutes at 2.5 is 30 — nearly a bath.

Aim for headroom, not an exact match

The usual advice is a first-hour rating at or slightly above your peak hour. Matching it exactly leaves nothing for a guest, a cold incoming supply in winter, or a heater that has lost some capacity to scale. Ten to twenty per cent over is sensible; far more than that means paying to keep water hot that nobody uses.

What this does not decide

Tankless units have no storage at all and are rated by flow at a given temperature rise, which is a different calculation and depends heavily on incoming water temperature. Heat pump water heaters have a recovery rate that varies with ambient air temperature. Both need their own sizing.

Frequently Asked Questions

What size water heater do I need?

Match the first-hour rating to your peak hour, not the tank to your household size. Two morning showers, hand washing and a dishwasher is about a 40 gallon peak hour, which a 40 gallon gas unit covers comfortably.

What is first-hour rating?

How much hot water a heater can deliver in one hour starting full: roughly 70 per cent of the tank plus the recovery rate. A 40 gallon tank recovering 60 gallons an hour has a first-hour rating of 88.

Is a bigger tank always better?

No. A 40 gallon gas unit with 60 gal/h recovery rates 88, while a 50 gallon electric with 40 gal/h rates 75 and an 80 gallon with slow recovery rates 81. Recovery does most of the work in the hour that matters.

Why can I only use 70 per cent of the tank?

Cold water enters the bottom as hot water leaves the top, and the two mix. Well before the tank empties, the outlet temperature has fallen below usable, so the deliverable share is around 70 per cent.

Should I size on daily use or peak hour?

Peak hour. A household using 200 gallons a day spread over sixteen hours never runs out, while the same household concentrating 40 gallons into one morning hour can empty a heater sized on the daily figure.

How much headroom should the rating have?

Ten to twenty per cent above your peak hour. An exact match leaves nothing for a guest, colder incoming water in winter, or capacity lost to scale, while a much larger unit costs money to keep water hot that nobody uses.

A 40 Gallon Heater Can Beat an 80 Gallon One

How the calculation works