"Is a 40 gallon enough for two bathrooms" cannot be answered as asked, because two bathrooms is not a demand. A guest bathroom used twice a week and an en-suite used every morning are the same bathroom count and completely different loads.
The question that can be answered is: what is the largest hour of hot water use in this house, and what can this heater deliver in one hour?
Peak hour, not bathroom count
Add up what genuinely happens in the busiest single hour.
| Use | Hot water |
|---|---|
| Shower, 10 minutes at 2.0 GPM | 14–16 gal |
| Bath | 20 gal |
| Shaving or washing at a basin | 2 gal |
| Dishwasher cycle | 6 gal |
| Clothes washer, warm | 10 gal |
Two ten-minute showers back to back is about 30 gallons. Add someone shaving and the dishwasher started on the way out of the door and it is nearer 38.
First-hour rating is the number to compare against
Every tank heater carries a first-hour rating on its yellow EnergyGuide label. It combines the usable hot water already stored with what the burner or element can add during that hour. It is the honest single figure for this question, and it is the one the gallon number keeps getting mistaken for.
Approximate first-hour ratings, at a typical 70°F rise:
| Heater | Storage | First-hour rating |
|---|---|---|
| 40 gal gas, 40,000 BTU | 40 gal | 70–75 gal |
| 50 gal gas, 40,000 BTU | 50 gal | 80–90 gal |
| 40 gal electric, 4,500 W | 40 gal | 50–55 gal |
| 50 gal electric, 4,500 W | 50 gal | 60–65 gal |
| 50 gal heat pump | 50 gal | 65–70 gal (hybrid mode) |
Read that table twice, because it contains the answer most people get wrong: a 40-gallon gas heater has a higher first-hour rating than a 50-gallon electric one. Gas puts heat in roughly twice as fast, so it refills while you are still drawing.
So: is 40 gallons enough?
On gas, for two bathrooms, almost always yes. A 70–75 gallon first-hour rating against a 38 gallon peak hour is comfortable, with room for a bath.
On electric, for two bathrooms in regular use, it is tight. A 50–55 gallon rating covers 38 gallons on paper, but the margin disappears in winter, when the incoming water is colder — you draw more per shower and the element refills more slowly, and both move the wrong way at once.
On electric with three or more people showering in the same hour, no. Go to 50 gallons, or accept spacing the showers out.
The lever nobody uses
A thermostatic mixing valve at the tank lets you store water at 140°F and deliver it to the house at 120°F. Because each stored gallon then carries more usable heat, a 40-gallon tank behaves closer to a 55-gallon one for the same money — and the mixing valve keeps the tap temperature safe.
This is frequently a better purchase than the next tank size up. It costs less, it fits the space you already have, and it works on gas or electric.
Raising the thermostat without a mixing valve gives you the extra capacity and a scald hazard with it, so do the two together or neither.
Where physical fit ends the discussion
Before deciding on 50 gallons, measure. A 50-gallon tall tank runs 58–64 inches high and 20–24 inches across; a 40-gallon tall runs 55–62 by 18–22. Post-2015 tanks are fatter than the ones they replace because the efficiency standards added insulation, and closets built around 1990s heaters routinely will not take the modern equivalent.
And measure the ceiling, not just the alcove: the anode rod is about 44 inches long and comes straight up out of the top. On a tall tank in a basement with seven-foot joists there is no room to pull it — and the anode is the single component that decides whether the tank lasts eight years or eighteen. Where headroom is short, fit a segmented rod at install time.
The short version
- Gas, two bathrooms: 40 gallons is normally plenty.
- Electric, two bathrooms, two people: 40 works, tightly, and struggles in winter.
- Electric, two bathrooms, three or more people: 50 gallons.
- Any case where the space is fixed: a mixing valve buys most of a size upgrade without a bigger tank.