Method, sources and limits
Every number on this site is a physical constant, a published fixture flow rate, or arithmetic on the two. There is no dataset, nothing is fetched, and nothing can go stale.
The calculation
- Add the simultaneous flow. Fixture flow rates in GPM, for the fixtures you expect to run at the same time.
- Find the temperature rise. Target temperature minus incoming water temperature.
- Multiply. BTU per hour = GPM x rise x 500.724. That constant is 8.3454 pounds per gallon times 60 minutes per hour, since a BTU raises one pound of water by one degree.
- Divide by efficiency for a gas unit, to get the input rating it needs.
- Select the smallest unit that covers it, and report what that unit actually delivers at this rise rather than at the rise it is advertised at.
Sources and constants
- 500.724 BTU per hour per GPM per degree F. Derived, not looked up: 8.3454 lb/gal x 60 min/hr. Published charts usually round this to 500, a difference of about a seventh of a percent.
- 3412.14 BTU per kilowatt hour. The definition.
- Fixture flow rates. Conventional published figures, within the federal maximums of 2.5 GPM for showerheads and 2.2 GPM for faucets.
- Incoming water temperatures. Based on the standard relationship between shallow groundwater temperature and mean annual air temperature, leaning towards winter because winter is the sizing condition.
- Unit capacities. The BTU input and kilowatt ratings residential equipment is actually sold in.
What the method cannot see
- Your actual incoming temperature. The per-state figures are guidance. A surface-water supply can run considerably colder in winter than groundwater would.
- Gas line capacity. A tankless unit commonly needs a larger supply line than the tank it replaces, and that is frequently the real cost of the project.
- Minimum activation flow. Most units need a small minimum flow before they fire, so a trickling tap may not produce hot water at all.
- Recirculation. A recirculation loop changes both the usage pattern and the energy picture, and is not modelled here.
- Pressure and pipe sizing. Flow assumes the plumbing can actually deliver it.
Why there is no data to refresh
Some sites in this portfolio carry committed datasets that need periodic refreshing. This one does not, deliberately. Physical constants do not change, fixture maximums change on a timescale of decades and with public notice, and equipment size lists are worth a look every couple of years. That is a maintenance profile of roughly nothing, which is the right shape for a site that should still be correct in five years without anyone touching it.
Frequently asked questions
How accurate is this tankless sizing calculator?
The arithmetic is exact: it is conservation of energy with published constants. The uncertainty is entirely in the inputs, chiefly your real incoming water temperature and how many fixtures genuinely run at once. Measure the first and be honest about the second and the answer is reliable.
Why do you use 500.724 instead of 500?
Because it is the derived figure: 8.3454 pounds per gallon times 60 minutes. The industry rounds to 500 and the difference is about a seventh of a percent, which changes nothing practical. There was simply no reason to round.
Does this account for gas line size?
No, and it is one of the most common surprises in a tankless project. A unit drawing 199,000 BTU often needs a larger gas supply line than the tank it replaces, and upsizing it can be a significant part of the installed cost. Ask about it before committing.