GPM and temperature rise
The arithmetic behind every tankless sizing chart, with your own numbers in it.
One equation governs all of this. Heating water takes 500.724 BTU per hour for every gallon per minute and every degree of rise, because a US gallon of water weighs 8.3454 pounds and a BTU raises one pound by one degree.
BTU per hour = GPM x temperature rise x 500.724
and rearranged, which is the form that matters when you are looking at a unit:
GPM delivered = (BTU input x efficiency) / (temperature rise x 500.724)
Most published charts round the constant to 500. That is accurate to about a seventh of a percent and makes no practical difference; this site uses the derived figure because there is no reason not to.
What each unit delivers, by rise
| Temperature rise | Flow delivered | What that covers |
|---|---|---|
| 30 F | 9.9 GPM | 4 showers at once |
| 40 F | 7.4 GPM | 3 showers at once |
| 50 F | 5.9 GPM | 2 showers at once |
| 60 F | 5.0 GPM | 2 showers at once |
| 70 F | 4.2 GPM | 2 showers at once |
| 80 F | 3.7 GPM | one shower |
| Temperature rise | Flow delivered | What that covers |
|---|---|---|
| 30 F | 11.1 GPM | 5 showers at once |
| 40 F | 8.4 GPM | 4 showers at once |
| 50 F | 6.7 GPM | 3 showers at once |
| 60 F | 5.6 GPM | 2 showers at once |
| 70 F | 4.8 GPM | 2 showers at once |
| 80 F | 4.2 GPM | 2 showers at once |
| Temperature rise | Flow delivered | What that covers |
|---|---|---|
| 30 F | 12.3 GPM | 5 showers at once |
| 40 F | 9.2 GPM | 4 showers at once |
| 50 F | 7.4 GPM | 3 showers at once |
| 60 F | 6.2 GPM | 2 showers at once |
| 70 F | 5.3 GPM | 2 showers at once |
| 80 F | 4.6 GPM | 2 showers at once |
Working it the other way
BTU input needed to deliver a given flow, at 0.93 efficiency:
| Flow | 30 F rise | 40 F rise | 50 F rise | 60 F rise | 70 F rise | 80 F rise |
|---|---|---|---|---|---|---|
| 1.5 GPM | 24,229 | 32,305 | 40,381 | 48,457 | 56,533 | 64,610 |
| 2.1 GPM | 33,920 | 45,227 | 56,533 | 67,840 | 79,147 | 90,453 |
| 3.0 GPM | 48,457 | 64,610 | 80,762 | 96,914 | 113,067 | 129,219 |
| 4.2 GPM | 67,840 | 90,453 | 113,067 | 135,680 | 158,293 | 180,907 |
| 5.0 GPM | 80,762 | 107,683 | 134,603 | 161,524 | 188,445 | 215,365 |
| 6.3 GPM | 101,760 | 135,680 | 169,600 | 203,520 | 237,440 | 271,360 |
Greyed figures are beyond the largest residential gas unit (199,000 BTU) and need two heaters or a smaller simultaneous-use target.