Tankless Water Heater GPM Calculator
Determine your peak hot water demand in gallons per minute (GPM) to properly size your tankless water heater.
Fixture Flow Rates
Understanding GPM Requirements for Tankless Water Heaters
Gallons Per Minute (GPM) measures your peak hot water demand and is the most critical factor in sizing a tankless water heater. Unlike tank heaters that store hot water, tankless systems must instantly heat water as it flows through the unit. Proper GPM calculation ensures your system can handle maximum demand without temperature fluctuations.
Why GPM Matters
Tankless water heaters are rated by:
- Flow Capacity: Maximum GPM the unit can heat
- Temperature Rise: How much it can heat the water at that flow rate
A unit rated for 7 GPM at 50°F rise might only deliver 3.5 GPM at 70°F rise. Your system must meet both your flow rate and temperature rise needs.
Standard Fixture Flow Rates
| Fixture | Standard GPM | Low-Flow GPM | Simultaneous Use Probability |
|---|---|---|---|
| Shower | 2.5 | 1.5-1.75 | High (especially mornings/evenings) |
| Bathroom Sink | 1.0 | 0.5-0.75 | Medium (during showers, teeth brushing) |
| Kitchen Faucet | 1.5 | 1.0 | Medium (meal times, cleanup) |
| Washing Machine | 2.5 | 1.5 | Low (usually runs independently) |
| Dishwasher | 1.5 | 1.0 | Low (often runs at night) |
Calculating Your Peak Demand
There are two approaches to determining your GPM needs:
- Fixture Count Method: Sum the flow rates of all fixtures that might run simultaneously
- Probability Method: Apply a usage factor (like 70%) to total possible flow
Our calculator uses a hybrid approach, allowing you to specify fixture counts and apply a usage pattern factor.
Typical Household GPM Requirements
Small Homes/Apartments
- 1 bathroom
- 1-2 people
- 3-4 GPM peak demand
Medium Homes
- 2 bathrooms
- 3-4 people
- 5-7 GPM peak demand
Large Homes
- 3+ bathrooms
- 5+ people
- 8-10 GPM peak demand
Temperature Rise Considerations
Your GPM capacity decreases as temperature rise increases:
| Temperature Rise | Effect on Capacity | Climate Examples |
|---|---|---|
| 50°F | 100% rated GPM | Southern states (groundwater 60-70°F) |
| 60°F | ~85% rated GPM | Central states (groundwater 50-60°F) |
| 70°F | ~70% rated GPM | Northern states (groundwater 40-50°F) |
Optimizing Your GPM Requirements
Reduce your GPM needs through:
- Low-flow fixtures: Modern showerheads can cut shower GPM by 30-40%
- Staggered usage: Avoid running dishwasher during shower times
- Point-of-use heaters: Supplement with small electric units for remote fixtures
- Recirculation systems: Reduce water waste without increasing flow rate
Frequently Asked Questions
What's the average GPM for a family of four?
Typically 5-7 GPM, depending on fixture types and usage patterns.
Can I reduce my GPM needs after installation?
Yes, by installing low-flow fixtures or behavior changes. Tankless systems are flexible to demand changes.
How does GPM relate to BTU for gas heaters?
Higher GPM requires higher BTU. Roughly 10,000 BTU per GPM at 50°F rise (e.g., 7 GPM needs ~170,000 BTU).
Why does my tankless heater struggle when two showers run?
Likely undersized for your actual GPM demand at your climate's temperature rise.
Should I size for worst-case or typical usage?
Size for worst-case if you want uninterrupted supply, or accept occasional limitations to save on equipment costs.
Conclusion
Accurate GPM calculation is essential for tankless water heater satisfaction. Use our calculator above to determine your peak demand based on your specific fixtures and usage patterns. Remember that your final system size should account for both GPM requirements and the temperature rise needed for your climate. When in doubt, consult with a professional plumber to assess your home's unique hot water needs.