Tankless Water Heater Calculator
Size the right on-demand water heater for your home.
Tankless Water Heater Sizing Guide
Gas units at 120°F target temperature
| Groundwater | Temp Rise | Max GPM @150K BTU | Max GPM @199K BTU | Fixtures |
|---|---|---|---|---|
| 40°F (North) | 80°F | 3.6 GPM | 4.8 GPM | 1 shower + 1 faucet |
| 50°F (Mid) | 70°F | 4.1 GPM | 5.5 GPM | 2 showers |
| 57°F (South) | 63°F | 4.6 GPM | 6.1 GPM | 2 showers + faucet |
| 62°F (Deep S) | 58°F | 5.0 GPM | 6.6 GPM | 2 showers + faucet |
| 70°F (Hawaii) | 50°F | 5.8 GPM | 7.7 GPM | 3 showers |
How We Calculate This
This tankless water heater calculator uses established formulas and industry-standard data to provide accurate estimates.
- Enter your specific values into the calculator fields above
- Our algorithm applies the relevant formulas using your inputs
- Results are calculated instantly in your browser — nothing is sent to a server
- Review the detailed breakdown to understand how each factor affects your result
These calculations are estimates based on standard formulas. For critical decisions, always consult a qualified professional.
How to Convert Oven Recipes to Air Fryer
This calculator sizes tankless water heaters by determining the total GPM demand from simultaneous fixtures and the temperature rise needed to heat groundwater to your desired temperature.
The basic rule:
- Flow rate (GPM) = sum of all simultaneously running fixture flow rates
- Temperature rise = desired hot water temp − incoming groundwater temp
- BTU required = flow rate (GPM) × temperature rise × 8.33 (weight of water) × 60 min ÷ efficiency
- Gas units are rated by BTU output (typically 150,000–199,000 BTU for whole-house)
- Electric units are rated by kW (typically 18–36 kW for whole-house)
Gas tankless heaters require proper venting (direct vent or power vent) and adequate gas line sizing (typically 3/4" minimum). Electric units need dedicated circuits (usually 2-3 circuits at 40-60 amps each). Always check local codes and have a licensed plumber install.
When Would You Use This Calculator?
This tankless water heater calculator is designed for anyone who needs quick, reliable estimates without complex spreadsheets or professional consultations.
- When you need a quick estimate before committing to a purchase or project
- When comparing different options or scenarios side by side
- When planning a budget and need to understand potential costs
- When you want to verify a quote or estimate you've received from a professional
- When teaching or learning about the concepts behind these calculations
Frequently Asked Questions
What size tankless water heater do I need for a family of 4?
A family of 4 typically needs 7-10 GPM with a gas unit rated at 150,000-199,000 BTU. This handles 2 showers and a faucet simultaneously. In cold climates (40°F groundwater), you may need the higher end. Electric units need 24-36 kW for equivalent performance.
Gas vs electric tankless: which is better?
Gas tankless heaters deliver higher flow rates (8-11 GPM) and handle more simultaneous fixtures. Electric units are simpler to install, have no venting requirements, and are more efficient (99% vs 80-97%), but deliver lower flow rates (3-8 GPM) and require significant electrical upgrades.
What is temperature rise and why does it matter?
Temperature rise is the difference between incoming cold water and your desired hot water temp. In Minnesota (40°F groundwater, 120°F desired), you need an 80°F rise. In Florida (70°F groundwater), only a 50°F rise. Higher rise means lower flow rate from the same unit.
Can a tankless water heater replace a 50-gallon tank?
Yes, but sizing matters. A 50-gallon tank delivers about 60-80 gallons in the first hour. A properly sized tankless unit provides unlimited hot water but at a fixed flow rate. A 199,000 BTU gas unit can match or exceed a 50-gallon tank for most households.
How much does a tankless water heater save?
Tankless heaters save 8-34% on energy costs depending on hot water usage. Homes using less than 41 gallons/day save 24-34%. Homes using 86+ gallons/day save 8-14%. Annual savings are typically $75-$150. The units last 20+ years vs 10-15 for tank heaters.
Do I need to upgrade my gas line for tankless?
Usually yes. Tank heaters use 40,000-50,000 BTU on a 1/2" gas line. Tankless units need 150,000-199,000 BTU, requiring a 3/4" line minimum (often 1" for long runs). A plumber must verify gas line capacity, meter size, and regulator before installation.