What size tankless water heater do I need for a family of 4?

For family of 4 with 2 bathrooms: Peak flow typically 5-7 GPM. At 70°F temperature rise: 175,000-245,000 BTU needed. Use our Tankless Water Heater Flow Rate Calculator for exact sizing.

How do I calculate flow rate for tankless sizing?

Add flow rates of all fixtures that may run simultaneously. Standard flow rates: Shower 2.5 GPM, Sink 1.5 GPM, Dishwasher 1.0 GPM, Washing Machine 2.0 GPM. Peak is not all fixtures at once.

What is temperature rise and why does it matter?

Temperature rise is the difference between inlet water temp and desired output temp. Higher rise requires more BTU/kW. Cold regions (45°F inlet) need larger units than warm regions (70°F inlet).

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tankless water heater sizing

How to Size a Tankless Water Heater: Complete Guide with Calculator

Learn how to properly size a tankless water heater. Step-by-step guide with tankless water heater calculator, flow rate calculations, and BTU requirements.

PlumberCalc Team

Updated 1/24/2026

Step-by-Step Guide

1. Calculate Peak Flow Rate

Determine maximum simultaneous hot water demand. Add flow rates of all fixtures that may run together: Shower (2.5 GPM) + Kitchen Sink (1.5 GPM) + Dishwasher (1.0 GPM) = 5.0 GPM peak demand.

2. Determine Temperature Rise

Calculate required temperature rise: Desired temp (120°F) - Inlet temp (varies by region: 45-70°F). For 120°F desired and 50°F inlet: 70°F temperature rise needed.

3. Calculate BTU/kW Requirement

BTU = Flow Rate (GPM) × Temperature Rise (°F) × 500. For 5 GPM at 70°F rise: 5 × 70 × 500 = 175,000 BTU. For electric: kW = BTU ÷ 3,412 = 51.3 kW.

4. Select Unit Size

Choose tankless unit that meets or exceeds calculated BTU/kW. Common sizes: 140,000-180,000 BTU for gas, 18-27 kW for electric. Always size up slightly for safety margin.

5. Verify Gas Line Capacity

Ensure gas line can supply required BTU. Most tankless units need 3/4" or 1" gas line. Use our Gas Line Sizing Calculator to verify adequate gas supply.