Sizing Guide
Tankless Water Heater Cost by Home Size
The right size tankless heater depends on your home's peak hot water demand. Undersizing leads to lukewarm water; oversizing wastes money. Here's what each home size needs.
Small (1–2 bathrooms)
Under 1,500 sq ft
Medium (2–3 bathrooms)
1,500–2,500 sq ft
Large (3–4 bathrooms)
2,500–4,000 sq ft
Extra Large (5+ bathrooms)
4,000+ sq ft
Quick Reference Table
| Home Size | Sq Ft | GPM Needed | Total Cost |
|---|---|---|---|
| Small (1–2 bathrooms) | Under 1,500 sq ft | 3–5 GPM | $1,000–$2,400 |
| Medium (2–3 bathrooms) | 1,500–2,500 sq ft | 6–8 GPM | $1,800–$3,800 |
| Large (3–4 bathrooms) | 2,500–4,000 sq ft | 8–10 GPM | $2,700–$5,000 |
| Extra Large (5+ bathrooms) | 4,000+ sq ft | 10+ GPM | $3,500–$7,500+ |
How to Calculate Your GPM Needs
GPM (gallons per minute) measures flow rate. To size your tankless heater, add up the flow rates of fixtures you'll use simultaneously:
- Shower: 2.0–2.5 GPM
- Bathroom faucet: 1.0–1.5 GPM
- Kitchen faucet: 1.5–2.0 GPM
- Dishwasher: 1.0–1.5 GPM
- Washing machine: 2.0–2.5 GPM
For example, running a shower (2.5 GPM) and kitchen faucet (1.5 GPM) simultaneously requires 4.0 GPM minimum. Add a 1 GPM buffer for safety, so you'd need a 5 GPM unit.
Temperature Rise Matters
Flow rate is heavily dependent on temperature rise — the difference between incoming groundwater temperature and your desired output (typically 120°F). In Florida (incoming 75°F), you need a 45°F rise. In Minnesota (incoming 37°F), you need an 83°F rise, requiring nearly twice the heating power for the same flow rate.
Always check the manufacturer's flow rate chart at your local temperature rise, not just the maximum GPM rating (which is usually at a low temperature rise).
When Multiple Units Make Sense
For extra-large homes (5+ bathrooms), a single tankless unit may not provide sufficient flow rate. Two options:
- Parallel installation: Two whole-house units plumbed in parallel, doubling available GPM. Cost: $5,000–$9,000 installed.
- Zone approach: Separate units for different zones (e.g., master suite, guest wing). Reduces pipe runs and wait time for hot water.