Water Heater Sizing Calculator
Tank-style sizing by first-hour rating (occupants); tankless sizing by GPM × temperature rise. Includes unit cost, install, and hidden tankless-retrofit costs.
Bedroom count for code reference
Tank swap: 2-4 hr. Tankless: 6-10 hr.
Gas line upsize / venting / 240V circuit / softener
Result
- Required FHR
- 75 gal first-hour rating
- Recommended tank
- 80 gal
- Unit cost
- $680
- Labor
- 4.0 hr · $380
- Hookup extras
- —
- Permit
- $80
- Total installed
- $1,140
This estimate is based on national average costs and may vary by region, project specifics, and market conditions. Use as a starting point for your bids.
Sizing rules: tank vs tankless
Tank-style sizes by first-hour rating (FHR) — gallons of hot water the unit can deliver in the first hour. DOE table:
| Occupants | FHR needed | Typical tank |
|---|---|---|
| 1 | 35 gal | 30 gal |
| 2 | 50 gal | 40 gal |
| 3 | 65 gal | 40-50 gal |
| 4 | 75 gal | 50 gal |
| 5 | 90 gal | 65-75 gal |
| 6+ | 100+ gal | 75-80 gal |
Tankless sizes by GPM at temperature rise. Take the max simultaneous flow (2 showers + a sink = 6.5 gpm) and find the unit’s rated capacity at your local groundwater-to-setpoint rise. Cold climates with 35-50°F groundwater see 30-50% capacity loss vs the manufacturer’s best-case rating.
Install cost benchmarks
- Tank gas (40-50 gal): $400-1,200 unit + $300-700 install = $700-1,900 total
- Tank electric (40-50 gal): $350-900 unit + $250-600 install = $600-1,500 total
- Tankless gas (whole-home): $1,000-2,500 unit + $1,200-2,500 install = $2,200-5,000 total
- Tankless electric (point-of-use or small): $400-1,200 unit + $400-1,500 install
- Heat-pump (hybrid): $1,500-3,500 unit + $400-800 install, often $500-2,000 in rebates
Hidden tankless retrofit costs
- Gas line upsize (1/2 → 3/4 in): $300-800
- Cat-3 stainless venting: $200-500
- 240V dedicated circuit (electric): $400-800
- Condensate drain (high-eff): $100-300
- Whole-home water softener (units demand soft water): $800-1,800
Frequently asked questions
How big a water heater do I need?
Tank sizing follows first-hour rating (FHR) by occupant count: 1 person → 35 gal FHR, 2 → 50, 3 → 65, 4 → 75, 5 → 90, 6 → 100. Then pick the standard tank that delivers that FHR with headroom (typical 30-40-50-65-75-80 gal sizes). A 4-person home needs ~75 gal FHR → 50 gal tank typically delivers that.
Tank vs tankless — what's the install difference?
Tank: $400-1,500 unit + $300-700 install. Replaces straight across in 2-4 hours. Tankless: $800-2,500 unit + $1,200-2,500 install. Requires bigger gas line (typically 3/4-in vs 1/2-in), dedicated circuit if electric (240V), category-3 stainless vent for gas. Tankless saves ~20% on annual energy bills but the payback period is 8-15 years.
How many GPM tankless do I need?
Add up simultaneous-use fixtures. 2 showers × 2.5 gpm = 5 gpm. Add a sink (1.5 gpm) = 6.5 gpm. Then check the tankless unit's rated gpm AT YOUR TEMPERATURE RISE — units lose 30-50% of rated flow at high temperature rises (60°F+ rise in northern climates). A 'whole-home' unit needs to be rated 8-11 gpm to handle most peak demand.
Heat-pump water heater — when does it make sense?
Best ROI in mild climates (zones 2-4) where the basement / utility room stays >55°F year-round. Heat pump pulls heat from ambient air — needs ~700 cu ft of unconfined air. 2-3× efficiency of resistance electric, often qualifies for $500-2,000 federal/state/utility rebates. Total install with rebate: often net $1,500-2,500. Skip in cold climates if installed in conditioned space.
What size gas line for a tankless water heater?
Most residential tankless need 3/4-in line at 1/2 PSI gas pressure. Verify with the unit's spec sheet — high-flow units (180k BTU+) may need 1-in gas line. Upgrading from 1/2-in to 3/4-in line is the most common hidden cost on tankless retrofits — budget $300-800 for line replacement if the existing tank ran on 1/2-in.
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