Water Heater Size Calculator
Size water heaters for your household needs. Calculate tank size, first hour rating, and compare tank vs tankless options.
Household Information
Tank Size
100 gallons
For 6.5 GPM with 65°F rise, you'll need a tankless unit rated for approximately 212 kBTU/hr (gas) or 62 kW (electric).
Tank vs. Tankless Comparison
| Feature | Tank | Tankless |
|---|---|---|
| Upfront Cost | $800-$1,500 | $1,500-$4,000 |
| Installation Cost | $300-$500 | $500-$1,500 |
| Annual Operating Cost | ~$227 | ~$159 |
| Lifespan | 10-15 years | 20+ years |
| Space Required | Large | Minimal |
| Hot Water Supply | Limited | Unlimited |
Hot Water Usage by Activity
- Look for the First Hour Rating (FHR) on the EnergyGuide label, not just tank size
- Consider peak usage times - morning showers, evening dishwashing
- ENERGY STAR certified models can save 10-20% on operating costs
- Hybrid/heat pump water heaters are most efficient but cost more upfront
- In cold climates, ground water is colder, requiring more heating capacity
About This Calculator
The Water Heater Size Calculator determines the right capacity water heater for your household based on family size, bathroom count, peak usage patterns, and fuel type preferences. Proper sizing ensures adequate hot water during morning rushes and high-demand periods while avoiding oversized equipment that wastes energy and money through unnecessary standby losses.
Water heater sizing centers on two key metrics: First Hour Rating (FHR) for tank heaters and gallons-per-minute (GPM) flow rate for tankless units. FHR measures how many gallons of hot water a tank can deliver during the first hour of use—this must meet or exceed your peak demand. For tankless heaters, GPM determines how many fixtures can run simultaneously at your required temperature rise.
In 2026, water heating accounts for 18-20% of home energy costs—the second-largest energy expense after heating and cooling. A standard 50-gallon gas tank costs $1,200-2,500 installed, while tankless gas units run $3,000-5,500. Heat pump water heaters (the most efficient electric option) cost $2,500-4,500 but save $300-500 annually on electricity. Federal tax credits of up to $2,000 are available for qualifying heat pump and solar water heaters. Enter your household details to calculate the optimal size and compare equipment options.
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How to Use the Water Heater Size Calculator
- 1Enter the number of people in your household (count regular occupants, not guests).
- 2Select the number of bathrooms with showers or bathtubs.
- 3Choose your preferred fuel type: natural gas, electric, propane, or heat pump.
- 4Toggle Advanced mode for custom settings: simultaneous showers, dishwasher timing, and laundry habits.
- 5Review the recommended First Hour Rating for tank water heaters.
- 6Check the recommended GPM for tankless options at your climate's groundwater temperature.
- 7Compare estimated operating costs and equipment prices for each fuel type.
- 8Consider heat pump options for maximum efficiency if you have electric-only service.
Formula
Peak Hour Demand = (Number of Showers × 18 gal) + (Dishwasher × 6 gal) + (Other uses × factor)Peak hour demand represents gallons of hot water needed during your busiest hour, typically morning shower time. The First Hour Rating (FHR) of a tank water heater must meet or exceed this demand. For tankless heaters, convert peak demand to simultaneous GPM by adding the flow rates of fixtures used at the same time.
2026 Water Heater Costs
Water heater prices vary by type, size, efficiency, and fuel source:
Tank Water Heater Prices (Installed):
| Size | Gas (Standard) | Gas (High-Eff) | Electric | Heat Pump |
|---|---|---|---|---|
| 30 gallon | $900-1,400 | $1,400-2,000 | $800-1,200 | N/A |
| 40 gallon | $1,000-1,600 | $1,600-2,200 | $900-1,400 | $2,000-3,000 |
| 50 gallon | $1,200-2,000 | $1,800-2,800 | $1,000-1,600 | $2,500-4,000 |
| 65 gallon | $1,500-2,400 | $2,200-3,200 | $1,300-2,000 | $3,000-4,500 |
| 80 gallon | $1,800-2,800 | $2,600-3,800 | $1,600-2,400 | $3,500-5,000 |
Tankless Water Heater Prices (Installed):
| Type | Unit Cost | Installed Cost | Notes |
|---|---|---|---|
| Electric point-of-use | $150-300 | $300-600 | Single fixture |
| Electric whole-house | $500-1,200 | $1,500-3,000 | Requires 200A service |
| Gas (mid-efficiency) | $800-1,500 | $2,500-4,000 | 0.80-0.85 UEF |
| Gas (high-efficiency) | $1,200-2,000 | $3,500-5,500 | 0.90+ UEF, condensing |
| Propane tankless | $1,000-1,800 | $3,000-5,000 | Similar to gas |
Annual Operating Costs (50-gallon equivalent):
| Fuel Type | Efficiency | Annual Cost | Notes |
|---|---|---|---|
| Natural gas (tank) | 0.60-0.70 UEF | $250-350 | Most common |
| Natural gas (tankless) | 0.80-0.95 UEF | $180-280 | 20-30% savings |
| Electric (tank) | 0.90-0.95 UEF | $450-550 | High electricity cost |
| Heat pump | 2.5-4.0 UEF | $150-250 | Most efficient electric |
| Propane (tank) | 0.60-0.70 UEF | $400-600 | Expensive fuel |
2026 Federal Tax Credits:
- Heat pump water heaters: 30% of cost, up to $2,000
- Solar water heaters: 30% of cost, no cap
- Applies to equipment and installation costs
First Hour Rating (FHR) Explained
First Hour Rating is the most important specification for tank water heaters—more meaningful than tank size alone:
What FHR Measures: FHR indicates how many gallons of hot water the heater can deliver in the first hour of heavy use. It depends on both tank capacity and recovery rate (how fast the unit reheats).
FHR vs. Tank Size:
| Tank Size | Gas FHR (Typical) | Electric FHR (Typical) |
|---|---|---|
| 30 gallon | 50-60 gallons | 40-50 gallons |
| 40 gallon | 60-75 gallons | 50-60 gallons |
| 50 gallon | 70-90 gallons | 58-70 gallons |
| 65 gallon | 85-110 gallons | 70-85 gallons |
| 80 gallon | 100-130 gallons | 80-100 gallons |
Why FHR Matters More Than Tank Size: A 40-gallon tank with excellent recovery can deliver 75 gallons in the first hour, while a 50-gallon tank with poor recovery might only deliver 60 gallons. Always compare FHR.
Peak Demand Calculation:
| Household Size | Bathrooms | Typical Peak Demand |
|---|---|---|
| 1-2 people | 1 | 30-40 gallons |
| 2-3 people | 1-2 | 40-55 gallons |
| 3-4 people | 2 | 55-70 gallons |
| 4-5 people | 2-3 | 70-85 gallons |
| 5-6 people | 3+ | 85-100 gallons |
FHR Sizing Rule: Peak Hour Demand ≤ First Hour Rating
Always choose a water heater with FHR at least equal to your calculated peak demand. Allow 10-15% buffer for guests or increased usage.
Hot Water Usage by Activity
Understanding usage per activity helps calculate your actual peak demand:
Average Hot Water Usage:
| Activity | Gallons | Duration/Notes |
|---|---|---|
| Shower (standard) | 18-25 gal | 8-10 minutes at 2.5 GPM |
| Shower (low-flow) | 12-16 gal | 8-10 minutes at 1.5 GPM |
| Bath (full tub) | 30-50 gal | Varies by tub size |
| Dishwasher cycle | 4-6 gal | Modern efficient units |
| Clothes washer (hot) | 25-40 gal | Full load, hot water |
| Clothes washer (warm) | 12-20 gal | Full load, warm water |
| Hand washing | 1-2 gal | Per minute of running |
| Shaving | 2-3 gal | Typical session |
| Food preparation | 3-6 gal | Typical meal prep |
Peak Hour Examples:
Family of 4, Morning Rush:
| Activity | Gallons |
|---|---|
| 3 showers (back-to-back) | 54 |
| 1 dishwasher start | 5 |
| Hand/face washing | 4 |
| Total FHR Needed | 63 gallons |
Family of 5, Heavy Usage:
| Activity | Gallons |
|---|---|
| 4 showers | 72 |
| 1 bath | 35 |
| Laundry start | 20 |
| Total FHR Needed | 127 gallons |
Empty Nesters, Light Usage:
| Activity | Gallons |
|---|---|
| 2 showers | 36 |
| Hand washing | 3 |
| Kitchen use | 4 |
| Total FHR Needed | 43 gallons |
Tankless Sizing (GPM Flow Rate)
Tankless water heaters are sized by gallons per minute (GPM) at a specific temperature rise:
Fixture Flow Rates:
| Fixture | Flow Rate (GPM) |
|---|---|
| Standard showerhead | 2.0-2.5 GPM |
| Low-flow showerhead | 1.5-2.0 GPM |
| Bathroom faucet | 0.5-1.5 GPM |
| Kitchen faucet | 1.5-2.2 GPM |
| Dishwasher | 1.0-2.5 GPM |
| Clothes washer | 1.5-3.0 GPM |
| Bathtub faucet | 3.0-4.0 GPM |
Temperature Rise Requirements:
| Climate Zone | Groundwater Temp | Rise Needed (to 120°F) |
|---|---|---|
| Southern US | 65-75°F | 45-55°F rise |
| Mid-Atlantic | 50-60°F | 60-70°F rise |
| Northern US | 40-50°F | 70-80°F rise |
| Alaska/Northern | 35-45°F | 75-85°F rise |
Tankless Sizing Chart:
| Simultaneous Use | GPM Needed | Gas Unit Size | Electric Unit Size |
|---|---|---|---|
| 1 shower | 2.0-2.5 | 4-5 GPM unit | 14-18 kW |
| 2 showers | 4.0-5.0 | 7-8 GPM unit | 24-27 kW |
| Shower + dishes | 3.0-4.5 | 5-7 GPM unit | 18-24 kW |
| 2 showers + dishes | 5.0-6.5 | 8-10 GPM unit | 27-36 kW |
| 3 simultaneous uses | 6.0-8.0 | 10+ GPM unit | May need 2 units |
Temperature Rise Impact: A tankless heater rated 8 GPM might only deliver:
- 8 GPM at 45°F rise (warm climate)
- 6 GPM at 60°F rise (moderate climate)
- 4-5 GPM at 77°F rise (cold climate)
Always check the manufacturer's flow rate chart for your specific temperature rise.
Tank vs. Tankless Comparison
Both technologies have advantages depending on your situation:
Tank Water Heaters:
| Pros | Cons |
|---|---|
| Lower upfront cost ($1,000-2,500) | Standby heat loss (15-25% energy waste) |
| Simple, inexpensive installation | Can run out of hot water |
| Works with any flow rate | Takes up floor space |
| No flow rate limitations | 8-12 year typical lifespan |
| Lower maintenance requirements | Larger units = more standby loss |
Tankless Water Heaters:
| Pros | Cons |
|---|---|
| Endless hot water | Higher upfront cost ($2,500-5,500) |
| 20-30% energy savings | GPM limitations in cold climates |
| Compact wall-mounted design | May need gas line upgrade |
| 20+ year lifespan | Annual descaling required |
| No standby losses | "Cold water sandwich" during use changes |
Best Choice by Situation:
| Situation | Recommended Type |
|---|---|
| Replacement on budget | Tank (same fuel type) |
| High hot water usage | Tank (larger capacity) |
| Small household (1-2 people) | Tankless (good payback) |
| Endless hot water priority | Tankless |
| Electric only, high rates | Heat pump tank |
| Vacation/rental property | Tankless (no standby) |
| Cold climate, heavy use | Large tank or dual tankless |
| Tight installation space | Tankless |
Payback Analysis (Gas Tankless vs. Gas Tank):
| Factor | Calculation |
|---|---|
| Extra cost | $1,500-3,000 |
| Annual savings | $70-150 |
| Simple payback | 10-20 years |
| With maintenance | Consider descaling costs |
Heat Pump Water Heaters
Heat pump water heaters (HPWHs) are the most efficient electric option, using 2-3× less energy than standard electric tanks:
How They Work: Heat pump water heaters extract heat from surrounding air (like a refrigerator in reverse) rather than generating heat directly. They work best in spaces with ambient air 40-90°F.
Efficiency Comparison:
| Type | UEF Rating | Annual Cost* |
|---|---|---|
| Standard electric | 0.90-0.95 | $500-550 |
| Heat pump (hybrid) | 2.5-3.5 | $180-220 |
| Heat pump (best) | 3.5-4.0 | $140-180 |
*Based on 64 gallons/day, $0.13/kWh
Heat Pump Requirements:
| Requirement | Specification |
|---|---|
| Space | 1,000+ cubic feet (10×10×10 room) |
| Temperature | 40-90°F ambient air |
| Clearance | 6-8 inches around unit |
| Drain | Floor drain nearby |
| Height | 6-7 feet (taller than standard tank) |
Operating Considerations:
| Factor | Impact |
|---|---|
| Noise | 40-50 dB (audible hum) |
| Air cooling | Cools and dehumidifies space |
| Cold spaces | Switches to resistance (less efficient) |
| Recovery rate | Slower than gas or resistance |
| First hour rating | Often lower than standard electric |
2026 Heat Pump Prices (Installed):
| Size | Price Range | Annual Savings |
|---|---|---|
| 50 gallon | $2,500-3,500 | $250-350 |
| 65 gallon | $3,000-4,000 | $280-380 |
| 80 gallon | $3,500-5,000 | $320-420 |
Federal Tax Credit: 30% of cost up to $2,000 (2024-2032)
Fuel Type Comparison
Your available fuel options significantly impact both equipment costs and operating expenses:
Natural Gas:
| Pros | Cons |
|---|---|
| Fast recovery rate | Requires gas line and venting |
| Lower operating cost than electric | Combustion safety concerns |
| Works during power outages | Pilot light energy waste (older units) |
| Best for high-demand households | Less efficient than heat pump |
Electric (Standard Resistance):
| Pros | Cons |
|---|---|
| Low installation cost | Highest operating cost |
| No venting required | Slow recovery rate |
| Simple maintenance | Requires 240V circuit |
| Safe operation | Doesn't work in power outages |
Electric (Heat Pump):
| Pros | Cons |
|---|---|
| Lowest operating cost | Higher upfront cost |
| Federal tax credit available | Needs warm ambient air |
| Dehumidifies space | Larger unit size |
| 2-3× more efficient | Slower recovery than gas |
Propane (LP Gas):
| Pros | Cons |
|---|---|
| Available without utility service | Highest fuel cost per BTU |
| Fast recovery rate | Requires tank storage |
| Good for rural areas | Fuel price volatility |
Operating Cost Comparison (Annual, 64 gal/day):
| Fuel Type | Efficiency | Fuel Cost | Annual Cost |
|---|---|---|---|
| Natural gas | 0.67 UEF | $1.20/therm | $290-350 |
| Natural gas (high-eff) | 0.90 UEF | $1.20/therm | $220-260 |
| Electric (resistance) | 0.93 UEF | $0.13/kWh | $480-540 |
| Electric (heat pump) | 3.0 UEF | $0.13/kWh | $170-210 |
| Propane | 0.67 UEF | $2.50/gal | $500-650 |
Fuel Availability Considerations:
- Check if natural gas is available at your property
- Electric requires 30-50A 240V circuit (resistance) or dedicated circuit (heat pump)
- Propane requires outdoor tank installation and regular deliveries
Installation Requirements
Proper installation ensures safety, efficiency, and code compliance:
Gas Water Heater Requirements:
| Requirement | Specification |
|---|---|
| Gas line | 3/4" minimum for most units |
| Venting | B-vent or direct vent (power vent) |
| Clearances | 6" from combustibles (varies) |
| Combustion air | Per code requirements |
| Seismic strapping | Required in earthquake zones |
| Expansion tank | Required in closed systems |
| T&P discharge | 6" above floor or to drain |
Electric Water Heater Requirements:
| Requirement | Specification |
|---|---|
| Circuit | 30A 240V (standard), 50A for larger |
| Wiring | 10 AWG for 30A, 6 AWG for 50A |
| Breaker | Dedicated circuit required |
| Clearances | Minimal (no combustion) |
| Access | Panel accessible for service |
Heat Pump Water Heater Requirements:
| Requirement | Specification |
|---|---|
| Space | 1,000+ cubic feet around unit |
| Ceiling height | 7+ feet recommended |
| Temperature | 40-90°F ambient |
| Drain | Floor drain for condensate |
| Circuit | 30A 240V dedicated |
| Noise consideration | Not in bedrooms |
Tankless Installation Requirements:
| Gas Tankless | Electric Tankless |
|---|---|
| 3/4" to 1" gas line | 150-200A service may be needed |
| Category III stainless vent | 2-3 dedicated 40-50A circuits |
| Condensate drain | 8 AWG or larger wiring |
| Clearances per manufacturer | Proper ground |
| Water softener recommended | Circuit breaker panel capacity |
Permit Requirements: Most jurisdictions require permits for water heater replacement. Typical inspection covers:
- Proper venting and clearances
- Gas line and connections
- Seismic strapping (where required)
- T&P valve and discharge
- Expansion tank (where required)
Maintenance and Lifespan
Regular maintenance extends water heater life and maintains efficiency:
Annual Maintenance Tasks:
| Task | Frequency | DIY/Pro |
|---|---|---|
| Check T&P valve operation | Annually | DIY |
| Inspect anode rod | Every 2-3 years | DIY/Pro |
| Flush sediment | Annually | DIY |
| Check for leaks | Monthly | DIY |
| Descale tankless | Annually (hard water) | DIY/Pro |
| Clean heat pump filter | Quarterly | DIY |
Anode Rod Replacement: The anode rod sacrifices itself to prevent tank corrosion. Replacement extends tank life significantly.
| Water Type | Replacement Interval | Cost |
|---|---|---|
| Soft water | Every 5-6 years | $25-50 DIY, $150-200 Pro |
| Normal water | Every 3-4 years | Same |
| Hard water | Every 2-3 years | Same |
Expected Lifespan:
| Type | Average Life | With Maintenance |
|---|---|---|
| Gas tank | 8-12 years | 12-15 years |
| Electric tank | 10-15 years | 15-20 years |
| Gas tankless | 15-20 years | 20+ years |
| Electric tankless | 15-20 years | 20+ years |
| Heat pump | 10-15 years | 15-20 years |
Warning Signs of Failure:
| Sign | Possible Cause | Action |
|---|---|---|
| Rusty water | Corroded tank/anode | Check anode, may need replacement |
| Rumbling sounds | Sediment buildup | Flush tank, may be too late |
| Water pooling | Tank leak | Replace immediately |
| Reduced hot water | Sediment or element | Flush or replace element |
| Pilot won't stay lit | Thermocouple | Replace thermocouple |
| Error codes (tankless) | Various | Consult manual, call pro |
Pro Tips
- 💡Compare First Hour Rating (FHR), not just tank size—a smaller tank with faster recovery can deliver more hot water per hour.
- 💡Set temperature to 120°F for efficiency, safety, and adequate hot water—140°F wastes energy and risks scalding.
- 💡Flush your tank annually to remove sediment that reduces efficiency and shortens tank life.
- 💡Check the anode rod every 2-3 years and replace when significantly corroded—this extends tank life by years.
- 💡Insulate the first 3-6 feet of hot and cold water pipes near the heater to reduce heat loss and shorten wait time.
- 💡Consider a heat pump water heater if you have electric service—savings of $250-400/year with tax credits available.
- 💡For tankless heaters, factor in your climate's groundwater temperature—cold climates reduce effective GPM significantly.
- 💡Check for utility rebates and federal tax credits before purchasing—savings can reduce effective cost by $500-2,000.
- 💡Install an expansion tank if you have a closed plumbing system (check valves or PRV)—it's usually required by code.
- 💡In hard water areas (7+ grains), install a water softener to extend water heater life and maintain efficiency.
- 💡Locate your water heater near high-demand fixtures to minimize hot water wait time and pipe heat loss.
- 💡Consider a recirculating pump with demand activation for instant hot water without the energy waste of continuous circulation.
Frequently Asked Questions
For a family of 4 with 2 bathrooms taking back-to-back morning showers, you need approximately 65-70 gallons FHR. This typically means a 50-gallon gas tank or 65-gallon electric tank. For tankless, you need 6-8 GPM to run 2 showers simultaneously. Consider usage patterns—if showers are staggered, a 40-gallon tank may suffice.
