Water Heater Sizing Calculator

Calculate the optimal water heater size for your home based on household size, usage patterns, and fuel type. Get recommendations for tank capacity and first hour rating. Supports both Imperial and metric units for US and international projects.

Household Information

Number of People in Household *

Number of Bathrooms *

Fuel Type

Hot Water Usage Level

Dishwasher (adds ~6 gallons/day)

Washing Machine (adds ~7 gallons/day)

Energy Pricing

Use Custom Rates

Using US Default Rate:

Electricity: $0.12/kWh

US national averages

Water Heater Sizing Calculations

Daily Hot Water Demand

Daily Demand = (People × Usage) + Appliances + Bathrooms

• Base Usage: 30-60 gal per person per day

• Dishwasher: ~6 gal per day

• Washing Machine: ~7 gal per day

• Extra Bathrooms: ~10 gal per bathroom beyond first

First Hour Rating (FHR)

FHR = Daily Demand × 0.28

• FHR: Peak hour hot water capacity

• 28%: Typical percentage of daily use in peak hour

• Critical: Must match household peak demand

• Units: Gallons per hour

More important than tank size for avoiding cold water issues.

Tank Sizing Formula

Tank Size = (FHR - Recovery Rate) + (Daily × 0.7)

• FHR: First hour rating needed

• Recovery Rate: Gallons heated per hour

• Daily × 0.7: Storage buffer for demand

• Minimum: 30 gallons for any household

Balances storage capacity with recovery performance.

Annual Operating Cost Calculation

Step 1: Energy per gallon = 500 BTU ÷ Energy Content

• Electric: 500 BTU ÷ 3,412 BTU/kWh = 0.146 kWh/gallon

• Gas: 500 BTU ÷ 100,000 BTU/therm = 0.005 therms/gallon

• Propane: 500 BTU ÷ 91,000 BTU/gallon = 0.0055 gal propane/gal water

• Hybrid: 500 BTU ÷ 3,412 BTU/kWh ÷ 2.4 COP = 0.06 kWh/gallon

Step 2: Annual Cost = Daily Gallons × 365 × Energy/Gallon × Rate ÷ Efficiency

Example: 273 gal/day × 365 × 0.146 kWh/gal × $0.12/kWh ÷ 0.95 = $1,838/year

Based on heating water 60°F (from cold tap to hot tank temperature).

Standard Tank Sizes & Applications

Small Households

• 30 gallon: 1 person, apartment

• 40 gallon: 1-2 people, condo

• 50 gallon: 2-3 people, small house

Compact units for space-limited areas

Medium Households

• 65 gallon: 3-4 people, family home

• 80 gallon: 4-5 people, larger family

Most common residential sizes

Large Households

• 100+ gallon: 6+ people, large family

• Multiple units: Very high demand

High-capacity solutions

Frequently Asked Questions

What size water heater do I need for my family?

Most families can figure this out pretty easily. A family of four typically needs a 50-65 gallon (190-250 liter) tank, but it really depends on your hot water habits. If you've got teenagers who take long showers or run the dishwasher and washing machine daily, you might want to bump up to a 65-80 gallon (250-300 liter) unit. The key is matching the First Hour Rating to your busiest hot water hour.

Why does First Hour Rating matter more than tank size?

Here's the thing - you can have a huge 80-gallon (300-liter) tank, but if it only heats 20 gallons (75 liters) per hour, you're going to run out of hot water fast during busy times. The First Hour Rating tells you how much hot water you'll actually get when everyone's showering, doing dishes, and running laundry at the same time. That's what really matters for avoiding those cold shower surprises.

Should I go with gas or electric for my new water heater?

If you already have a gas line, go with gas - it's usually cheaper to operate and heats water much faster. Electric units cost less upfront and are easier to install, but they'll hit your electric bill harder and take longer to reheat the tank. Gas units heat about 40 gallons (150 liters) per hour compared to electric's 23 gallons (87 liters), which makes a big difference for large families.

Can I use this calculator with metric measurements?

Absolutely. Just hit the metric toggle button and all the usage levels, appliance additions, and results will show in liters instead of gallons. The calculator automatically handles the conversions, so you get accurate sizing recommendations whether you're thinking in gallons or liters. Perfect for international projects or if you just prefer metric.

How much space do I need around my water heater?

You need enough room to work on it safely. Gas units need proper clearances for combustion air and venting - usually 6 inches (15cm) on sides and back, 18 inches (45cm) in front. Electric units are more forgiving but still need access for maintenance. Hybrid heat pump models need the most space since they pull air from around the unit, so give them plenty of breathing room.

What's this hybrid heat pump water heater everyone talks about?

Think of it as a super-efficient electric unit that pulls heat from the air instead of just using electric coils. They cost more upfront but can cut your water heating costs by 60-70%. The catch? They're slower to recover than gas, need more installation space, and work best in warmer climates. Great for energy-conscious homeowners who aren't in a huge rush.

Can I install a water heater myself or do I need a pro?

Electric units are more DIY-friendly if you're comfortable with electrical work, but gas units really need a professional. You're dealing with gas lines, venting, and safety issues that can be dangerous if done wrong. Most local codes require permits and inspections anyway. Even if you're handy, having a pro do it protects your warranty and ensures everything's up to code. If you're in the US and need an electrician for the electrical connections, you can find qualified professionals at GetElectricalJobs.com.

How long should a water heater last before I replace it?

Most tank water heaters last 8-12 years, depending on water quality and maintenance. If you're starting to get rusty water, hearing weird noises, or constantly running out of hot water, it's probably time to start shopping. Don't wait until it completely fails - water heater leaks can cause serious damage to your home.

What maintenance does a water heater actually need?

Not much, but it makes a huge difference. Flush the tank once a year to remove sediment buildup, test the pressure relief valve, and check the anode rod every few years (it prevents tank corrosion). If you have hard water, you might need to flush more often. These simple steps can easily add 3-5 years to your water heater's life.

Why is my new water heater taking forever to heat up?

Probably because it's undersized for your actual usage. A lot of people focus on tank size but ignore recovery rate. If your family's hot water demand exceeds what the unit can reheat per hour, you'll always be waiting. This is especially common when switching from gas to electric without accounting for the slower heating speed.

Should I go with a tank or tankless water heater for my situation?

Tanks are usually the better choice for most families - they're cheaper upfront, easier to install, and handle multiple simultaneous uses without breaking a sweat. Tankless makes sense if you're tight on space, have very low usage (vacation home), or want the energy savings from not keeping a tank hot 24/7. But remember, tankless units cost 2-3 times more initially and may need bigger gas lines or electrical upgrades.

How do you calculate the annual operating cost?

We use proper thermal calculations, not rough estimates. To heat 1 gallon of water 60°F requires 500 BTU (1 gallon = 8.33 lbs × 60°F × 1 BTU/lb/°F). For electric, that's 500 ÷ 3,412 = 0.146 kWh per gallon. For gas, it's 500 ÷ 100,000 = 0.005 therms per gallon. Then we multiply by your daily usage, 365 days, energy rates, and adjust for efficiency. We use different default rates for US ($0.12/kWh) vs international users ($0.25/kWh) since energy costs vary dramatically by country. You can also enable "Use Custom Rates" to enter your exact local energy prices for the most accurate cost estimate.

Complete Your Hot Water System Design

Water heater sizing is just the beginning. Design an efficient hot water system with these professional calculators:

Pipe Sizing•Water Flow Rate•Water Pressure Drop•Unit Converter