What Size Water Heater Do I Need?

When considering what size water heater do I need, here are a few things to think about:

  • The number of people in your household (if you share your house with teenagers who seem to spend hours in the bathroom, you'll understand why this is important!),
  • The number of sinks, basins, and showers in your home and how often you will be using more than two at a time (this affects how much water is heated and the capacity you will need) and lastly
  • The climate in which you live (this changes the ground temperature and how quickly your water heats up).

You also need to know which type of system you wish to install so here’s a little about the different types of heaters available. So, here are the basics (for more a more detailed look, please read our article dedicated to choosing a water heater) –

What Size Tankless Water Heater Do I Need? /What Size Water Heater Tank Do I Need?

When considering what type of water heater to chose there are two main options, either a tankless or on-demand system or a storage and heat pump system.

  • Storage and heat pump tank – comprise of a large tank where the water is heated and stored. Typically, these are whole house systems which store and then distribute the water as needed.
  • Tankless or On-demand systems are also whole house units, but they don’t store hot water. Water is heated and distributed as required. As they only heat the water as and when it is needed they can be more economical to run and typically take up less space than a tank.

Click here for an informative graphic on the other options available (including solar) depending on where in the US you live.

A common question people often ask is ‘what size water heater do I need to replace a 50 gallon water heater?’ Well if you are considering replacing a larger water heater such as a 50 gallon+ you may wish to consider looking at a tankless water heater. This will cut down on the amount of space needed to locate your water heater.

What Flow Rate Do I Need?

To calculate your households flow rate, you first need to know how many fixtures use hot water you will use simultaneously. So, add up the wash hand basins, sinks, and showers in your home. Don’t forget to add any small washrooms you may have in the basement etc.

To get a picture of how much flow rate you require – work out how many hot water faucets or showers you want to run at the same time. For example, in the morning you may wish to run two showers and the downstairs cloakroom hot water. This then is the amount of hot water you need to have available in your house for your morning routine.

A simple way to work out the flow rate is to place a five-gallon bucket beneath a facet or under a shower. Time how long it takes to fill the bucket, and this is the flow rate of that facet (or shower). For example, if it takes 3 minutes to fill the bucket, then the flow rate = 15 gallons per minutes or 15 gpm. Note this is a guide only, each facet or shower may have a different flow rate. But, it will give you an indication of the average rate within your home.

Below is a table of the average flow rate of different fixtures/appliances around the home.

Fixture / ApplianceAverage Flow Rate (GPM)
Bathroom Faucet0.5-1.5
Kitchen Faucet1.0-2.5
Clothes Washer1.5-3.0

So if you plan on having 2 showers running in the morning whilst the clothes washer is on then on average you would be looking at a flow rate of between 3.5-7.0 GPM. Therefore you would need a water heater with a 3.5-7.0 GPM minimum.

What Capacity Water Heater Do I Need?

Here’s a simple guide from The Home Depot to work out approximately how many gallons of water you may need depending on the size of your household:

Gallon CapacityHousehold Size (people)

A question often asked is ‘what size water heater do I need for a family of 5’ for example. Using this helpful table above we can see that you’d be looking at installing a 50 gallon or more tank (or tankless) unit which should comfortably provide hot water for the whole family.


What Fuel Type Should I Purchase?

There are four main fuel types of water heaters, the kind you choose will depend on how much you wish to spend, your monthly available fuel cost you want to pay, the type of fuel you want to use, among other things.

Fuel TypeOutside Ventilation Required?FeaturesPower Outage Issues?
GasYesUsually cost less to operateGas units aren’t affected by power outages
ElectricNoHeat water quickly; requires no gas piping.Can be affected by power outages
Liquid PropaneYesGenerally, heat more water per hour than a standard electric modelLiquid propane units aren’t affected by power outages
Hybrid ElectricNoAlthough these models have a higher upfront cost, they typically heat water faster and are more economical to run.Can be affected by power outages.

Click here for further information on these fuel types from Energy.gov including helpful hints on how to keep your fuel bills down.

What About Cost?

When considering which hot water system to purchase and install cost will often be high on everyone’s list, in fact it’s often the first thing that people consider, sometimes before the basics such as which system is most suited to the number of people in my household. But, obviously for those on a budget it’s an extremely important factor.

To find out more about what to look for when choosing an appliance, Energy.gov has a wealth of information on i’s site including this guide to the initial and the running costs you should think about. Read it here.

A cautionary note – once you’ve answered the question of “what size water heater do i need”, don’t be tempted to purchase a larger unit than is required for your family size. You will not only spend more on the unit purchase price but also on ongoing fuel costs such as gas, propane or electricity. Buy the unit that comfortably provides enough water for your household.

What Size Water Heater Do I Need – Conclusion

To help you further in your decision here’s a great, You Tube video to help you decide the right water heater for you, presented by The Home Depot. We hope our article on what size water heater do i need helps you in your decision.

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