HOT WATER TANK SIZING

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This calculator estimates storage tank size and heater input based on actual fixture demand, not just a rough “number of bedrooms” rule of thumb. It lets you build up a picture of daily hot water use from showers, sinks, laundry, dishwashers and tubs, then turns that into:

• Total daily hot water volume
   
• Peak hour draw (how much gets used in the busiest hour)
   
• A design storage volume and suggested nominal tank size (US gal)
   
• The temperature rise (ΔT) from incoming cold to stored hot water
   
• The daily heating energy needed, plus the required heater input in kW and BTU/h
   

You can size for either natural gas/propane or electric heaters. For gas, the tool also shows an approximate BTU/h “band” (e.g. typical 40–50 gal, high-output, or tankless territory). For electric, it estimates the required current at 240 V and 208 V so you can see what kind of electrical service the heater would need. It’s all based on simplified but transparent assumptions, making it a great early-design and “sanity check” tool before you dive into manufacturer charts and code books.

1. Start with the optional Residential Wizard
   If you’re sizing a typical home, fill in:
    
   • Number of people
      
   • Number of bathrooms
      
   • Usage level (low / average / high)
      
   • Whether there’s a large soaking/whirlpool tub (and its volume)
      
   • Whether there’s a dishwasher and clothes washer using hot water
      
   • Click “Apply residential wizard to fixture table” and the calculator will populate the 5 fixture rows with reasonable flows, quantities, and “% of day at flow” values, and set a peak-hour percentage. You can still tweak everything afterward.
      

1. Refine using the Detailed Fixture Demand table
   In the Detailed Fixture Demand section, each row represents a typical fixture or group of fixtures. For each row, enter:
    
   • A fixture name (e.g. “Showers”, “Lav sinks”, “Kitchen sink”, “Laundry”, “Dishwasher”)
      
   • Flow in US gpm
      
   • Quantity of that fixture
      
   • Estimated % of day at flow (how much of a 24-hour day that load runs at or near full hot flow)
      
   • You can also use the Preset dropdown to quickly load patterns like “Small residential – 1 bath”, “Typical residential – 2.5 bath”, “Small office”, or “Small restaurant / café”. Presets overwrite the rows and peak-hour percentage, which you can adjust afterward.
      

1. Set peak-hour, temperatures, and heater details
    
   • % of daily hot water used in busiest hour: often 30–50% for residential/light commercial.
      
   • Incoming water temperature (°C): use a winter design value (e.g. 2–7 °C in much of Canada).
      
   • Tank setpoint / storage temperature (°C): often 49–60 °C, with mixing valves tempering down to fixtures.
      
   • Choose heater type (gas or electric) and enter efficiency (%) if known.
      
   • Set effective heating hours per day – how many hours per day the heater realistically runs to reheat the tank (e.g. 2–4 h).
      

1. Run the calculation and interpret the results
   Click Calculate to see:
    
   • Total daily hot water volume in litres and US gallons
      
   • Peak-hour draw based on your busiest-hour %
      
   • A design storage volume roughly equal to that peak-hour volume
      
   • A suggested nominal tank size (picked from common US gallon sizes)
      
   • ΔT, daily kWh to the water, and required heater kW and BTU/h
      
   • For gas: an approximate BTU/h band (small tank, typical residential, tankless range, or commercial scale)
      
   • For electric: the estimated amperage at 240 V and 208 V
      
   • Use Reset to clear everything and start fresh, or rerun with different usage assumptions to compare “low”, “average”, and “high” scenarios.
      

This calculator is meant as a preliminary sizing and education tool. It won’t replace the National Plumbing Code of Canada, fuel/electrical codes, or manufacturer sizing procedures—but it will give you a transparent, demand-based starting point so your conversations with suppliers, engineers, and inspectors start from solid numbers instead of rough guesses.