16 calculators
Water Heater Calculators
Quick answer
Water heater sizing reduces to two numbers: peak hour hot water demand, which decides the first hour rating to buy, and temperature rise, which decides how much flow any tankless unit can actually deliver. The 16 calculators below work through those and everything that follows from them, from gas pipe capacity to expansion tank size to annual running cost.
Every calculator here shows its formula, renders a worked example into the page itself so it reads correctly with no scripts running, and labels each figure as a published standard or a planning convention. Where the inputs fall outside the published relations, they return no answer and say why rather than extrapolating.
Sizing
- What size water heater Work out what size water heater a household needs from its peak hour hot water demand, with the first hour rating to shop for and the t Peak hour demand = sum of (activity gallons x how many times
- Peak hour demand Add up the hot water your household uses in its busiest hour, activity by activity, using the gallons published in the DOE first hour r Peak hour demand = (showers x 10) + (baths x 20) + (shampoos
- First hour rating Estimate the first hour rating of a storage water heater from its tank size and its burner or element rating, and see how much of it is FHR = (tank gallons x usable fraction) + recovery in one hou
- Temperature rise Work out the temperature rise a water heater has to produce from your inlet water temperature, and see what that rise does to a tankles Rise = outlet temperature - inlet temperature. Flow at a new
- Recovery rate Work out how many gallons per hour a water heater reheats from its burner input or element rating, and how long it takes to bring a col Gas GPH = BTU/h x efficiency / (8.33 x rise). Electric GPH =
Tankless
Install
- Gas pipe size Find the natural gas pipe size a water heater needs from its BTU input and the run length, using the published NFPA 54 low pressure cap Required CFH = BTU/h / 1,030 BTU per cubic foot. Then read t
- Breaker and wire size Find the breaker and conductor size an electric water heater needs, applying the NEC continuous load rule, the 75 C ampacity table and Amps = watts / volts. Design amps = amps x 1.25 (continuous
- Expansion tank size Size a thermal expansion tank from your water heater capacity, static supply pressure and relief valve setting, using the published acc Expansion volume = tank gallons x (cold density / hot densit
- Drain pan size Find the drain pan size a water heater needs from its tank diameter, with the minimum drain line size and what pan depth actually buys Minimum pan inside diameter = tank diameter + 2 inches. Then
Running cost
- Operating cost by UEF Work out what a water heater costs to run per year from its UEF, your fuel price and your daily hot water use, for electric, gas, propa Delivered BTU per day = gallons x 8.33 x rise. Input BTU = d
- Payback period Work out how long a more efficient water heater takes to pay back its extra cost, for a tankless upgrade or a heat pump water heater ag Annual saving = cost at the old UEF - cost at the new UEF. P
- Recirculation loop Work out what a hot water recirculation loop costs in standing heat loss, and how much a timer or a thermostat saves against running it Loss per day = loss per foot per hour x loop length x hours
Maintenance
- Descaling interval Work out how often to descale a tankless water heater from your water hardness and outlet temperature, with the grains per gallon to pa Interval = base interval for the hardness band, reduced by a
- Anode rod life Estimate how often to inspect a water heater anode rod from your water hardness, the rod material, whether the water is softened and th Inspection interval = base interval for the material, reduce
Efficiency
Researched guidance, not professional advice. These are planning figures from published test procedures, code tables and manufacturer specifications. A water heater is a pressure vessel: never cap, plug or remove the temperature and pressure relief valve. Gas line, flue and vent work belongs to a licensed plumber or gas fitter, because improper venting produces carbon monoxide. Turn off the breaker before touching an element, and never energise an element that is not submerged.