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Free Online Hot Water Heater Sizing Calculator

Size a domestic or commercial hot water storage heater from occupancy, demand and temperature rise. IPC basis. Free online tool.

📐 Standard: IPC
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Hot Water Heater Sizing Calculator
Reference: IPC
🌡️ PLUMBING
Size a domestic or commercial hot water storage heater from occupancy, demand and temperature rise. IPC basis. Free online tool.
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Calculation confidence: Planning-levelSuitable for early design, estimating and feasibility checks. Verify the final design against the governing code and a licensed engineer before construction or procurement.
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ℹ About This Calculator

A storage water heater must hold enough hot water for peak demand and recover fast enough between draw-offs. This calculator sizes the storage capacity and heater rating from the number of users, hot-water use per person and the temperature rise, following IPC practice.

Sizing balances two things: storage volume for the peak-hour draw and recovery rate - how quickly the element reheats the tank. A small tank with a large element (fast recovery) suits continuous commercial use; a larger tank with a modest element suits domestic morning peaks. Undersizing either one leaves users running cold mid-shower.

Hot Water Heater Formula

IPC

Q_energy = m × Cp × ΔT
Recovery (L/h) = (P × 3600) / (Cp × ΔT) Q_energy = energy per fill, kJ
m = hot-water mass, kg (≈ litres)
Cp = 4.186 kJ/kg·K
ΔT = hot − cold temperature rise, °C
P = heater element power, kW

Worked Example

Example: A home of 4 using 45 L each per day at 60 °C from 25 °C cold water needs 180 L/day, with a peak-hour draw near 40% (~72 L). A 100 L storage geyser with a 2 kW element recovers about 2000×3600/(4.186×35×1000) ≈ 49 L/h, comfortably covering the peak with reserve in the tank.

Hot Water Demand Reference (IPC)

Storage capacity follows the peak-hour demand, not just the daily total. Typical hot-water allowances:

OccupancyHot water (L/person/day at 60°C)
Residence40 – 45
Hotel (per bed)60 – 90
Hospital (per bed)65 – 90
Office5 – 8
Restaurant (per meal)6 – 8

Storage vs Instantaneous

Storage heaters suit peaky domestic loads - a tank rides through the morning rush. Instantaneous (tankless) heaters suit low, continuous or point-of-use draws and save standing losses, but need a high power input for the same flow. For most US homes a 15-100 L storage geyser is standard; larger buildings use central storage with recirculation.

Indicative allowances - confirm against IPC and the actual usage pattern.

Frequently Asked Questions

What size geyser do I need for a family of 4? +
For a typical US family of four, a 100-150 L storage water heater covers the morning peak comfortably, or a 25 L unit per bathroom for point-of-use. The right size depends on how many showers run close together; use this calculator with your actual peak-hour demand.
How is water heater recovery rate calculated? +
Recovery rate (litres per hour) = element power in watts × 3600 ÷ (4.186 × temperature rise × 1000). A 2 kW element raising water by 35 °C recovers about 49 L/h. Higher power or lower temperature rise gives faster recovery.
Storage vs instant water heater - which is better? +
Storage heaters handle peaky demand well and use a lower power supply, but lose some heat standing. Instantaneous heaters avoid standing losses and never run out, but need a high electrical or gas input for the same flow rate. Choose storage for domestic peaks, instant for continuous point-of-use.
What temperature should a storage water heater be set to? +
Around 60 °C is typical - hot enough to limit Legionella growth and give useful mixed-water volume, without excessive standing loss or scald risk. Thermostatic mixing at the outlet then delivers a safe usage temperature.
How much hot water does one person use per day? +
In residences about 40-45 litres per person per day at 60 °C is a common design figure; hotels and hospitals use 60-90 L per bed. This calculator lets you enter the actual per-person figure for your occupancy.
What standard covers water heater sizing in the US? +
IPC covers electric storage water heaters, and IPC (Plumbing Services) gives hot-water demand allowances by occupancy. Together they guide storage capacity, element rating and installation.
Does cold water temperature affect heater sizing? +
Yes. A lower incoming cold-water temperature means a larger temperature rise, so more energy per litre and slower recovery. Winter cold-water temperatures should be used for the worst-case sizing.
How do I size a central hot water system for a hotel? +
Use the per-bed hot-water allowance (60-90 L/day), apply a peak-hour and diversity factor, then size central storage plus a recovery boiler and a recirculation loop to keep outlets hot. This tool sizes the storage and recovery; the recirculation is designed separately.

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →