ApplianceCalc

Water

Water Heater Size Calculator

Estimate a starting storage tank size range from household demand, showers, dishwasher use and laundry.

Empirical household demand estimate
1

Your appliance details

Units

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2

Your results

Approximate tank size range80–140 L
Estimated peak-hour hot water111 L
Assumed shower flow8 L/min

What this means

Use this range to begin comparison, then choose a first-hour rating that covers peak-hour demand with your installer.

Show calculation
  1. Peak demand = people × 5 L + showers × duration × 8 L/min × 0.7 + dishwasher cycles × 6 L + laundry cycles × 20 L.
  2. Estimated demand = 110.6 L; tank screening range = 80–120%, rounded to 10 L.

Important notes

Empirical estimate: actual requirements depend on conditions this simplified model cannot measure.

All per-use amounts and the 70% hot-water fraction are estimated. Flow rate, incoming temperature, mixing and simultaneous use change demand.

Tank volume is not first-hour rating. Recovery rate and usable stored volume need separate verification.

Calculation basis: Empirical household demand estimate

Hot-water demand is concentrated in busy periods rather than spread evenly through a day. Describe a peak-use period to estimate an initial tank-capacity range. Tank volume alone is not enough to select a unit: recovery and the first-hour rating determine how much hot water it can deliver while refilling.

How to use

  1. Enter household size and the number and duration of showers in your busiest period.
  2. Include dishwasher and laundry use that overlaps that period.
  3. Use the estimated range as an initial shortlist, then check first-hour delivery, shower flow and installer guidance.

How the calculation works

Formula

Assumed peak demand = showers × minutes × 8 L/min × 0.7 hot-water fraction + 6 L per dishwasher use + 20 L per laundry use + 5 L per occupant. Tank range = 0.8–1.2 × estimated demand. All these allowances are site-selected empirical assumptions; this does not calculate a certified first-hour rating.

Example

Two 8-minute showers at a hypothetical 8 L/min use 128 liters of mixed water. If 60% comes from the hot supply, they draw about 77 liters of hot water before adding other uses. Both the flow and hot fraction are assumptions.

Factors affecting the result

  • Incoming temperature and tank setpoint change the fraction of hot water in a shower.
  • A fast-recovering tank can deliver more than its stored volume within an hour.
  • Appliance hot-water demand varies with model and cycle; cold-fill machines may heat internally.

Frequently asked questions

Is first-hour rating the same as tank size?

No. Tank size describes stored volume. First-hour rating describes delivery over a busy hour, including the effect of recovery.

Can I use the result to choose a tankless unit?

No. Tankless selection needs simultaneous flow and temperature rise. Ask an installer to check those requirements.

Sources & methodology

Calculation basis: Empirical household demand estimate. Methodology

Last updated: September 15, 2026