What size hot water cylinder do I need?
Answer two quick questions and get the recommended size. No sign-up, no math.
For 3 people with a gas system. Your busiest hour uses about 136 L of hot water, so look for a cylinder with a First Hour Rating (FHR) of at least that.
Estimate only — confirm against the unit's spec label and your local code.
How hot water cylinder sizing works
The size you need depends on how much hot water your home uses in its busiest hour — think back-to-back morning showers plus a dishwasher or laundry. The key number on a hot water cylinder's label is the First Hour Rating (FHR): how much it can deliver in one hour. Match the FHR to your busy-hour demand and you won't run cold.
Gas units reheat water much faster than electric ones, so a gas one can use a smaller cylinder for the same demand. Tankless (on-demand) hot water cylinders are sized differently — by flow rate — and we'll add that calculator next.
Reference & education only — not professional advice. Confirm against the unit's spec label, your local code, and a licensed professional. Last reviewed 2026-06.