How much hot water does my cylinder deliver in the first hour?
Enter the cylindersize, the heat input and your temperature rise — you'll get the first-hour hot water delivery, worked out as the usable stored water plus one hour of recovery, instantly, in gallons or litres.
Assumptions (tap to fine-tune)
Temperature rise = your hot setting minus the incoming cold water (e.g. 50 °C − 10 °C = 40 °C). Recovery efficiency and usable storage % are typical planning figures — adjust them to your unit's spec label.
A full 150 L gas cylinder can deliver about 311 L of hot water in the first hour — the 105 L already hot in the cylinder plus the 206 Lit reheats in that hour. Match this to your busiest hour's demand so you don't run cold.
Estimate only — the FHR on a unit's label comes from a standardized test; confirm against the spec label.
First hour rating = usable storage + one hour of recovery. Recovery = heat input × efficiency ÷ (the energy to raise that volume of water through your temperature rise). These are physics and labelled planning figures, not code values, and there are no prices.
Common questions
- What does first-hour hot water mean?
- The First Hour Rating (FHR) is how much hot water a full hot water cylinder can deliver in the first hour of heavy use. It is the hot water already stored in the cylinder that you can draw before cold dilutes it, plus the extra the hot water cylinder reheats during that hour (its recovery). It is the most useful sizing number because it reflects a real morning rush better than the cylinder's nominal size alone.
- How is first hour rating calculated?
- FHR = usable storage + one hour of recovery. Usable storage is roughly 70% of the cylinder's nominal size, because the outlet cools as cold water flows in. Recovery is the heat input × efficiency divided by the energy needed to raise that volume of water through your temperature rise. A 40-gal (150 L) gas heater with a 40,000 BTU/hr burner works out to roughly 83 gal (about 314 L); a 50-gal (190 L) electric one to roughly 61 gal (about 230 L). The calculator above does the arithmetic for you.
- What first hour rating do I need?
- Match the FHR to your busiest hour, not your daily total. Add up the hot water your home uses in its heaviest hour — back-to-back showers (about 20 gal / 75 L each), plus a dishwasher or laundry — and choose a hot water cylinder whose FHR meets or beats that figure. A family taking several morning showers in a row typically wants an FHR in the 60–80 gal (230–300 L) range.
- Why does a gas cylinder deliver more in the first hour?
- Because gas burners pour in heat far faster than electric elements, so a gas unit recovers much more water during the first hour. In the formula a typical 40,000 BTU/hr gas burner recovers around 55 gal (about 207 L) an hour, while a 4.5 kW electric element recovers only about 26 gal (about 98 L). That is why a smaller gas cylinder can out-deliver a larger electric one in the first hour.
- Is first-hour delivery the same as the cylinder size?
- No. Capacity is just the water held in the cylinder; the FHR adds the recovery, so it is almost always larger than the nominal size — and how much larger depends on the heat input. Two cylinders of the same size can have very different first-hour ratings, which is why it pays to read the FHR, not only the litres or gallons on the front.
Want the full explanation? Read the first hour rating guide → Not sure what size to buy? Try the hot water cylinder size calculator →
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-06.