Hot Water Wait Time Calculator (Pipe Length and Water Wasted)
How long you wait for hot water at the tap and how much water goes down the drain meanwhile, from the pipe's bore and length and the tap's flow.
Results
The dashed line marks the value you entered.
What this tool does
The water standing in the pipe has gone cold, so you wait until the whole length between heater and tap has been flushed through: time = pipe volume ÷ flow. A 14 mm bore holds about 0.15 litre per metre, so a 12 m run holds roughly 1.8 litres and takes around 18 seconds at 6 L/min — more if the pipe is wider or the tap trickles. The calculation assumes the water moves as a plug and ignores the pipe warming up, so reality is slightly longer. Shortening the run, insulating it or using a recirculation loop attacks the real cost, which is the water and the heat lost to the drain.
Formula
water in the pipe = π × d² ÷ 4 × length · wait = volume ÷ flow
Variables
| Symbol | Meaning | Unit |
|---|---|---|
d | Inside diameter of the pipe | mm |
l | Pipe run from the heater to the tap | m |
q | Flow at the tap | L/min |
u | Times a day you wait for hot water | — |
w | Water price per m³ | — |
S | Wait for hot water | s |
V | Water held in the pipe | L |
DL | Water run off each day | L |
YM | Water run off in a year | m³ |
YC | Cost of that water in a year | — |
Worked example
- Inside diameter of the pipe14 mm
- Pipe run from the heater to the tap12 m
- Flow at the tap6 L/min
- Times a day you wait for hot water6
- Water price per m³2.5
- Wait for hot water18 s
- Water held in the pipe1.85 L
- Water run off each day11.1 L
- Water run off in a year4.0 m³
- Cost of that water in a year10.11
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.