How to compare water companies fairly
Companies run very different numbers of overflows. A company with more overflows will usually have more spills in total, even if each one spills less.
What to do
Compare the median spills per outlet, the share of outlets with many spills and the change for outlets monitored in both years. Use totals only with the outlet count beside them.
Totals mislead
In 2025, United Utilities listed 2,270 outlets and Thames Water listed 581. Their totals differ for that reason alone. Always read a total with the number of outlets behind it.
Use the median
The median is the middle outlet. A few outlets with very high counts do not pull it up. In 2025 the company medians ranged from 6 to 21 spills per outlet.
Compare like with like across years
The number of outlets in the return changed between years as monitors and records were added. Compare only outlets with a count in both years, and leave out years when the monitor worked less than 90% of the time.
Weather matters
Overflows operate in heavy rain, so counts tend to rise in wet years and fall in dry ones. A fall in one year does not show on its own that a company improved its network.