Would a water meter be cheaper?

An unmetered bill is based on the property's rateable value — a valuation of the house, not a measure of the water used. A metered bill is based on the water actually used. So the question is really: does this household use more or less water than its house is deemed to be worth?

The rule of thumb

If there are more bedrooms than people in the house, a meter is more likely to be cheaper. If there are more people than bedrooms, it is more likely to be dearer.

It is a genuinely useful starting point and it is repeated everywhere, including by the water companies themselves. It is also only a proxy, and it is worth being clear about why.

Why it is only a rule of thumb

The rule uses bedrooms to stand in for two different things at once: how much the property is valued at, and how many people live in it. Both approximations break in ordinary situations.

Rateable values are historic. They were set decades ago and do not track what has happened to the house or the street since. Two identical houses can carry noticeably different unmetered charges.
Behaviour beats bedroom count. One person taking long daily baths, running a power shower, or watering a garden through a dry summer can use more than three people who do none of those things.
A garden changes the picture. Sprinklers and hosepipes are the largest discretionary use in most metered households, and they are invisible to a bedroom count.
Standing charges differ. A metered bill carries its own fixed element, so a very low-usage household does not pay nothing.

How to answer it properly

The honest comparison needs three figures, and every one of them is obtainable without anybody's permission:

The current unmetered charge. It is printed on the water bill.
The household's actual usage. If a meter is already fitted, two readings a month apart give it directly. Reading the meter takes a minute. If there is no meter, the supplier's own calculator will estimate from occupancy and habits.
The supplier's current tariff — the price per cubic metre for water and for wastewater, plus the standing charges. Suppliers publish these, and they are set regionally rather than nationally, so a figure from one company does not transfer to another.

Usage multiplied by tariff, plus the standing charges, against the unmetered charge. That is the whole calculation, and it is arithmetic rather than judgement.

A calculator is coming here

Water Pages is building a comparison that does this arithmetic against the household's own supplier and tariff, rather than a national average. It is not live yet. Until it is, the supplier's own calculator and the figures above will answer the question, and this page will say so plainly when that changes.

Changing your mind

Switching to a meter is not necessarily permanent. Households in England and Wales that opt for a meter generally have a period during which they can ask to revert to unmetered charging, and there are separate arrangements for households where a meter would cause hardship, including assistance schemes for large families and for certain medical conditions.

The length of that period and the qualifying rules for assistance are set by the supplier within the regulatory framework, and they differ between companies. Both are worth confirming with the supplier in writing before agreeing to anything, because both are the kind of detail that is easy to be told informally and hard to rely on later.

On sourcing. This page deliberately carries no specific tariff, allowance or time-limit figures. Those are set regionally, change annually, and a number quoted without its source and its date is worse than no number. Where Water Pages publishes one, it will carry both.