Fire hydrant signs – a successful metric conversion

Metric Views looks at a successful but unsung project that took place during the early years of the UK’s metric transition. We speculate what might have occurred had this job been the responsibility, not of local government, but of the UK Department for Transport (DfT), and we draw attention to a paradox.

A visitor arriving from abroad could be forgiven for wondering if Britain is losing the plot. First, there are delays in the air, waiting for a landing slot. Then it is necessary to queue for several hours for a passport check. Finally, on the drive into London, there are unfamiliar symbols on the traffic signs (surely ‘m’ for distance can not be metres, and what can be the meaning of quotation marks, apostrophes and ‘yds’?).

Metric Views is not qualified to comment on the problems with landing slots or passport checks, but we believe that the archaic measures on our traffic signs are an embarrassment and difficult to explain away to a visitor. It is some consolation, however, that not all signage in the public domain appears stuck in a time warp.

If you live in a house, you will probably have a fire hydrant sign within 100 metres of your front door. They show fire fighters the location of a water supply, and are particularly useful when the hydrant cover is obscured, perhaps by snow or a parked vehicle.

Imperial hydrant sign

hydrant sign on concrete post

Wikipedia provides a succinct summary of the facts about fire hydrant signage:

‘In the United Kingdom and Ireland, hydrants are located in the ground. Yellow “H” hydrant signs indicate the location of the hydrants. Mounted on a small post or nearby wall etc., the two numbers indicate the size of the water main (top number) and the distance from the sign (lower number). Modern signs show these measurements in millimetres and metres, whereas older signs use inches and feet. Because the orders of magnitude are so different (6 inches versus 150 mm) there is no ambiguity whichever measuring system is used.’

This does not appear to have deterred at least one supplier from incorporating units into the design:

Hydrant signs with units

Note that the upper number relates to the size of the water main serving the hydrant and not the size of the hydrant itself. All hydrant fittings are standardised, perhaps partly as a result of an incident on the night of 14 November 1940, when it is said that fire appliances were sent from Birmingham to assist with a conflagration in Coventry only to discover that the two sets of hydrant fittings did not match.

During the ten years after 1975, most hydrant signs in Britain were changed from Imperial to metric. The changeover continues to this day as surviving Imperial signs require replacement. The job has been done by local government, without fuss and at minimal cost, as part of the national project to provide the UK with a single, simple and universal measurement system, about which we comment later.

One can imagine what would have happened if this job had been the responsibility of the DfT:

1. Regulations requiring Imperial measures on hydrant signs would have been left in place, with no provision for local discretion on units.

2. The conversion would have been postponed for 25 years, ‘to ensure that most fire fighters had first received a metric education’.

3. After the point had been passed when a majority of fire fighters had been taught metric measures at school, an inflated estimate of the cost of the changeover of hydrant signage would have been prepared. Say, one million signs at £250 per sign, plus 10% to allow for an underestimate in the number, plus 25% for supervision, plus 65% for ‘optimism bias’, plus £125 million for preparatory work, publicity and project management: about £725 million. Not affordable, of course.

4. A permanent exclusion of fire hydrant signs from any requirement to go metric would then have been sought.

There is a paradox in all of this. The national plan to provide the UK with a single, simple and universal measurement system was a principal reason for the metric changeover of fire hydrant signage, whereas it is just one of several reasons why it is in the UK national interest to convert road traffic signs¹. Yet it was lack of vision at the DfT that undermined some of the benefits of the progress with the metric changeover elsewhere in the UK economy. And now we see the consequences including, for example, excessive reliance on London’s financial sector for our prosperity, and performance in mathematics among 15 year-olds that is no better than the Organisation for Economic Co-operation and Development (OECD) average².


1. Reasons for converting road traffic signage include:

A single easy system (also a principal reason for converting hydrant signs).
Consistent information for drivers.
Consistency between highway signage and Ordnance survey maps.
Compatibility with the Highway Code.
Compatibility between vehicle manuals and road signage.
Better understanding of incident location description on motorways.
Consistency for industry.
Cheaper cars.
Greater safety for cross border traffic, to and from the UK.
Fewer bridge strikes.
Support for the UK’s reputation abroad as a place where it is easy to do business and also for its position as a major player in the global economy.

These are explained further in a booklet “Metric signs ahead”, available as a free download from

2. OECD (2010), PISA 2009 results: executive summary. (The OECD programme for international student assessment (PISA))

“Korea, with a country mean of 546 score points, performed highest among OECD countries in the PISA 2009 mathematics assessment. The partner countries and economies Shanghai-China, Singapore and Hong Kong-China rank first, second and third, respectively. The OECD countries Finland, Switzerland, Japan, Canada, the Netherlands, New Zealand, Belgium, Australia, Germany, Estonia, Iceland, Denmark, Slovenia and the partner countries and economies Chinese Taipei, Liechtenstein and Macao-China also perform significantly above the OECD average in mathematics.”

The UK performance score on the mathematics scale was 492, close to the OECD average of 496. The US score was 487.

Further information can be found at:                                                       

16 thoughts on “Fire hydrant signs – a successful metric conversion”

  1. Yes. UK fire hydrant signs as an example of advancing metrication in the UK. The unambiguous conversion is definitely a factor. The target user group is another (employees versus public). Thanks for reminding us of positive stories like this.


  2. Derek says: “Support for the UK’s reputation abroad as a place where it is easy to do business and also for its position as a major player in the global economy.”

    Apologies if I’ve told this story before, but this statement brings to mind a lost foreign contract that a colleague of mine experienced many years ago. Now long retired, he was a mining engineer turned software programmer, specialising in machines used in mining operations. Travelling the world, he eventually secured a contract to provide the software for some bespoke mining machines being made in Japan, and to be used in Australia, both countries of course metric. His client was a large Japanese multi-disciplinary manufacturer, who normally did their software in-house, but had been persuaded to outsource the programming to my colleague based on his mining expertise.

    The last step before signing the contract was for the head of engineering of his client to visit him in the UK, and check that he had a proper business (he did, with a plush office suite in Berkshire and around a dozen employees). He picked up his client at Heathrow, who had flown in from Japan. The client was shocked to see that, apparently, Britain was not metric, judging by the roadsigns. Much of the subsequent conversation centred about whether the code would be programmed in SI from the outset. My colleague assured his client that all engineering done in the UK was in SI, yet he was unable to convince his client, based on the lack of visible metrication in the UK, that he wouldn’t program it in Imperial and then convert it.

    The contract was in the millions, with the promise of more to come – and all lost because technical and business visitors to this country think that we are living in the past, based on what they FIRST see when they get off a plane or a car ferry: nothing but Imperial road signs. I wonder just how much foreign trade we are constantly losing as a result?


  3. Only a few of our most public uses of measurement remain imperial; such as road signs, pints of beer and milk, and personal use of height and weight. As such some people, who do not need to use measurements in their professional lives, continue to be surprised when they delve into fields that switched to metric decades ago.

    Just the other day a member of the House of Lords raised a question about motorway marker posts (which have been metric since at least the early 1970s, reflecting the units that highway engineers use). Reading between the lines, he was obviously horrified to discover that these posts are marked with metric values.

    I wonder how long it will be before another politician raises a similar question about fire hydrants. Don’t they realise how silly it makes them look? What a shame the most public uses of measurement weren’t metricated first. It would have made the whole process much quicker and less controversial, and we would all have reaped the benefits of a single national measurement system a lot sooner.


  4. The question and reply immediately following the one about the metric distance markers in the House of Lords was this:

    Roads: Kilometre Measurements

    Asked by Lord True

    To ask Her Majesty’s Government whether they have any plans to adopt the kilometre in place of the mile as the prime measure of distances by road in the United Kingdom.[HL16910]

    Earl Attlee: This Government have no plans whatsoever to adopt the kilometre as the unit of distance on roads in the United Kingdom. To do so would require the metrication of all traffic signs indicating speed as well as distance, for which diverting funding from high priority areas is not considered justifiable, or indeed desirable.

    Notice that the question was just about distances. The government sneakily throws in the issue of converting speed limits, which the Irish demonstrated and which logic confirms is quite separate from the issue of converting distance signs. Note also the assertion that the metrication of road signs is considered by the government as being not desirable. This is how warped the current government’s view is on the matter.

    The story told by John Frewen-Lord about his colleague who lost a contract with a Japanese company because their immediate impression from the road signs they was was that the UK is not metric is the mirror image of the experience one has in Ireland. It also reinforces the observation that metricating road signs significantly accelerates the growth of the metric mindset in the population.

    This is confirmed in both Ireland and Canada, both of which converted road signs to metric and both of which have seen “kilometers” completely displace “miles” for all practical purposes within only a few years following the conversion. In fact, it is quite telling that, despite Canada’s closeness to the much larger USA and the omnipresence of US television and radio in Canada, the residents of that country have no idea what a mile is or how hot or cold something is in degrees Fahrenheit because the road signs are metric and the local and national media use metric exclusively.


  5. I’ll leave it to any Canadian readers to be more specific about the general case, but I once ran into an interesting glitch when talking to a couple of Canadian students from Ontario who wanted to have a go at surfing whilst in Britain (Ontario being something like 1000km from the sea….).

    I suggested they might like to borrow or buy wetsuits because the waters round here (even in August or early September) are rarely going to touch 18°. “Er – sorry” said one of the Canadians “You have any idea what that would be in Fahrenheit, eh?”

    I didn’t. “What?” I exclaimed “I thought you Canadians did stuff in metric?”

    “We do.” came the reply. “Mostly. Except water temperatures. We tend to do that in Fahrenheit!”

    D’oh! This was my first encounter with “A Very Canadian Mess”. I don’t know if they ever got to try out surfing, but if they did they’ll have found out pretty quick what 18° feels like!


  6. You think it’s bad where you are, it took me all morning over here just to figure out how to switch the settings on my stove and microwave (as it is still quite uncommon in the USA) to Metric.


  7. @Wild Bill

    Sorry I missed your post about Canadians doing water temperature in Fahrenheit. That does strike me as very odd since the Canadians I’ve known who work down here in the USA all have complained about having trouble dealing with temperature in Fahrenheit when it comes to weather reports.

    Very odd indeed that water temps would be an exception for divers!


  8. @Wild Bill

    I can’t confirm it as I have never discussed surfing with a Canadian; however, it seems likely and could be blamed on the US being a bad influence. Most of the population of Ontario is near the Great Lakes.

    There are a number of marine weather buoys on the Great Lakes (all pulled for the season due to ice). Some are operated by NOAA, some by Environment Canada. We use °F, they use °C. The US probably operates about 75% of these stations (eyeballing a chart, not counting). In addition, the US uses satellite data to prepare water temperature maps of the entire Great Lakes. As you might guess, they are °F. Since the US provides a preponderance of the available data, they may have adapted to the units we offer.

    There are a number of shore stations that also provide marine weather. A quick survey showed none observe water temperature, except the Belle Isle station in the Detroit River. With buoys pulled, the satellite data is the only Great Lakes Water temperature available until the thaw and opening of navigation.


  9. Why does that hydrant sign have the pipe diameter measured in millimetres, but the distance in moles?


  10. jgh’s comment

    The SI symbol for the mole is ‘mol’.
    Are you referring to the capital letter M?
    Of course, the distance in metres (symbol lowercase m) should have been used.


  11. The UK is still in the EU (European Union, previously called The Common Market) but may or may not leave.
    The EU wanted the UK to be fully metric but a decision by the UK to have a choice; hence one example with roads-signs still being in Miles rather than Kilometres.


  12. Ray O:

    The truth is that the UK started its programme of conversion to metric units several years before we joined the European Economic Community. The UK itself had every intention of completing that process. The EEC gave the UK various opt-outs and periods of time to complete the change which the UK had itself initiated. But give a country enough rope and it will hang itself. And that is precisely what the UK seems to have done with the fine mess it has landed itself in with its measurements. And if not hanged itself, certainly tied itself up in knots. I hope I live long enough to see the nation reunited with or without Brexit and to see it have and use the measurement units children are taught in school.


  13. Not sure it has been that successful I found one that had still not been converted in 2020. Also I found a Hydrant plate about 2m up a telephone post with the bottom digit indicating the Hydrant was 1m away. I looked 1 m up & down the post and no sign of the Hydrant. I eventually found it about 10m away !


  14. Having looked round the area there are several hydrants signs with imperial labels including one where the stick on metric lettering is peeling off so it has reverted to imperial. These are all in the LFCDA area.


  15. In my area there are still a number of old imperial signs. The ones that were converted used a slide-in plastic insert for some bizarre reasoning, didn’t take long for the inserts to be removed. One is painted in the middle of the road with no readings, easier than replacing the concrete post that got broken (now a nice little trip up hazard for the unwary walker).
    However, to be fair I think the whole system would be available to the services via an in-cab display (one would hope, 2020?) so the road markers would now be nothing but a physical reference point.


  16. ‘ BrianAC says: 2020-06-09 at 06:42’ ,

    I am not sure how it works in practice but when the appliance requires more water there may be only one or two people available to run down the road with the standpipe, key & hoses. Thus they need an individual GIS to tell them where the hydrant actually rather than relying on the one in the fire engine that may be up to 90 m away.

    Also best not to rely on GPS for your GIS as there are circumstances where it can fail or be deliberately interfered with. A detailed map in the appliance togher with a visible marker plate seems a better idea to me.


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