Why What3Words is not suitable for safety critical applications

What3Words is a widely promoted system that is used for sharing a location using just three words. For example, ///wedge.drill.mess is located in Hyde Park.The globe is divided into 3m squares, each of which has a unique three word address to identify it.

Many UK police, ambulance and fire services advocate the use of What3Words to report your location in an emergency. The idea is that it is easier to communicate three words than it is to read out a grid reference, and that a position is more helpful than an address in many situations.

Due to a series of design flaws in What3Words, I do not believe it is adequate for use in these safety critical applications.

I initially believed that What3Words prevented simple mistakes causing errors; it wasn’t until a friend found two addresses under 10 miles apart that I considered that this might not be true. I decided to look into it.

So how bad is the problem? Pretty bad.

Easily Confused Words

The What3Words word list is 40,000 words long. It is important that words in this list cannot be confused, otherwise they may be communicated incorrectly. For example band/banned, bare/bear, beat/beet are easily confused.

What3Words acknowledge this themselves.

The problem is that their “best” doesn’t appear to be very good.

A quick inspection of their word list finds the following words that sound very, very similar to one another:

wants         once
recede        reseed
census        senses
choral        coral
incite        insight 
liable        libel 
ordinance     ordnance 
overdo        overdue 
picture       pitcher 
verses        versus
secretary     secretory
assets        acids 
arrows        arose
clairvoyance  clairvoyants
collard       collared
confectionary confectionery
disburse      disperse
equivalence   equivalents
incidence     incidents
incite        insight
incompetence  incompetents
independence  independents
innocence     innocents
instance      instants
intense       intents
lightening    lightning
ordinance     ordnance
parse         pass
pokey         poky
precedence    precedents
purest        purist
variance      variants

There are also a huge number of plurals. Out of the 40,000 words, 7,697 also exist in their plural form. That means that 15,924 out of the 40,000 can be confused by misreading, mishearing, or mistyping a single letter “s”. That’s 40% of the available words!

Over 75% of What3Words addresses contain words that can be confused in this way.

What3Words themselves said that “people confuse plurals only about 5% of the time when hearing them read out loud”. That gives an overall chance that a What3Words address is confused 1 in 27 times!

And this doesn’t take into account simple typing errors like flip and slip.

Broken Algorithm

What3Words acknowledge that two locations with similar addresses being confused is a problem. Indeed, if ///limit.broom.flip and ///limit.broom.slip were in the same town, that would lead to confusion.

They state that their solution is to space these confusing addresses “as far apart as possible”.

As far apart as possible turns out to not be very far.

In this small blue area below, there 255 confusing address that result in another location less than 5km away.

If we increase the error to 20km, the situation gets far, far worse. There are now 3,268 locations that can be confused, just by adding or removing a character “s” to one of the words.

To put this into perspective, we can calculate the odds that you land on one such square. There are 1,456,332 individual 3m squares in that blue box.

With a maximum acceptable error of 5km, that means there is a 1 in 5712 chance you land on one of these squares.

With a maximum acceptable error of 20km, that means there is a 1 in 446 chance you land on one of these squares.

These odds vary depending on where you are, but in most urban areas of the UK that I have checked, there is a better than 1 in 1000 chance that a square has an address that can be confused with another under 20km away.

For several areas in London, there is a better than 1 in 25 chance you find a confusing pair located within the M25.

What3Words implied the odds were closer to 1 in 2.5million. There’s a very big difference between what I’ve seen and what they are saying.

The error when you make a mistake can range from as little as 10m to as much as 20,000km. There is no way to determine how far away the other address is.

It is a matter of opinion as to what an acceptable level of error is, but there are examples of mountain rescue being sent to a location almost 40km away because of a single character confusion. 20km may not be an issue in a city, but it certainly is for an accident in the mountains.

This issue is inherent in the way the What3Words algorithm was designed. You can read more about this here.


Here are some examples of how badly this could go wrong. Without significant additional information, that the person may not be able to give, you cannot determine which of the two addresses is correct.

“There’s been a train derailment, on the Clyde.” – 1.54km



“One of our party has fallen to the west of the ridge on the way up to Beinn Maol Chaluim, visibility is poor.” – 1.83km



Just to give you an idea of the terrain around here – that short distance could be a serious delay.

“I think I’m having a heart attack. I’m walking at North Mountain Park. Deep Pinks Start.” – 1053m.

(Try reading both out)




By making a single character error in a What3Words address, there is a significant chance that the location will change to another that is less than 5km away. This level of error is dangerous and difficult to detect.

In my opinion, this makes it unsuitable for use in emergency situations.

Even What3Words themselves have identified that confusing pairs would be an issue, and have stated, tens of times, that they designed the system so that they were not close together. This is simply not the case – confusing pairs frequently exist in close proximity.

You need to ask yourself why What3Words pitch their system in this way. Why state that their system doesn’t suffer from these issues, when it does? Why did they decide that this is an issue, and claim it was fixed?

The same kind of errors happen if we make mistakes with most grid reference systems, phone numbers, or addresses. This is a known issue. We read back phone numbers to check they are correct. If I read out my address as “Southampton, Scotland”, there is an obvious means of detecting an error. What3Words has been sold as immune to these errors, so people don’t check for them. This is where the risk creeps in.






18 thoughts on “Why What3Words is not suitable for safety critical applications

  1. Permalink  ⋅ Reply

    Loz Archer

    April 29, 2021 at 9:05pm

    Good discussion, and interesting examples used of local similarities.

    Perhaps the scheme could be extended to optionally calculate a fourth redundant word or character for situations like this, which is used to validate the previous three words like a checksum. That’d make an interesting project for a Masters student somewhere: what4words!

  2. Permalink  ⋅ Reply


    April 29, 2021 at 9:33pm

    And the solution is to move the market 3m to the side and report the next 3 words. Are they adjacent? Bingo, you’re good to go. 5 or 500 miles away? Get the next three words, or ask them to repeat one or other codes.

  3. Permalink  ⋅ Reply

    John Halstead

    May 2, 2021 at 4:02pm

    Use oslocate in the UK, its much easier to use, and far better for mountain rescue.

    • Permalink  ⋅ Reply

      William Taylor

      May 12, 2021 at 8:03am

      I think the main problem with that approach is that errors are very likely only discovered when the emergency team arrive at the ‘wrong’ location, unless 6 words are used as a new standard.

  4. Permalink  ⋅ Reply

    Duncan Bayne

    May 3, 2021 at 2:58am

    While solid, I don’t think the evidence you’ve presented supports your conclusion, because it neglects the failure modes of any alternatives.

    Is it _more_ dangerous than, say, reading GPS coordinates? Are errors in GPS coordinates easier or harder to detect? How does it compare with, say, trying to describe unfamiliar surroundings to a dispatcher?

    I don’t know the answers to these questions, myself. It may be that What3Words still turns out to be the wrong choice. But I don’t think anyone should decide for or against it – or any other technology! – in isolation.

    • Permalink  ⋅ Reply


      May 3, 2021 at 7:49pm

      I disagree. This isn’t about a comparison between What3Words and other systems. It’s that What3Words falls far short of the claims.

      • Permalink  ⋅ Reply


        May 6, 2021 at 11:42am

        That’s not the claim you are making, the claim is that it “isn’t suitable for safety critical applications”.

        Duncan’s point is valid – Assuming there is no perfect solution, the most suitable solution for a safety critical solution is the one that is the best compromise between various factors. So in order to establish that it isn’t suitable, we really need to establish if there is a better alternative option.

        • Permalink  ⋅ Reply


          May 7, 2021 at 5:47pm

          It really isn’t suitable. The chance of having to read out your location twice is 1 in 27 according to their own stats.

          A better system would be the one they claimed to have. It’s possible, so why not fix this one?

      • Permalink  ⋅ Reply

        Gwyn Evans

        May 6, 2021 at 12:42pm

        the problem is that you then go on to judge it as absolutely “unsuitable” – no indications of relative usefulness, simply that since there’s a chance that a misleading location might be given, it’s de facto beyond the pale and should be discarded… Really?

        • Permalink  ⋅ Reply


          May 7, 2021 at 5:46pm

          Yes, it’s not suited to emergency applications.

          You’ve read into it that it should be discarded.

      • Permalink  ⋅ Reply

        Duncan Bayne

        May 6, 2021 at 10:04pm

        I don’t think it’s valid to claim that X isn’t a suitable solution for Y, without evaluating it against other candidate solutions, including maintaining the status quo. The only claim that matters in the context of safety-critical applications is whether it’s better or worse than other available solutions.

        It’s pretty clear that What3Words *does* fall short of its claims, but that doesn’t *necessarily* make it worse than the alternatives. It’s also clear that What3Words are a bunch of litigious bullies, but that’s also irrelevant to the central claim here.

        • Permalink  ⋅ Reply


          May 7, 2021 at 5:42pm

          Another candidate solution would be what3words that met the claims they made, such as no homophones or dangerously confusing pairs.

          They created the impression that the system needs to have errors around 1 in 2.5 million and that errors needed to be on the other side of the earth.

  5. Permalink  ⋅ Reply

    Stephen K

    May 5, 2021 at 12:44am

    Check out Connect Rocket. They have a feature called locate which sends a text message to the Subject/Patient/Bystander. A simple tap on their part returns GPS coordinates within seconds. Our search and rescue team (BC Canada) uses it as do several other teams in our area. It’s very fast and efficient – has worked well for us.


    • Permalink  ⋅ Reply

      Stephen K

      May 5, 2021 at 12:49am

      There was also another tool made by a local SAR team member called Your Location. A little harder to use but another option. Believe it is free to use.


      • Permalink  ⋅ Reply

        Jeremy GH

        May 8, 2021 at 1:17pm

        Which was a developement of an app ‘SARLOC’ produced and used within the UK mountain rescue community.

        And most modern phones have ‘Advanced Mobile Location’ (in various forms) which will automatically send an SMS/text message with your location to the emergency services, when you call the emergency number (999/112/911)

Leave a Reply to Stephen K Cancel reply

Your email will not be published. Name and Email fields are required.

This site uses Akismet to reduce spam. Learn how your comment data is processed.