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.






Why What3Words is not suitable for safety critical applications