That Brexit bill might be more than £39 billion (but it also might be less)
The problem is, economies are unpredictable. Which means big bills like this are too.
In the latest you-won't-know-til-you-know reveal from the Brexit negotiations, it turns out the Brexit divorce bill UK and EU politicians spent ages negotiating over may still change in amount over the next two years – for reasons that neither the UK nor the EU can really control.
What it means: Basically, although there are lots of sophisticated ways for economists to estimate how things are going to go in the next few days / months / years, you can't predict with 100% certainty what's going to happen in any economy, because, well, that would be like fortune-telling and you just can't do it.
Unfortunately, that applies to the value of the Brexit bill as much as anything else. A watchdog called the National Audit Office has pointed out today that all sorts of factors could still change the final amount. For one, if the value of the pound (or the euro) changes dramatically over the next year, that could shift the total. Secondly, part of the bill is made up of money the UK will owe the EU for their annual budget in 2019/20 – but the amount each country owes is based on how strong its economy is, so depending on how Britain's doing next year, that could shift too.
Slightly creepily, another factor is the life expectancy of EU officials. The biggest portion of what the UK owes the EU is pensions for them, so obviously the cost of that depends on how long they live. Economics just got a little too personal.