Why AI Chatbots Sometimes Get the Maths Wrong

A friend told me last week that she had asked an AI chatbot to work out her rent increase. The answer was tidy, explained and confident. Its only problem was that it was off by about 40. This happens more often than people think, and understanding why helps you use these tools more wisely.

Language models are not calculators

The language models behind tools like ChatGPT, Claude and Gemini are trained to predict the most likely next piece of text. To them, numbers are pieces of text too. When a model sees "25 percent of 1,875", it does not compute the way a calculator does; it produces an answer that resembles patterns it saw in training. The result is often right, but small slips can happen, especially in long, multi-digit or multi-step calculations.

The real danger: convincing mistakes

A calculator gives a wrong answer when you press the wrong key, and you usually notice. A language model can present a wrong result in fluent sentences that look like careful working. If one intermediate step is off, the rest of the explanation still reads consistently, so the mistake slips by. With money, such as loan payments, tax or compensation, that can turn into a real cost.

What newer tools change

Some AI products can now run code in the background or call a calculation tool when maths is needed, which makes the result far more reliable. But you cannot always see whether that feature kicked in for a given chat, and free versions or some settings may not use it at all.

So how should you use them?

Ask for the method, check the number. AI is very good at explaining a formula or working out which calculation you need. Check the figure yourself with a calculator.
Ask for the steps. Request the working, not just a single number, so it is easier to spot where something went wrong.
Do a sense check. Obvious oddities, like a 25 percent rent rise producing a figure four times the old rent, stand out. Make a rough estimate in your head too.
Leave critical figures to official sources. Tax rules, insurance contributions and compensation rules change, and the model's knowledge may be out of date.

In short

AI is a good companion for understanding what to calculate. Just do not hand it the final number. Learning the formula from it and doing the maths on a calculator gets you the best of both.