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Home » ChatGPT versus PTC Mathcad: units, reliability and rounding

ChatGPT versus PTC Mathcad: units, reliability and rounding


    The Problem

    Recently, a European colleague asked American teammates how to express the dimensions 3’ 1/8” x 3’ 1/8 (a 36.125-inch square) in centimetres. She explained that American metrics made her feel lost. It was a very simple unit conversion request. The answer is 91.7575 cm x 91.7575 cm.

    ChatGPT attempt one

    Another colleague took to ChatGPT, the viral artificial intelligence (AI) chatbot, searching for an answer to this simple question. This was the output he received:


    ChatGPT’s confident tone in its first response led both of my colleagues to believe 93.98 cm was correct. However, 36.125 inches, equivalent to 93.98 centimetres, assume a conversion rate of 2.6015 centimetres for one inch. This is factually wrong, and ChatGPT made the same conversion error twice in one sentence. One inch is equal to 2.54 centimetres. I noted my colleague’s use of ChatGPT and its incorrect response.

    ChatGPT attempt two

    As a big believer in ChatGPT’s prowess and usefulness, my colleague asked ChatGPT to modify the answer:


    Unfortunately, ChatGPT’s second response did not help. With 93.66 cm, ChatGPT now believes that the conversion ratio of one inch to centimetres is 2.5923 centimetres per inch. That’s still incorrect.

    In attempt two, ChatGPT does recognise that it has made a mistake, but that begs the question of why it is giving out faulty information in the first place. It then gave out another erroneous answer as a correction. Clearly, ChatGPT doesn’t know what mistake it made to begin with. I wouldn’t count on AI taking over engineering calculations any time soon.

    ChatGPT attempt three

    My colleague believed that the third time would be the charm and rephrased the question to:


    With his third bizarre response, my colleague finally admitted that his “confidence in ChatGPT JUST DIED!” Remember, ChatGPT has now provided four different answers to the same problem: 93.98 cm, 93.66 cm, 91.7675 cm and 93.6775 cm in the same answer. All of those are wrong. In the end, ChatGPT never provided the correct answer of 91.7575 cm.

    ChatGPT is correct that 3 feet and 1/8 inches are 36.125 inches. ChatGPT is also (finally) correct that the conversion rate is 2.54 cm/inch. However, ChatGPT is incorrect that 36.125 inches * 2.54 cm/inch= 91.7675 cm. (Remember, it’s 91.7575 cm.)

    Evidently, ChatGPT totally goes off the rails when it tries to “round to the nearest hundredth” and to the nearest tenth, even though it wasn’t prompted to do so. It also appears to use this new, rounded answer as the basis of its 93.6775-centimetre answer. Apparently, 91.7675 rounded to two decimal places is 93.68. This is wrong on both sides of the decimal place. What a mess.

    PTC Mathcad Prime: Only One Try Needed

    It’s time to turn to real math software: PTC Mathcad Prime. Unlike ChatGPT or Excel spreadsheets, Mathcad Prime is unit-aware and balances units. It has unit conversion ratios built in, and you can rely on Mathcad Prime to give you the correct result the first time, as tens of thousands of businesses and universities already do today.

    Unlike ChatGPT, if you recalculate the same Mathcad Prime worksheet multiple times, you’ll get the same consistent, correct response every time.


    Mathcad knows that 2.54 centimetres go into one inch and that three feet plus 1/8 of an inch are 36.125 inches. Mathcad knows that it can be converted to 91.7575 centimetres, and it will display the length in centimetres just by changing which unit is at the end of the expression, whether it’s inches or centimetres.

    Mathcad customers often express how useful it is to work with purpose-built engineering and mathematics software that recognises units, including the potential to catch errors if there is a unit mismatch. With Mathcad Prime, the software catches your errors and provides clear feedback on what those errors are—a world of difference compared to my colleague having to plead with ChatGPT to stop making errors.

    Regarding rounding numbers, in Mathcad, you can change the Display Precision of a math expression in the Math Formatting tab, which will change the decimal places displayed in the results if you want only one or two decimals. Display Precision doesn’t affect the accuracy of the results if you perform further calculations. If you want to affect future precision by rounding the result, you can use Mathcad Prime’s built-in rounding and truncation functions, as shown in the screenshot above.

    Our customers often rave about Mathcad’s document-oriented approach and natural math notation, making it easy for their stakeholders to read through a Mathcad worksheet and understand every step of the process. The same can’t be said for “black boxes” software, like an Excel formula with several cell references or ChatGPT with whatever process it used to come up with four uniquely wrong answers.

    Use software for the purpose it was built for!

    Source: PTC