How much water would it take to fill St. Paul's Cathedral?

  | James Innes


How much water would it take to fill St. Paul's Cathedral?

Alternative and related questions:

How many bottles of red wine are drunk in France on Christmas Day?

The meaning behind the question:

This is definitely not a general knowledge question! It is a question specifically designed to test your reasoning skills. The interviewer wants to see how you approach the problem, how able you are to identify the relevant factors and, having identified the relevant factors, how you use them to calculate your answer. They’re not expecting you to be able to give them a precise figure. They’re mainly looking to see how you rise to the challenge of attempting to formulate an answer.

Your answer:

You’d be forgiven for pausing for a second to think your answer through. This is most definitely not an easy question! Try to keep a clear head and identify what factors will lead you to an answer. You’re not expected to be an expert on St. Paul’s Cathedral – nor on French wine-drinking habits. The key is to try to think through the question logically and to convey your thoughts to the interviewer in an ordered fashion.


That’s a difficult question! If we assume that we have already plugged up any potential leaks then the answer primarily hinges on a precise calculation of the internal volume of St. Paul’s Cathedral. I don’t know St. Paul’s Cathedral very well but I know it’s a complicated piece of architecture. In order to answer the question reasonably precisely I’d need to see plans of the building so that I could break it up into a number of different shapes, measure them and calculate their volumes accordingly. I’d also have to make a deduction for interior furniture, etc. although I would expect that to be fairly minor.

If your work involves having to handle complex calculations of this nature then you might want to take your answer to the next level by actually having a stab at the correct figure:

At a guess, I’d say St. Paul’s is roughly 200m long, 50m wide and 50m high – making a total of half a million cubic metres. Features such as the dome will add to this figure but, likewise, internal furnishings, pillars and walls will reduce it. With 1,000 litres in a cubic metre, half a million cubic metres equates to 500 million litres. Without more precise data that would be my best estimate. Seeing the plans of the building would be useful but another technique would be to buy a scale model from a souvenir shop or suchlike. I could then determine an upper limit by plugging any leaks, placing it in water, measuring how much water it displaced and then scaling this figure up.

Word of warning:

Having answered such questions, don’t make the mistake of asking what the correct answer is. The interviewer probably won’t know and you might just make them feel a little foolish!

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