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Comment by Symbiote | Hacker News Reader

Any time the calculation is scaled up (or down), or involves numbers at different orders of magnitude.

If we have 2mm of rain in Copenhagen this afternoon, and my yard measures 8m×12m, how much water must drain away?

  2mm × 8000mm × 12000mm = 192 000 000mm³
                         = 192 000cm³ 
                         = 192 dm³        = 192 L 
                         = 0.192 m³

How much rain falls on the whole city?

            2mm × 180km² = 0.002 m × 180 000 000 m²
                         = 360 000 m³
                 approx. = 360 000 tonnes.

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Sure, but when do you need to do that?

What about if it isn't water you are calculating but some other substance? You use a look up table to find the factors - we need to look up a but more in the table, but it isn't a big deal.

Are you serious, they literally just gave you an example lol.

I always hear that argument about "but when will I need it?" If you're using this system then you're always going to use it. It comes up all the time, it's natural so you don't think about it too much.

Does it mean it's better? Well yes, but it doesn't mean other measuring systems aren't good. You don't need it, but you'll want it once you learn it.

He gave an example I've never needed in my life other than for trivia.

This is a good opportunity to employ steelmanning.

The first is a real example from a friend whose drains were blocked during some construction work, where rainfall from his garden was running into his basement during a storm. Would the water reach the electric cables?

The second is obviously relevant for the people responsible for rivers, storm drains, sewers, dams, hydropower and so on. When we start terraforming Venus, we'll need to add the conversion factor for sulphuric acid rain.

Maybe you don’t build very much stuff?

Parsing/reading error of the day:

> my yard measures 8m

You use a very long unit of measure.

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