 In this video we'll continue to look at unit conversions but we'll try some more complex examples. So let's take this example. At 25 degrees Celsius water has a density of 0.997 grams per milliliter and we want to convert this density to grams per litre. So here we're converting from a compound unit of grams per mill to another compound unit, grams per litre. This may seem a bit weird and different to our other examples but exactly the same conversion principles apply. First we need to write the value that we know as a fraction along with its unit. Here the compound unit is grams per milliliter. Per means divided by. So grams per milliliter is more properly written like this. So we'll put the value up the top and the compound unit next to it with grams on top and millilitres underneath. Now we don't need to change the grams, we just want to change the denominator from mills to litres. So what we need is the conversion factor from mills to litres. Now we know that 1000 millilitres is equivalent to 1 litre and we want to cancel out the millilitres so they're going to go on the bottom. And now we cancel our units and we complete the calculation and we get that the density is 997 grams per litre. Which if you think about it makes sense. If 1 milliliter has a mass of 0.997 then 1000 millilitres, that's a litre, must have a mass of 1000 times that which is 997. It's always good to do a little common sense check of your calculations when you've done a conversion.