There are few things more frustrating that getting half way through a recipe to find half of the ingredients listed in milliliters instead of ounces – forcing you to wash off your hands, locate a phone and google a quick conversion – but why do we do use two different units of measurement in the USA? In fact - the USA officially only uses the imperial system, making it only one of three countries in the world who aren’t using or taking steps towards converting to the “metric” system that is used by the vast majority of the world. Due to this, most things you buy in the US will be listed with both the metric and imperial measurements – but when it comes to things such as cooking at home – any European, Mexican or Asian cookbooks, for example, will provide the measurement information in the metric system, potentially posing a dilemma for budding chefs who aren’t also a dab hand at mental arithmetic. Where do Ounces Come from? The use of the fluid ounce began in Scotland and England to measure wine, water and ale – however the size of the ounce tended to vary based on the system of fluid measure, creating a number of different “named” ounces including; the tower, troy and avoirdupois ounce. The old system was flawed further, with the use of “allowances” which meant “a unit of measure was not necessarily equal to the sum of its parts” – due to an age-old-custom of allowing for the guessed weight of the package or packing materials. In 1824 – to put a stop to the confusion - the British Parliament decided the “Imperial Gallon” would be equivalent to the volume of ten pounds of water. The imperial gallon can be divided into four equal parts – or quarts – that could be further divided into two pints, the pints into four gills and the gill into five ounces. 160 imperial fluid ounces make up one imperial gallon – therefore making an imperial fluid ounce of water equivalent to around and later, after the introduction of liters, However, the American government decided to stick with the older measurement which was based on the old wine gallon used in England prior to changes in 1824 and contains around 231 cubic inches – this unit of measurement is now known as the “US gallon”. Due to this slight difference the US fluid ounce is around 4% larger than the imperial fluid ounce, measuring around rather than the imperial – however, as the use of the imperial ounce around the world has decreased, the US gallon is often the measurement in question. So why liters, centiliters and milliliters? When reading recipe from outside of the US it is unlikely you will find anything not in the “metric” or “modern” system, however, milliliters and liters have been around a lot longer than the imperial system that is often considered to be the “old” form of measurement. First derived in France in 1795, the liter was introduced as a new “republican unit of measurements” and equal to one cubic decimeter. One liter is the equivalent of the volume of a cube with 10cm sides – and the mass is almost exactly the same as a one kilogram. Whilst the official system in the US is the United States customary units – which is primarily based on the imperial system – due to the use of the metric system in the majority of the rest of the world, most things will be listed with both measurements, including; speed, liquids, body weight and length. What is the Easiest way to Convert OZ to ML? You never know when you might need to convert OZ to ML – or the other way around – but it is a handy trick to have up your sleeve (especially if you spend a lot of time outside of the USA). Once you know the conversion, you be able to make an educated guess on the conversion – even when you don’t have a calculator or Google to hand. ounces go into one milliliter – or one ounce is equivalent to milliliters. The easiest way to convert ounces to milliliters, is to round the ounce up from to 30ml – and multiply it by the number of ounces you have – this will let you know, give or take, approximately how many milliliters so many ounces is. Working it out the other way around can be a lot more difficult – rather than multiplying the number of oz by ml – you’ll need to divide it.