Convert l to ml

A litre is a thousand cubic centimetres. It is not strictly an SI unit — the SI unit of volume is the cubic metre — but it is accepted for use with SI and is what fuel, drinks and engines are measured in.

A millilitre is exactly one cubic centimetre, which is why medicine and laboratory work quote both and mean the same thing.

Going from litres to millilitres moves the decimal point 3 places to the right and changes nothing else. There is no factor to remember and no rounding to decide: the digits stay in the same order and only their position changes.

1,234 l is 1234000 ml — same digits, moved along. That is worth knowing because it is the one kind of conversion you can check at a glance: if the digits of the answer are not the digits you started with, something other than the conversion has happened to them.

The reason to leave the litre behind is that its useful range runs out at about a fifth of itself. Above 200 ml the litre is a comfortable unit and the decimals are readable; below that the numbers turn into 0.15, 0.075, 0.005, and each of those is a place where a decimal point can move without anybody noticing.

Millilitres remove the problem by removing the decimal. 150, 75 and 5 are integers of different lengths, which makes them different at a glance in a way that 0.15 and 0.015 are not. The conversion is not doing arithmetic so much as choosing a notation in which a mistake is visible.

A medicine spoon is manufactured to hold 5 ml and a dosing syringe is graduated in single millilitres, which is why medicines are prescribed at that scale rather than in fractions of a litre. Both instruments deliver what they claim, and a dose specified to the millilitre can be given to the millilitre.

A teaspoon out of a kitchen drawer is not one of those instruments. Cutlery is designed to a shape rather than to a volume and varies between sets and between patterns within a set, and the variation is a large fraction of a small dose. That is the reason dosing instructions name the supplied device: the unit conversion is exact and the household spoon is what makes it meaningless.

Solutions are specified per litre and administered per millilitre, and moving between the two is the same thousand. A stock at 5 grams per litre is 5 milligrams per millilitre; one at 250 mg per litre is 0.25 mg per ml. The prefixes do the whole conversion, which is the property that makes metric concentrations tractable.

The customary equivalent has no such property, and comparing the two shows what the prefix is worth. An ounce per gallon has to be turned into a ratio before it means anything at another batch size, while a figure per litre already is one. Anything specified per litre can be scaled to any volume by multiplying, with no factor to look up.

Serving sizes cluster on the divisors of a thousand for a reason. A litre gives exactly five 200 ml servings and exactly four at 250. At 330 ml it gives three with 10 ml left in the bottle, at 175 ml it gives five with 125 over, and at 150 ml it gives six with 100 over.

That remainder is why bottle sizes and serving sizes were chosen together rather than independently. A 750 ml bottle is six 125 ml servings; a 1.5 litre bottle is six 250 ml ones. Where a portion size does not divide the container, somebody is going to be left with an amount too small to serve and too large to discard, and the arithmetic above is what predicts it.

Dilutions are where the millilitre figure has to be exact rather than approximate. A one-in-twenty dilution from a litre of stock takes 50 ml of stock made up to a litre; one in two hundred takes 5 ml. At the second of those, an error of half a millilitre is ten per cent of the active quantity, and half a millilitre is well within what a jug delivers.

The instrument therefore has to match the number. Volumes above about 100 ml can be poured into a graduated vessel, between 10 and 100 ml a measuring cylinder is the honest tool, and below 10 ml a syringe or pipette. Converting the litre figure into millilitres is what makes it obvious which of the three the quantity requires.

A litre bottle holds a nominal litre rather than a measured one. Packaging in Europe and the UK marked with the estimated sign — a small ℮ next to the quantity — is filled under the average quantity system: the batch must average the declared volume, and individual packs may fall below it by a defined tolerance.

For anything domestic the difference is irrelevant, and for anything measured out of the bottle it is worth knowing which end of the arithmetic it lands on. Dividing a litre bottle into four 250 ml portions assumes exactly a thousand millilitres are in it, and the last portion is the one that absorbs whatever the fill tolerance was.

A litre figure that was itself a conversion carries its rounding into every portion. A capacity given as 4.7 litres was probably five US quarts, and dividing 4,700 ml into doses spreads a 32 ml rounding error across all of them. A figure given as 3.8 litres was a US gallon, and the real number is 3,785.4.

The rule that follows is to divide from the most precise figure available rather than from the tidiest one. Where the litre number is the original measurement, use it; where it is a rounded translation of something customary, go back to the customary number, convert once, and divide the result.

The two phrasings describe different quantities and the difference is exactly the volume of everything else in the vessel. Making a solution up to one litre means adding liquid until the total reads 1,000 ml, so 50 ml of concentrate takes 950 ml of water. Adding a litre to 50 ml of concentrate gives 1,050 ml and a solution five per cent weaker than intended.

Volumes are also not perfectly additive when the liquids are dissimilar. Mixing alcohol and water produces slightly less volume than the two parts summed, because the molecules pack more closely together, so a mixture measured by adding components and one made up to a mark are not the same even in principle. For anything domestic the effect is beneath notice; for anything specified to a concentration, making up to the mark is what the instruction means and the only way to hit the figure.