A magnum is one bottle in your count and two at dinner. A half bottle is one record and half a decision. Formats break the assumption that a bottle is a bottle, and they break it in three separate places: the count, the rack and the drinking window.
The names are regional, the milliliters are not
Magnum is the one name that means the same thing everywhere: 1.5 liters, two standard bottles. After that the naming splits by region. A three liter bottle is a double magnum in Bordeaux and a jeroboam in Champagne and Burgundy. A Bordeaux jeroboam is larger again, and six liters is an Imperial in Bordeaux and a Methuselah in Champagne. Both Sotheby's and Decanter publish the naming tables, and they disagree with each other in exactly the places the regions do.
This is why the practical rule is to record the volume, not the word. Put 1500 or 3000 in the size field and keep the name in a note if you enjoy it. A number sorts, filters and adds up. A name has to be looked up before it can be trusted.
What a format does to your count
Keep one record per format. Six 750 ml bottles and two magnums of the same wine are two lines, not nine bottles, because they will be opened on different occasions and they may not even taste the same on the same day. The reasoning is the same as for a case split between rooms, covered in the split case guide.
Notice which question you are asking. "How many bottles do I have" and "how much wine do I have" diverge as soon as formats are involved. Nine bottles that include two magnums are eleven bottles of wine by volume. For buying decisions the volume number is the honest one.
What a format does to your racks
Most racks, cabinets and fridge shelves are built around a standard Bordeaux bottle. A magnum will not fit many of them, a Burgundy shoulder eats the neighboring slot, and half bottles roll around in a space made for something bigger. The capacity guide covers measuring this before buying a cabinet.
Give large formats their own location rather than pretending they live in a row. A shelf, a box or a floor space with a name is easier to keep accurate than a row you have to remember is really only half a row.
What a format does to the wine
Larger bottles are generally held to develop more slowly, and half bottles faster, because of the ratio between wine and the small amount of air under the closure. Treat that as a reason to check a large format later than the standard bottle rather than as a guarantee about a particular wine.
If you keep drinking windows, adjust them per format rather than per wine, and record why. The drinking window guide covers keeping an estimate visible as an estimate.
Wine and vintage: Volume in milliliters: Format name, if you want it: Location that suits the shape: Window compared with the 750 ml version: Glasses it will pour:
Serving, in glasses rather than liters
A 750 ml bottle pours five generous glasses. A magnum pours ten, which is the real reason to open one at a table of eight rather than two standard bottles: one wine, one moment, no debate about which bottle was better. The party planning guide covers the arithmetic for a full evening.
Half bottles do the opposite job. They are the format for a weeknight, for one drinker, or for tasting a wine you are not ready to commit a full bottle to.