The Supply Ceiling

The supply ceiling is the name Treaties & Tradeoffs uses for an analytical frame: the limit of distance and time that an army or a trade route could sustain, worked out under assumptions that are stated one by one. It is used to check which routes or options were physically possible with the means at hand.

The assumptions behind it

A supply ceiling is only as good as its inputs, so every article that uses the term lists them, each with a source:

• how many people had to be fed;
• the ration each person needed;
• the carriers available, such as porters, pack animals, wagons or ships;
• how much each carrier could move;
• the fodder the animals themselves consumed;
• what could be found or bought along the way.

From those inputs follows a limit: how far the force or the goods could travel, and for how long, before the carriers would consume a large share of what they carried. Change an assumption and the ceiling moves, which is why the assumptions are always shown.

Where the term applies

The supply ceiling applies only to the movement of material goods that can be counted and weighed, such as grain, fodder, fuel and cargo. It says nothing about morale, ideas or money, and this site does not stretch it to cover them. When an article compares the spread of an idea or the flow of funds to a supply line, that is a figure of speech, and the supply ceiling is never applied to it.

A ceiling is also a limit, not an explanation of an outcome. Showing that a route was near its limit does not show that supply decided a campaign. Articles that use the term set the calculation beside the other conditions of the case, such as season, terrain and the choices of the people involved, and name the counterexamples.

When the term is not used

If the assumptions cannot be found in sources, the ceiling cannot be worked out, and the article does not use the term. A rough guess presented as a limit would be exactly the kind of unsourced number this site avoids. See How a replay works for how figures and units are handled, and the known-then test for the wider method of judging decisions on what was known at the time.

Where to see it worked

The Roman legion replay works out a ceiling of time only: how many days of grain a foot soldier carried on his back. The Berlin Airlift replay works out a ceiling as tonnes a day the aircraft could lift, with the flying hours it assumes set out one by one.