How are military deployment costs calculated?

When the US military deploys to someplace we often get cost estimates of that deployment. But those cost estimates can vary wildly. It need not be a military thing either. We can see the same when it comes to deploying civil emergency services.

My question is, do they subtract the cost they accrue to just have these people on standby from the cost of putting them into action?

For example, a firehouse has people on staff and fire equipment on hand. That cost is there even if they never go anywhere and just sit and do nothing. Same for the military. We pay soldiers whether they sit on base in Alabama or are in Afghanistan. We pay for tanks which may sit idle.

This could have some considerations closer to us. For example, imagine you are in an accident and have a medical airlift to a hospital. How do they calculate that cost to you?

As you say how exactly it is calculated varies and can give varied answers.

While I don’t know the specifics, as far as I know military personnel in active warzones get additional money. That may not be part of pay, might be called “bonus” or “benefit” or something else

Tanks don’t sit idle anyway, they are driven a certain number of times a year, even if sitting in storage.
Think of your car. There are certain costs related to its usage. You burn fuel. Wear and tear as parts breakdown and get replaced. Same with a tank, except on a much higher level.

Fire departments are a bit different, in that they probably account at the station/unit level, and the cost of going on fire calls is seen as higher fuel consumption, maybe more coffee expenses, etc… I imagine the air ambulances have a certain level of activity baked into their budget, but there’s almost certainly some cushion or mechanism to go beyond that if necessary.

For the military, there’s probably multiple very dry, very detailed cost accounting procedure manuals and memos that describe exactly how the Federal government, Department of Defense, and individual services account for all their various deployment costs. Beyond that, all of those government agencies employ legions of bean counters to determine that stuff.

If I had to guess, there’s likely a “base” cost associated with a unit, which is some combination of payroll and then some basic maintenance, fuel, etc… costs that are associated with whatever the “normal” level of activity is. That “base” cost probably doesn’t vary much, and is probably pretty much the same across units - i.e. one squadron of F-16s costs much the same as another at the same level of flying, etc… Same with companies of infantrymen, ships of the same class, etc… This is likely modified by where they’re stationed, because of environmental reasons, cost of living differences, supply difficulties, etc… meaning that a squadron stationed in say… Dallas would have lower costs than a squadron stationed in Greenland (but not deployed). And for certain types of units, a certain activity level is “baked in”- helicopters fly so many flight hours a month, ships sail so many nautical miles, and so forth.

I’ll go a bit further and guess that when a unit’s deployed, the accounting is probably done at a different level and from a different budget. So if that squadron of F-16s from Dallas is deployed to some random Polish Air Force field as a show of force versus Russia, then the accounting may be done at a Wing level or higher, and likely comes out of some sort of deployment budget meant to fund the actual deployment of the units separate from their everyday garrison costs.

So for a six month period where they’re deployed, they’re counted as consuming that “base” cost for just existing, and also whatever deployment costs they incur are counted against whatever deployment budget there is.

That’s my guess as to how it works;

Costs for a deployment include:
Transportation to and from the deployed location. This is for shipping equipment, vehicles and personnel. It will involve everything from chartered civilian airlines, fuel for military transport aircraft, contracted rail and overland shipment as well as ocean-going cargo vessels. Much of a deployment uses contract and chartered civilian transportation in addition to or in lieu of military transport.
Fuel for everything at the deployed location.
Housing costs. Even if it’s a tent, there are costs associated with housing all the Soldiers. Electricity, heat, a/c, etc.
Repair and maintenance costs.
Costs of additional supplies.

Those are just direct costs associated with a deployment. There are other costs like the additional pay and entitlements for each person, etc. These may or may not be factored into to calculations, depending on who’s asking. It’s not usually something that’s even considered, because the military unit doesn’t need to pay for that directly. That comes out of the big Army budget rather than the unit’s deployment budget.

An idle tank burns no fuel, shoots no rounds, and doesn’t break. Plus, the tank needs to actually get from Alabama to Afghanistan which costs a significant amount of money. These are some of your deployment costs. If the tank is destroyed by the enemy or rolls into a ravine, the costs will go way up compared to just sitting idle in a motorpool.