Three costs, and the smallest one is the one people name
Ask anyone what it costs to hold a piece of stock for a month and they will talk about storage. On the figures the page opens with, storage is $6.00 and it is not the largest line: twenty minutes a month of relisting, renewing, answering and declining comes to $7.33 at a $22 hour, and beats the shelf. The cash inside the item adds $0.90 on top, which is small only because $45 is tied up — that is the line that grows fastest as the pieces get more expensive, and on a $400 item held at the same rate it dominates the other two.
Adding them up gives a figure per month, and that figure is the honest price of patience. It is also why the same item can be worth holding in a quiet season and not worth holding when you have four sales coming up and nothing to spend at them: nothing about the item changed, the value of the tied-up cash did.
The probability is already in the decision
Leaving the odds field out would not remove the assumption, it would set it silently to one hundred per cent. Nobody actually believes that, which is why the page asks. The weighted line then does something the head does badly: it prices the outcome where the higher number never turns up and you take less than you were offered today, months later, having paid to keep it in the meantime.
That fallback price is the field worth thinking hardest about. It is frequently lower than the offer currently on the table, because the buyer who made that offer has bought something else by then and the season has moved on. If you find yourself entering a fallback higher than today offer, ask what the mechanism for that is.
What the break-even price is telling you
The page prints the price the higher outcome would have to reach, at your own odds, for the wait to come out level with taking the offer now. It is a more useful number than the difference, because it is directly comparable with something you can observe — you can look at what similar items have actually sold for and see whether that price shows up. If it does not appear in the sold listings at all, the wait is not being priced against a market, it is being priced against a hope.
None of this is a recommendation. There are entirely reasonable reasons to hold something the arithmetic says to sell, including that you will use it, that you enjoy owning it, or that you think the price estimate here is wrong. The page prices one of the considerations, not all of them.
Related pages
Inventory turnover does this at the level of a whole stock rather than one item, and days-on-hand is the aggregate version of the same idea. The reorder calculator covers the opposite mistake, holding too little. The buy price ceiling is where the tied-up cash figure comes from, and storage unit sizing is worth a look if the space line is real rather than notional.
Questions people ask
What should I put for what the cash is worth per year?
What you would actually earn by putting the money into the next purchase instead, expressed as a yearly percentage. If a typical buy returns half of what you put in and you turn it over three times a year, that figure is high, and the holding cost of a slow item is correspondingly high. If the money would sit in an account doing nothing while you wait, set the field to zero — that is a legitimate answer and the page will simply drop the line. The default is a placeholder, not a rate.
Is the price I paid a loss I should be counting?
No, and the page deliberately does not subtract it from either side. It is spent either way, so including it would move both columns by the same amount and change nothing about the comparison. It appears once, as the cash tied up, which is a different thing entirely — not money lost but money unavailable. The two get confused constantly, usually in the direction of refusing an offer because it is below what was paid, which is a fact about the past rather than about the offer.
How do I estimate the chance the higher price happens?
The most concrete way is to look at what actually sold rather than what is listed. If the price you are holding out for shows up regularly in completed sales for the same thing in the same condition, your estimate can be high; if it appears once and everything else clears well below, it should be low. Whatever you choose, the useful exercise is to run the page at your estimate and again at half of it, because the difference between those two answers tells you how much of the plan is resting on the guess.
Does this tell me to sell or to hold?
It does not, and it is not built to. It prices one consideration — money, over the period you named, at the odds you gave — and there are others it cannot see. You may want the item. You may know something about the buyer pool that has not reached the numbers. Your estimate of the higher price may be better than any calculator input. What the page does is stop the holding cost being invisible, because it is the only part of the comparison that never sends a bill.
Why is the relisting time worth more than the storage?
Because storage is often shared and marginal — an item on a shelf you already own and already heat costs close to nothing extra — while the twenty minutes a month is time that could not be spent on anything else. Over a year that is four hours per item, and the person holding thirty items is holding a part-time job they did not apply for. That is the specific reason a large slow stock feels heavier than its storage bill suggests it should.