Tool
Chain Cost Calculator
Add up everything one new purchase is likely to pull in after it, and see the total in months of saving.
How to use it
Name the thing you are thinking of buying and its price. Then list what it will make look wrong: the rug that will look tired beside a new sofa, the shoes that will not suit a new suit. Give each a rough replacement price and say how likely you are to buy it. Add what you save in a month to see the total as months of saving. The result updates as you type, and the Diderot effect page explains why the list tends to grow.
The result
The form needs JavaScript to recalculate. Without it, the result below stays on the couch example.
This $1,400 velvet sofa is a $3,200 decision if you buy everything that follows.
Weighted by how likely you are to buy each item, expect to spend about $2,249, 1.6 times the sticker price.
At $400 a month, the expected total is 5.6 months of saving, and the whole chain 8 months.
The couch: an example with round, made-up prices that show the shape of a chain. They are not quotes for any real product.
The chain
velvet sofa
rug
coffee table
cushions and a throw
side table and lamp
curtains
media console
Where to stop
Buying in the order listed: what you have spent after each step, and what stopping there keeps in your pocket.
| Stop after | Spent by then | Stopping here saves |
|---|---|---|
| 1. velvet sofa | $1,400 | $1,800 |
| 2. rug | $1,750 | $1,450 |
| 3. coffee table | $2,150 | $1,050 |
| 4. cushions and a throw | $2,270 | $930 |
| 5. side table and lamp | $2,520 | $680 |
| 6. curtains | $2,700 | $500 |
| 7. media console | $3,200 | $0 |
How the numbers work
- Worst case is the price plus the full cost of every item on the list: what you pay if you buy the whole chain.
- Expected chain counts each item at a share of its cost: all of it when you are certain to buy it, 60 percent when it is likely and 30 percent when it is a maybe. The purchase itself always counts in full. These weights are our rule of thumb, not a measured probability.
- Where to stop takes the items in the order you listed them and shows the running total after each one, with what you keep by stopping there instead of buying everything.
- Months of saving divides a total by what you save in a month.
What this does and does not tell you
It shows the size of the decision before you make it, and where the cheapest place to stop is. In most chains that is straight after the first purchase, before anything that still works gets replaced to match.
It does not know your prices; every figure is yours. It leaves out costs that keep coming, such as a subscription or higher bills, unless you enter a year of them as one item. And it does not say whether the purchase is worth making. A run of purchases you plan on purpose, with a budget and an end date, is a project you chose. The seven rules that stop a chain help with the ones you did not.
Questions readers ask
Why does the calculator weight items by how likely I am to buy them?
Not every follow-on purchase happens. Some chains stop on their own once the new thing stops feeling new. Counting a maybe at 30 percent of its cost keeps the expected total from assuming the worst, and the worst-case figure beside it shows the ceiling if you do buy everything.
What should I count as part of the chain?
Anything you would replace or add because the new purchase makes it look wrong or feel incomplete, rather than because it has worn out. Things the purchase truly needs in order to work, such as ink for a printer, count too, as certain. Leave out what you would buy anyway.
Do you keep what I enter?
No. The form works in your browser and sends nothing to us; the result is in the page address so you can share it. Reloading the page or opening a shared link sends that address to our host like any other page request. Nothing goes into your browser storage.
How do I save or share a result?
Copy the page address. It carries every figure you entered, so anyone who opens it sees the same chain, and a bookmark brings it back later.