Build a small tool or keep paying for one: the cost of each over your horizon and the month the build breaks even.
Arithmetic only — not tax, accounting, legal or investment advice. Check against current rules.
Build vs buy break-even: Build cost is hours at the hourly rate. The built tool then costs upkeep hours plus hosting each year, and breaks even in the first month its cumulative cost falls to the purchase price's. Undiscounted.
Quoted from the toolbelt spec, the document this tool was built against.
Negatives are parenthesized because that’s what a statement does. The rest is four promises.
Amounts are integers in the currency's smallest unit, end to end. A float never touches a cent.
Rounded once, at the end. Rounding each operand is how a split stops summing to the whole.
The arithmetic lives in one unit-tested module, and the extension uses the same one.
There is no request to make. What you type stays in this tab and is gone when you close it.
A calculator answers the question once. These carry the answer into a product, name what it computed, and argue the method.
Worked example. Type over it.
Result
| Year | Buy, cumulative | Build, cumulative |
|---|---|---|
| 1 | 9,000.00 | 8,040.00 |
| 2 | 18,000.00 | 10,080.00 |
| 3 | 27,000.00 | 12,120.00 |
Undiscounted, and it leaves out the vendor's price rises and your own overruns. A tool built for one job is not the whole product: list what you would lose before you count the saving.