Cost Per Decade

The buy-it-for-life buyer's actual question is narrow and answerable: is paying four times as much once cheaper than rebuying?

Across the competitor pages we fetched while researching this site, not one publishes cost-of-ownership arithmetic, a warranty comparison, or an assessment of whether spare parts can actually be bought. Those are the three inputs to that decision, and all three are public documents. So this page is the answer, with the working shown.

The formula

Cost per decade = live price / comparison horizon in decades

That is it. The interesting part is not the division; it is where the horizon comes from and what it does and does not mean.

The horizon is a comparison basis, not a prediction

This is the part to understand properly, because it is what stops the number being a guess dressed up as arithmetic.

We are not predicting how long your tool will last. Nobody can do that, and a site that claimed to would be inventing the most important input. The horizon is a normalization constant assigned from cited evidence, so that every tool on a page is divided by the same basis when the evidence is the same, and by different bases when the evidence differs.

What that buys you is a like-for-like comparison. A tool with a published lifetime warranty and a tool with nothing published are not the same proposition, and the horizon is how that difference becomes visible in a number instead of staying in prose nobody reads.

The tiers

One definition we apply differently, and publish

“Renewable wear part” means one of two things: either replacement parts are genuinely purchasable (a plane iron, a saw blade), or the tool has no separately consumable part at all.

That second clause matters. A chisel is monolithic: the edge is the tool and sharpening renews it. Scored the usual way — “are spare parts listed?” — a chisel would rate as high rebuy risk for having no spares to sell, which is precisely backwards. It has none because nothing about it wears out separately.

We publish that definition rather than applying it quietly, because a rating system whose rules are hidden is not a rating system, it is an opinion with a number attached.

Worked example

Take two bench planes. Plane A is $220 and its maker publishes a limited lifetime warranty on the casting: tier A, 30-year horizon, so $220 / 3 decades = about $73 per decade. Plane B is $28 with no warranty and no parts listing found: tier D, 10-year horizon, so $28 / 1 decade = $28 per decade.

The cheap plane wins on this metric, and that is not a bug.It is the number telling you the truth: if the cheap plane really does last ten years, it is cheaper per year than the expensive one. The reason to buy the expensive one is not cost per decade. It is that it works better today, needs less setup, and carries a rebuy risk of Low against the cheap plane's High — which is the same evidence saying the ten-year figure is a floor value with nothing behind it.

A metric that always agreed with the expensive option would be a marketing device. This one does not, and that is how you can tell it is measuring something.

Rebuy risk: the number's companion

Cost per decade tells you what a tool costs per unit time if it lasts its horizon. Rebuy risk tells you how confident the evidence is in that horizon.

• Low — lifetime warranty, and renewable wear part, and resharpenable.
• Medium— two of the three.
• High— one or none.

Read them together. A low cost per decade with a High rebuy risk is a cheap tool with nothing documented behind it. A higher cost per decade with a Low rebuy risk is a tool the evidence actually supports.

What this cannot tell you

Four things, stated plainly because a metric without limits is a sales tool.

Whether the tool is any good. Cost per decade knows nothing about how a plane cuts. A well-made tool and a badly made one at the same price and tier post identical numbers.

How long your specific tool will last.That depends on how you use it, store it and maintain it, far more than on the maker. Storage in particular does more for the fifty-year outcome than purchase price does — see rust removal and prevention.

The value of using a good tool. Thirty years of pleasant work with a plane that fits your hand is not in the arithmetic, and it is not nothing.

Anything, when there is no live price. If our price data is stale the figure is suppressed rather than estimated, because computing it from a remembered number would make it fiction.

Why we can be confident in the inputs

Every input is a document you can open. The price comes from the Amazon Product API, date-stamped, and vanishes after 48 hours. The warranty term comes from the maker's own warranty page, cited with the date checked. The parts availability comes from the maker's parts listing.

A reader can re-derive every number on this site. That reproducibility is the whole trust argument, and it is a stronger one than a “why trust us” panel — because you do not have to.

The warranty research that feeds the tiers is on warranties and repairability, and the full method is on the methodology page.