# Repair or Replace

> Fix it or buy new? The 50% rule plus a cost-per-year check, with a clear verdict. Enter the age and expected lifespan of a broken item, the repair quote, the replacement price and running costs. The tool applies the common 50% rule of thumb and compares the cost per remaining year of repairing versus replacing, including the risk of another failure, and gives a reasoned verdict.

Part of usefulweb.site (https://usefulweb.site/). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://repairorreplace.usefulweb.site/ (interactive tool at https://repairorreplace.usefulweb.site/tool)
- JSON API: POST https://repairorreplace.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://repairorreplace.usefulweb.site/api/v1/tool)
- OpenAPI: https://repairorreplace.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://repairorreplace.usefulweb.site/mcp - tool `repair_or_replace`

## Inputs
- itemName (text): Item
- currency (text): Currency - Shown in results only; no conversion is performed.
- itemAgeYears (number): Age of the item
- expectedLifespanYears (number): Typical lifespan - Typical total life of this kind of item; also used for the new one unless set below.
- replacementPrice (number, required): Price of a new replacement
- repairQuote (number, required): Repair quote
- lifeAfterRepairYears (number): Expected extra life after repair
- newItemLifespanYears (number): Lifespan of the new item (optional)
- runningCostOldPerYear (number): Running cost of the old item - Energy, water, consumables.
- runningCostNewPerYear (number): Running cost of the new item
- failureProbabilityPercent (number): Chance of another failure after repair - Your estimate for the extra life period; an assumption, not a statistic.
- nextRepairCost (number): Cost of that next repair (optional) - Defaults to the current repair quote.
- ruleThresholdPercent (number): Rule-of-thumb threshold

## Method
- 50% rule of thumb: if the repair quote is more than the threshold (default 50%) of the replacement price, replace; otherwise repair. This is a common heuristic, not a law or standard.
- Expected extra repair cost = P(another failure) × next repair cost (defaults to the current quote).
- Repair cost per year = (repair quote + expected extra repair cost) ÷ expected extra life + running cost of the old item per year.
- Replace cost per year = replacement price ÷ lifespan of the new item + running cost of the new item per year.
- Break-even repair quote = (replace cost per year − old running cost) × extra life − expected extra repair cost (floored at 0).
- Final verdict follows the cost-per-year comparison, because it accounts for remaining life, failure risk and running costs; the 50% rule is shown as a cross-check.