# Tolerance Stack-up Calculator

> Estimate the worst-case and statistical range of an assembly gap. Add and subtract dimension chains with asymmetric deviations to calculate the worst-case and root-sum-square range of a clearance or overlap.

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://kerfwisetolerancestack.usefulweb.site/ (interactive tool at https://kerfwisetolerancestack.usefulweb.site/tool)
- JSON API: POST https://kerfwisetolerancestack.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://kerfwisetolerancestack.usefulweb.site/api/v1/tool)
- OpenAPI: https://kerfwisetolerancestack.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://kerfwisetolerancestack.usefulweb.site/mcp - tool `kerfwise_tolerance_stack`

## Inputs
- dimensions (textarea): Dimensions: name, nominal, lower deviation, upper deviation, direction - One row per dimension. Use the same unit for every row. Deviations are signed (for example -0.2 and 0.2, or -0.020 and -0.007 for a shaft). Direction is +1 for additions and -1 for subtractions.

## Method
- Add each nominal dimension with its direction. For the worst-case minimum, use lower deviations for added dimensions and upper deviations for subtracted dimensions; reverse for the maximum.
- For the statistical (RSS) interval, centre every contributor in its band, add the centres with their directions, and combine the half bands as the square root of the sum of squares. This assumes independent, normally distributed sizes whose bands are ±3σ, so about 99.7 % of assemblies fall inside the interval.
- All dimensions use the same unit. Neither interval accounts for geometric tolerances, temperature or assembly deformation.

## Limits
- Planning estimate only. Confirm tolerances for the actual design and manufacturing process.