Tolerance Stackup Calculator

Work out how much slack — or interference — a chain of toleranced dimensions leaves once they're all assembled together. Enter each contributing dimension, and get the assembled nominal size plus both a worst-case and a statistical (RSS) tolerance range, with a breakdown of which dimension is actually driving the variation.

How to use it

1. Pick your units — millimeters or inches. Every value you enter below is in that unit.
2. List the dimension chain, one dimension per line: Label, Direction, Nominal, Tolerance.
   • Direction is + if the dimension adds to the stack, - if it subtracts — for example, a bore diameter is + while the shaft and any parts filling that bore are -.
   • Tolerance is the ± half-width. A dimension toleranced ±0.05 mm is entered as 0.05.
   • The label is optional — leave it out and the row is just numbered.
3. Read the Stackup Result for the assembled nominal size and both tolerance ranges, and the Contributor Breakdown to see which dimension eats the most of the total variation.

Worst-case vs. statistical (RSS)

Worst-case tolerance is the simple sum of every contributor's tolerance — the range the assembly can never exceed, because it assumes every dimension lands at its limit at the same time. It's the safe, guaranteed bound, but it gets pessimistic fast as the chain grows.

RSS (root-sum-square) tolerance is √(t₁² + t₂² + t₃² + …) — the statistical range assuming each dimension's tolerance is an independent, normally distributed ±3σ band. It's almost always tighter than worst-case, because it's extremely unlikely every part in a chain lands at its extreme simultaneously. RSS is the number most designers actually use for interchangeable-parts production; worst-case is what you fall back to for safety-critical or low-volume fits where every unit has to work.

Reading the contributor breakdown

Each row shows two shares:

• Worst-Case Share — that dimension's tolerance as a percentage of the total worst-case tolerance. Linear; a tolerance twice as wide gets twice the share.
• RSS Variance Share — that dimension's squared tolerance as a percentage of the total variance. Because RSS squares each tolerance before summing, the widest tolerance in the chain dominates the RSS range far more than it dominates the worst-case range — this is the number to look at when deciding which dimension to tighten to shrink the stack.

Limits

This assumes every dimension in the chain is independent (no correlated errors between contributors) and, for RSS specifically, that each one is centered on its nominal with a roughly normal distribution — reasonable for a mature, capable process, optimistic for anything running near its control limits. It's a design-stage sanity check, not a substitute for a full GD&T tolerance analysis on a safety-critical stack.

Privacy

Everything runs in your browser — no dimensions or tolerance data leave your machine.

Related tools

For sizing an interference fit once you know the stack, see the Press Fit Interference Calculator. For the weight of the parts in the chain, use the Plate Weight Calculator.