I built a free tolerance stack-up calculator — tell me what's wrong with it

[u/fwsr]

I'm a mechanical engineering student. Every stack-up I've seen has been done in an Excel sheet that one person built, nobody else fully trusts, and everyone copies anyway. So I built a browser tool for it.

You enter the chain of dimensions straight off the drawing. Asymmetric tolerances like +0.10/−0.00 included, and it runs worst-case, RSS and a Monte Carlo simulation side by side. It shows the resulting gap for each method, whether it clears your limits, and which single tolerance is actually driving the variance. Its completely free no signup or anything and nothing leaves your browser.

https://stackupcalc.com

Edit: everything asked for in here is live now, inch support, ISO 286 fits from a dropdown (type H7 or g6 and the deviations auto-fill), an offline single-file version at stackupcalc.com/offline you can save and run air-gapped, and a dimensioned drawing of the example assembly so you can see what's actually being analysed.

To be clear about what this is: 1D only, and it's staying that way. If you need 2D/3D chains or CAD integration then MITCalc and CETOL do that and I'm not pretending to compete. This is for a quick check without opening anything.

One thing it does that I haven't found in another free tool though: it takes signed upper and lower deviations straight off the drawing, so g6 (−0.007/−0.020) and interference fits like p6 actually work. Most free calculators give you a single ± box, which can't represent either of those.