How accurate is this calculator?

The calculator output is a budgeting range, not a quote. The low, high spread (-20% / +50%) intentionally absorbs the things you can’t see from outside your codebase: undocumented customisations, vendor extensions that have gone silent, hidden cron-orchestrated integrations, and the dependency-resolution chaos that only surfaces when you actually run composer update.

From my own data on 200+ shipped upgrades:

• ~78% of projects landed inside the calculator’s low, high range without a change order.
• ~17% needed a small change order (median +12%) usually for an extension swap-out the audit caught.
• ~5% needed a substantial replan, almost always when the client had hidden a major customisation from the audit step (rare, but it happens).

To turn the calculator output into a hard number, run a paid 1-3 day audit ($1.5k, $3k). Post-audit my fixed quote falls within ±5% of the audit estimate.