Productivity & time laws · law 088
Ninety-Ninety Rule
By Shivam Bhandari · Reviewed 31 October 2026 · How we write
what it means
The first 90% of a project takes 90% of the time; the remaining 10% takes the other 90%.
How Ninety-Ninety Rule works
The joke adds up to 180% on purpose. The last details — testing, edge cases, polish, approvals — are routinely underestimated and take as long as everything before.
For any project, plan seriously for the ‘last mile’ rather than treating it as nearly done.
where it comes from
Credited to Tom Cargill of Bell Labs and popularised by Jon Bentley's ‘Programming Pearls’ column in Communications of the ACM in 1985.
Ninety-Ninety Rule examples in business
Software
An app is ‘almost ready’ for months while bugs and edge cases are fixed.
Store fit-out
The shop looks complete, but signage, licences, and final inspections take weeks.
Content
A report draft takes a week; editing, design, and sign-off take another.
How to apply it
- 1Plan explicitly for testing, polish, and approvals.
- 2Define ‘done’ clearly at the start.
- 3Track remaining tasks, not percentage complete.
- 4Launch a smaller version sooner rather than chasing perfection.
common misreading
It is an exaggeration for effect — but the underestimation of finishing work is very real.
ask yourself
What is ‘90% done’ in your business right now, and what is really left?
Frequently asked questions
What is Ninety-Ninety Rule?
The first 90% of a project takes 90% of the time; the remaining 10% takes the other 90%. The joke adds up to 180% on purpose. The last details — testing, edge cases, polish, approvals — are routinely underestimated and take as long as everything before.
Who came up with Ninety-Ninety Rule?
Credited to Tom Cargill of Bell Labs and popularised by Jon Bentley's ‘Programming Pearls’ column in Communications of the ACM in 1985.
How do you apply Ninety-Ninety Rule in business?
1. Plan explicitly for testing, polish, and approvals. 2. Define ‘done’ clearly at the start. 3. Track remaining tasks, not percentage complete. 4. Launch a smaller version sooner rather than chasing perfection.
What is a common misunderstanding of Ninety-Ninety Rule?
It is an exaggeration for effect — but the underestimation of finishing work is very real.