How much slack should you add?

Estimates run long — that's Parkinson's lawParkinson's law: work expands to fill the time allotted to it. Slack in an estimate tends to get used up, so tasks rarely come in early even when they could., not bad luck. Instead of guessing a padding factor, feed in how past tasks estimated versus how long they really took, and read off a buffer sized to your own track record.

Past tasks
TaskEstimateActualSlip
1.6×
1.1×
2.33×
1.13×
1×
1.67×
Your buffer
Confidence80%
+67%Buffer · p80The slip ratio at your chosen confidence. At 80%, this many of your past tasks would have finished inside the padded estimate.
1.67×Multiply by
5 / 6Ran over estimate
Aestimate → plan for16.7 days.

Your typical task slips to 1.36× of its estimate; at 80% confidence you'd budget 1.67×. Push the confidence higher to cover more of the bad days — at the cost of more padding you may not use.

buffer = the slip ratio at the confidence you pick · padding from your own history