Everyone padded their estimate. Where did the padding go?
Ask six leads for a duration and you get six numbers with safety quietly built into each one. The project genuinely needs some of that safety. The trouble is where it sits: hidden inside individual tasks it gets spent whether it was needed or not, because it belongs to nobody and nobody hands it back. Critical chain planning takes it back into the open.
Method guide for Google Sheets Critical Chain Pro engine
What critical chain planning is
You give every task two durations. The focused duration is the fifty-fifty estimate, what the task takes when nothing goes wrong and nobody is protecting themselves. The safe duration is the padded number the lead first offered. The difference between them is that person's safety, and adding it up is usually the first uncomfortable moment.
The tool then finds the critical chain, the longest path through the plan on focused durations, and puts a project buffer at the end of it. Paths that feed the chain get their own smaller feeding buffers, so a late feeder does not push the chain itself. What you commit to the outside world is the chain plus the project buffer, and from then on you track the buffer rather than the task list.
The questions it answers
- How long is this plan really, once the padding is out of the tasks?
- How much buffer does it need, and where should it sit?
- Are we in trouble yet, or consuming buffer at about the rate we expected to?
When you do not need buffers
Skip it when your estimates are not padded, which is rarer than people think but does happen where durations are measured rather than asked for. Skip it when the plan is short enough that one buffer at the end is the whole idea and you do not need a method to size it.
Skip it when the real constraint is a resource rather than a chain of dependencies. This tool finds the chain as the longest path on focused durations and does not level resources, so if the same team is committed to two paths at once, spotting that stays with you. It also needs two honest numbers per task: if the leads give you the same figure twice, there is no safety to pool and nothing for the method to do.
A worked example
Ten activities of a data center migration, in working days, with two durations on each. The safe estimates total 117 days against 72 days of focused work, so 45 days of padding are scattered across ten tasks.
| Activities | 10, with predecessors |
| Focused durations | the fifty-fifty estimate, 72 days in total |
| Safe durations | the padded number first offered, 117 days in total |
| Buffer method | RSEM, with the 50 percent rule one click away |
The answer
One run pulls the padding into the open:
- Padded estimates implied
- 117 days
- Critical chain
- 49 days, on focused durations
- Project buffer (RSEM)
- 11.5 days
- Commitment
- 60.5 days, chain plus buffer
The padded estimates implied 117 days. The chain on focused durations is 49, RSEM sizes the project buffer at 11.5 days, and the commitment is 60.5 days with the safety in one place where you can watch it burn. Two feeding paths get buffers of their own, 8 days into migration wave 1 and 5 days into the cutover. The same plan under the 50 percent rule carries a 24.5-day project buffer, because that rule works from the length of the chain while RSEM works from the safety each task actually gave up. Neither is universal: on the single-task feeding path the order flips, and the 50 percent rule gives the smaller number.
Figures from the shipped How Much Buffer Does This Plan Need? template, reproduced by running the same code the add-on runs, under both buffer methods.
What you get back in the sheet
- A task table on the sheet: id, name, predecessors, focused duration, safe duration.
- The chain, its length, and the buffered commitment, with a feeding buffer for every path that joins the chain.
- Two sizing methods. The 50 percent rule halves the chain and ignores the safe column entirely, which is what you want when you do not trust the padded numbers. RSEM takes the root of the summed squares of the safety given up, which is leaner on a long chain and is what you want when you do.
- A fever chart. Plot buffer consumed against chain completion across your status updates and see whether the project is green, yellow or red. The zones default to yellow running from 10 to 70 percent and red from 30 to 90 percent as the chain goes from start to finish, and both boundaries are editable.
- A written reading of the result. Every run ends with a card titled “What this means”: the probability that matters, the biggest driver, and what to tighten first. On the free tier the tool writes it from its own figures. On Pro you also get an AI reading of the same figures, written by a language model, and See what was sent shows the whole payload: “Ratios, shares, counts and cell references only. No cell values, no labels, no names, no formulas.”
Free or Pro
This is a Pro engine, one of five. Every free install includes five full-quality runs of any Pro engine: the same engine with nothing switched off, at any model size, on your own numbers. The five are one allowance shared across all five Pro engines, not five for each. A run counts only once it has produced a report, so a cancelled or failed run costs you nothing.
After that, this engine asks you to upgrade and nothing else does. Every statistics, forecasting and machine-learning tool stays free on every plan, and optimization and what-if stay free with limits set by the method rather than the plan: 2,000 decision cells on Simplex LP, 32 changing cells in a scenario. Nothing you have built stops working. Pro also removes the “Made with Sortia” footer from generated report tabs.
The other four Pro engines are Monte Carlo risk simulation, Decision trees, Schedule Risk (Monte Carlo CPM) and Optimization under uncertainty. Pro is $199/year, and a Day Pass covers seven days for $9 if you have a single decision to make.
Templates that use it
Each one loads into your sheet with the numbers already in place, and every figure on its page was computed by the tool itself.
- The worked example above, in fullHow Much Buffer Does This Plan Need?
- The same plan question answered by simulation instead, if your estimates are three-point rather than paddedWhat Finish Date Can I Commit To?
- Free, and the right tool once the work is under way and you have actualsProject Health Check (EVM)
The other 3 models in the library that run critical chain planning:
Try it in your own sheet
- Open Sortia in Google Sheets and choose Start from a template.
- Pick one of the models above, and it loads with the inputs filled in.
- Change the assumptions to fit your situation and press Run.
Never used Google Sheets? Start here goes the whole way, in seven steps, and assumes nothing.
Other methods: Monte Carlo Decision trees Schedule risk Optimization under uncertainty Statistics Machine learning Forecasting Optimization What-if analysis