How to use the critical chain buffer calculator
- List the tasks on the chain with a safe estimate and an aggressive estimate in the same unit, separated by commas.
- Choose the buffer method: cut and paste, which takes half of the safety removed, or the root of summed squares.
- Change the unit label in the advanced section if you like. The default is days.
- Read the buffer, the planned duration, the time saved against the safe total and both methods side by side.
Formula
safety removed = safe estimate - aggressive estimate, for each task
cut and paste buffer = 50% of the total safety removed
root of summed squares buffer = sqrt(sum of (safe - aggressive)^2)
planned duration = sum of aggressive estimates + buffer
The aggressive estimate is the time a task takes if things go well, and the safe estimate allows for trouble. Critical chain planning removes the padding from tasks and puts a shared buffer at the end.
Five tasks
The sample tasks have safe estimates of 10, 20, 8, 12 and 5 days, which add up to 55, and aggressive estimates of 6, 12, 5, 7 and 3 days, which add up to 33. The safety removed is 22 days.
By cut and paste the buffer is 11 days, so the planned duration is 33 + 11 = 44 days, 11 days less than the safe total. By the root method the squares are 16, 64, 9, 25 and 4, which add up to 118, and the root is 10.86 days.
Why a shared buffer
Padding every task hides the safety where it is easily wasted, because work expands to fill the time. Pooling the safety into one buffer at the end of the chain is shorter than the sum of the padding, because it is unlikely that every task will run long at once.
The cut and paste method is simple and conservative. The root method gives a smaller buffer when there are many tasks, because the safety of independent tasks adds up more slowly than a plain sum.
Using the buffer well
A buffer size is only the start. During the project, watch how much of the buffer has been used compared with how much of the chain is done, and act when the buffer is being used faster than the progress. Both methods are rules of thumb and the root method assumes task delays are independent.
Estimate one uncertain task with the Three Point Estimate Calculator, simulate the whole schedule with the Monte Carlo Project Duration Simulator, and reserve money for risks with the Contingency Reserve Simulator.
Assumptions and limits
- All tasks are on one chain of work that follows one after another. Parallel paths and resource conflicts are not modeled.
- Aggressive estimates must be 0 or more and no more than the safe estimates.
- Up to 100 tasks are read. Numbers can have decimals.
- The unit is a label only and does not change the arithmetic.
- Nothing typed here leaves your browser.
Frequently asked questions
Which method should I use?
Cut and paste is the easiest to explain and gives a larger buffer. The root method is lighter for chains of many tasks. Use the one your team already knows, and compare the two in the stats.
What is a safe estimate?
The time you would quote to be nearly sure of finishing, often with a margin for problems. The aggressive estimate is the time that is achievable if there are few interruptions.
Why is the root buffer smaller?
Squaring and then taking the root of the sum grows more slowly than adding up and halving when there are many tasks. It assumes delays do not all happen together.
Can the buffer be negative?
No. Aggressive estimates cannot exceed safe ones, so the safety removed is zero or more and so is the buffer.