Which day can you promise and still be right nine times in ten?

Five phases with a low, likely and high estimate each. The sheet adds the likely values and gets 63 days, and 63 days holds in 19 in 100 runs. The deadline with 90% odds is day 74.

Work Starter Monte Carlo Pro engine

Install free How Monte Carlo simulation works

After you install, this is the model to open.

What deadline gives 90% odds of finishing?

In your spreadsheet, click the Sortia icon in the strip of icons down the right-hand edge. No strip? Click the arrow at the bottom-right to open it. You can also use Extensions, then Sortia, then Open Sortia.
Click Start from a template and put that name in the search box.
Pick the card with that name and click Load this template. It arrives on a new tab with real numbers already in it.

This one runs on a Pro engine, and every free install includes five full-quality runs on your own numbers, shared across all five Pro engines rather than five for each. After that, Pro is $199/year.

What it does

Five phases, each with a low, likely and high estimate in days, run one after the other. The sheet adds the likely values and gets 63 days, and 63 is the deadline most people would promise, because it is the number the sheet shows. Click Run on the 20,000 trials the template loads with. The seed is set to 7 so your first run reproduces these figures exactly.

Total duration comes back with a mean of 67.3 days and a median of 67.1, and the promise of 63 days holds in 19 in 100 runs. The deadline with 90% odds of finishing is the ninetieth percentile, 73.7 days, so promise day 74: finishing by day 74 happens in 91 in 100 runs, and day 73 is 88 in 100. If the promise has to hold at 95% odds it is day 76, because the ninety-fifth percentile is 75.6.

The gap between the deadline the sheet suggests and the deadline with 90% odds is eleven days, and none of it is padding. It is what five ranges that each lean long add up to, and it is invisible on a sheet that holds one number per phase. The tornado says where the eleven days come from. Build leads at 0.73 and carries 56% of the spread, Design is next at 0.45, and the other three phases together carry less than a quarter.

Second run: narrow Build in the Risk Analysis panel to 22, 25 and 32 days and rerun. The ninetieth percentile moves earlier and the deadline with it, and that is the only kind of edit that moves it, because a deadline with odds attached is a percentile of the work, not a promise about it. What the model cannot tell you: the phases run one after another with nothing in parallel, the five ranges are drawn independently when in practice a late build is followed by late testing, so the true tail is a little longer than this one, and scope that changes mid-project is not inside any of the ranges.

To make it yours, retype the phases and their three estimates, set the same ranges on the matching draws in the Risk Analysis panel, and read your deadline off the ninetieth percentile.

The model

It arrives on a tab called Template: Deadline With 90% Odds, carrying these columns:

Low (days)
Likely (days)
High (days)
Duration drawn (days)

with the model computed beside the data:

Total duration (days)	63
Days to spare against the promise (days)	0

Once it is in your sheet

The model arrives with real numbers in it and runs as it stands, so you can press the button first and understand it second.
Change the numbers to yours. The sheet marks which cells are inputs and which hold formulas, and most labels carry a note explaining the row.
Press the run button at the bottom of the panel. It is labeled for the tool you are in, and the result lands on its own tab, with a written reading of it beside the figures.

Never used Google Sheets? Start here goes the whole way, in seven steps, and assumes nothing.

Next question

Every model like this one, and the method behind them: Monte Carlo simulation.

Install free All 379 templates