Free Online Construction Cost Risk Calculator: Monte Carlo Estimate

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Each package cost gets a range instead of a point, the site factor multiplies the lot to represent weather, access, and everything a site does to a plan, and 10,000 runs later the estimate is a distribution. The gap between the P50 and the P90 is your contingency, computed instead of guessed.

The estimate is a distribution

Every estimator knows the single number is fiction; this tool makes the fiction honest. Give each package a minimum, likely, and maximum cost, let the site factor carry the things no bill of quantities captures, and run the job 10,000 times. The histogram is what the project actually is: the P10 you might get with luck, the P50 you should budget, and the P90 that tells you what contingency really costs. The tornado ranks where the money risk lives, which is where the firm quotes should be chased first.

Histogram of Monte Carlo construction cost outcomes with optimistic, expected, and conservative markers, beside a tornado chart ranking site factor and package costs
Ten thousand versions of the job: the tender you win, the budget you run, the contingency you carry.

The percentile gap replaces the flat percent contingency: when P90 minus P50 is 12 percent of the job, a 5 percent contingency is a decision to be surprised.

Reading the tornado

  • The site factor usually tops the chart, which is the quantified version of every veteran saying the site decides the money.
  • A package with a wide bar and no firm quote is the next phone call; a narrow bar is already managed.
  • Ranges should come from real quote spreads and history, not symmetric guesses; the tool is only as honest as its inputs.
  • Rerun as quotes firm up and watch the distribution tighten, which is progress you can show a client.

What this tool is for

Setting contingency defensibly, comparing tender risk between jobs, and teaching why two identical bills can deserve different prices.

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