What is the chance this project blows its budget?

A budget is a promise. If your cost estimate is uncertain, overrunning the budget isn’t a maybe — it has a measurable probability. Know it before you commit to a number you can’t defend.

With a budget of 18,000 USD against an expected cost of about 14,500 USD, you have about a 5.3% chance of going over budget. Cost stays at or below 18,000 USD in only about 94.7% of outcomes.

Try it with a real decision

Start with the example. Make it yours.

The numbers below are an editable example. Replace them with your own cost estimates; the answer updates as you type. Everything stays in this browser.

New to three-point estimates?

Optimistic: what it takes when things go well, without assuming a miracle.
Most likely: the outcome you would expect most often on similar work.
Pessimistic: a realistic difficult outcome. Include uncertainty, not an arbitrary markup.

Keep all three in the same units and put the most likely value between the other two. P80 means an estimated 80% chance of staying at or below a number under this model; it is not a promise.

These quick tools use a normal approximation, not simulated samples. The approximation can extend beyond your optimistic and pessimistic bounds, particularly for skewed estimates. For bounded task sampling, dependencies and separate risk events, use the full app.

Cost estimate & budget

Cost — optimistic (O)

Cost — most likely (M)

Cost — pessimistic (P)

Your budget (B)

Currency label label only — no rates

Results

Probability cost > budget5.3%

Your budget18,000 USD

Cost you can expect to stay at or below at each confidence (normal approximation)
Confidence	P50	P80	P90	P95
Cost level	14,500 USD	16,324 USD	17,277 USD	18,064 USD

If your budget lands below the P80 figure, more than 20% of outcomes exceed it. A “safe” budget is one that clears the confidence you actually need.

Formula

P(cost > budget) = 1 − Φ((budget − mean) / σ)

cost at P = mean + z·σ, with mean = (O + 4M + P)/6, σ = (P − O)/6

Normal approximation: probabilities use a normal approximation to a Beta-PERT cost estimate — a fast, honest single-estimate shortcut.

Test an alternative

What changes if…?

Keep these results, then adjust an estimate above to compare. Your baseline stays in this tab until you clear it or reload. Values are rounded for display.

Plain text with your assumptions, key results and any saved baseline. No account needed.

No baseline yet. Keep a result you would like to compare.

Continue with the whole project

Find the assumptions behind your budget risk

Give individual tasks their own estimates and model identifiable events separately. Use the app to examine how those assumptions affect the project cost distribution.

This calculator summarizes one cost range. A narrow range cannot stand in for missing risks. All outputs remain conditional on the assumptions you enter.

Start with a project template Explore the full app →

Anyone can generate a budget number in seconds; knowing whether to trust it is the skill. Recommended reading: How to Remain Valuable When Intelligence Becomes Cheap .

What is the chance this project blows its budget?

Cost estimate & budget

Results

Formula

What changes if…?

Find the assumptions behind your budget risk

Product

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