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.
Cost estimate & budget
Results
| 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.
Related tools
- Will this fixed-price project make money?
- What must we quote just to break even?
- Turn a three-point estimate into P50/P80/P90
- P50 vs P80 — what the numbers actually mean
One budget and one cost distribution is a start. When many tasks and risk events compound, the real overrun risk is higher — model it properly in the full app, or see the methodology.