Turn one three-point estimate into honest P50, P80 and P90 numbers

Most underpriced bids and missed deadlines start from quoting a single optimistic number. A PERT estimate lets you say “probably about 65 hours, and to be 80% sure we plan for about 75” — a range you can defend instead of a coin flip.

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 duration 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.

Three-point estimate → honest percentiles

Your best single-point estimate is about 65 hours. To have an 80% chance of staying at or under a number, plan for about 75 hours.

Optimistic (O) Best realistic case

Most likely (M) Most probable outcome

Pessimistic (P) Worst realistic case

Units hours, days, $, seats…

Planning values

P50 · single-point estimate 65 hours

P80 · 80% chance we stay at/below 75 hours

P90 · 90% chance we stay at/below 80 hours

PERT mean 65 hours

Standard deviation (range/6) 12 hours

Meaning of the language: P80 = an estimated 80% of outcomes fall at or below this value under the normal approximation. Budgeting at P50 leaves you ~50% likely to go over.

See the formula and assumptions

PERT mean = (O + 4M + P) / 6 — Beta-PERT, weighting λ = 4

standard deviation ≈ (P − O) / 6

P50 ≈ mean · P80 ≈ mean + 0.84·σ · P90 ≈ mean + 1.28·σ

Normal approximation: the P50/P80/P90 above are computed from a normal approximation to the Beta-PERT distribution — a fast, honest approximation for planning. It is a single-estimate shortcut; for a full multi-task, multi-risk view use the app below.

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

When one estimate becomes a whole project

Split the work into individual tasks, give each task its own range, and connect tasks that must finish before others can start.

This calculator treats one estimate in isolation. Adding P80 values from separate tasks does not produce a project P80. All outputs remain conditional on the assumptions you enter.

Start with a project template Explore the full app →

One honest range is worth more than a confident point estimate — and knowing which estimate to trust is the durable skill. Recommended reading: How to Remain Valuable When Intelligence Becomes Cheap .

Turn one three-point estimate into honest P50, P80 and P90 numbers

Three-point estimate → honest percentiles

Planning values

What changes if…?

When one estimate becomes a whole project

Product

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