Excel Prob Function

What this does

PROB works on a distribution you define yourself: a column of possible outcomes and a column of their probabilities. It then totals the probability of outcomes falling within bounds you give. That makes it the right tool for scenario modelling, where the distribution comes from judgement or historical frequency rather than a named statistical form — sales scenarios with assigned likelihoods, project durations from expert estimates, demand tiers with observed frequencies. The probabilities must sum to exactly 1, or it returns #NUM!, which is a useful check on the model itself. Keep the inputs visible and clearly labelled and the whole thing stays auditable — anyone who opens the file later, including you, can see at a glance exactly what feeds the result and change one assumption without hunting through the formula. Treat “excel prob function” as a small repeatable workflow rather than a one-off click you hope to remember next time. Use a small test block before the live file, so any surprise in the affected formula shows up while it is still harmless. When a formula is involved, keep the inputs labelled beside it, reference cells instead of typing values, and apply number formatting only after the result checks out. That turns a calculation you can defend to a CFO or an auditor into a method you can reuse, explain, and defend when the workbook leaves your screen.

A worked example

Sales scenarios of 100, 200, 300 and 400 in A2:A5 with probabilities 0.1, 0.4, 0.35 and 0.15 in B2:B5: =PROB(A2:A5, B2:B5, 200, 300) returns 0.75, the chance of landing in the middle band. The chance of the worst case alone: =PROB(A2:A5, B2:B5, 100). The expected value, for comparison: =SUMPRODUCT(A2:A5, B2:B5). PROB works on a judgement-based distribution, which is what scenario planning actually has rather than a textbook one. One habit worth forming early: name the cells that hold your inputs, so the formula reads in plain language instead of a string of cell addresses. A reviewer — or you in three months — can then follow the logic without decoding what B7 and D2 were supposed to mean, which is most of what makes a sheet maintainable.

In Google Sheets

Google Sheets handles this almost identically to Excel. The formula syntax above is the same, and the menu lives under a slightly different label rather than a ribbon tab. Use the platform toggle at the top of the page to switch every keyboard shortcut between Windows and Mac, and expect at most cosmetic differences in naming. The aim was to get you unstuck fast and leave you a little more capable than a copy-paste would. The answer is at the top and the detail above shows why it holds — so the next time a colleague asks, you can answer without reaching for search. Treat “excel prob function” as a small building block rather than a chore. Once the inputs sit in their own cells and the formula reads from them, the same setup answers a dozen related questions with a tweak, and Excel keeps every dependent figure current as the data changes. The tool above is there so you can rehearse and verify before committing anything to a real workbook; the steps and worked example are there so the logic sticks. Get it right once and it stops costing you time — it starts saving it, every time the question comes back around.

Common mistakes

• Probabilities that do not sum to 1, which returns #NUM! — usually a rounding issue in the model.
• Omitting the upper limit when a range was meant, which asks about a single value instead.
• Using it where a named distribution fits better; a binomial or Poisson process should use its own function.

Frequently asked questions