Free Calculator

How Long Will This Project Really Take?

Your gut says three weeks. Your gut is running the planning fallacy. Enter the real effort, how much of each person you actually have, and an honest buffer, and get a date you can commit to without dreading it.

Total work if one person did it all, in weeks

13 people12
10%60%100%

"Full time" with other duties is usually 60-70%

0%40%70%

20% for routine work, 40-50% for anything new

Realistic calendar time

0 weeks

Projected finish

–

if you start today

Optimistic (no buffer)

0 wk

the version in your head

Effective weekly output

0

person-weeks per calendar week

Buffer adds

0 wk

reality tax, not padding

The Estimate in Your Head vs the One You Should Commit To

The optimistic number assumes full allocation and nothing going wrong. The realistic number assumes people have other jobs and reality has a vote. Guess which one actually happens.

Optimistic

0 weeks

what you want to promise

Realistic

0 weeks

what you should commit to

What This Number Means for You

Commit to the realistic date, out loud.

Allocation is the hidden variable that wrecks plans.

A project does not slow down because people are lazy; it slows down because "on the project" quietly means 60 percent of a person whose calendar is full of other things. Before you promise a date, confirm the allocation is real. The team capacity calculator shows whether those hours even exist.

Protecting focus time shortens the timeline for free.

Higher real allocation is just fewer interruptions and meetings stealing project time. Raising each person from 50 to 70 percent shortens the project without adding anyone. The context switching cost calculator shows what those interruptions cost.

About the numbers: Effective weekly output = people × allocation (in person-weeks completed per calendar week). Optimistic weeks = effort ÷ output. Realistic weeks = optimistic × (1 + buffer). Finish date adds the realistic weeks to today. The model assumes work parallelizes cleanly, which large teams rarely do, so treat many-people estimates as a floor. It does not model dependencies, sequencing, or a single specialist bottleneck. Planning estimates for a defensible date, not a project plan.

Frequently Asked Questions

Why is the realistic timeline so much longer than my estimate?
Two reasons that compound. First, people are almost never 100 percent allocated to one project: meetings, other work, and overhead mean the same person contributes maybe half a day of project work per day. Second, the planning fallacy is real and well documented: humans systematically underestimate how long their own tasks take, because we plan for the version where nothing goes wrong. The buffer exists to price the version where something does, which is every version.
What allocation percentage is realistic?
Lower than you think. Someone "on the project full time" who also attends team meetings, answers messages, and handles the occasional other request is realistically at 60 to 70 percent. Someone splitting time across two projects is often at 40 percent on each, not 50, because context switching taxes both. If in doubt, estimate what they actually touch the project in a normal day and convert that to a percentage.
How big should the buffer be?
For work the team has done many times before, 20 percent. For work with real unknowns or new technology, 40 to 50 percent is honest, and even that can be optimistic. The buffer is not padding you sheepishly admit to; it is the part of the estimate that accounts for reality, and removing it does not make the work faster, it just makes the deadline wrong earlier.
Does adding people make it faster?
Up to a point, then it reverses. More people means more coordination, more onboarding onto the project, and more communication paths, which is why a famous rule in software holds that adding people to a late project makes it later. This calculator models the linear speedup from more hands, but treat large teams on one project skeptically: two focused people often beat four distracted ones.
Should I share the buffered number or the optimistic one?
The buffered one, always, and out loud. Managers who commit to the optimistic estimate to look responsive are simply choosing to deliver the bad news later, when it is more expensive and less forgivable. A realistic date you hit builds far more trust than an aggressive date you miss, and the person you most need to believe your estimates is your future self.
How does this relate to team capacity?
They are two halves of the same planning. The team capacity calculator tells you how many hours exist in a period across everything. This one tells you how long a specific project takes given the slice of those hours you assign to it. Set the capacity budget first, then schedule projects inside it, and you stop the classic mistake of committing to more projects than the hours can carry.

A Realistic Date You Hit Beats an Aggressive One You Miss.

Confirm the allocation, keep the buffer, and commit to the number reality will actually produce.

Calculator: Team Capacity → Guide: Time Management →

Related: Context Switching Cost → Related: Knowledge Transfer Time →

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