What Is a PERT/CPM Chart?
A PERT/CPM chart is a network diagram used in project management to plan and track a project made up of interdependent activities. Each activity is shown as a box (or node) with its own duration, and arrows connect an activity to the ones that depend on it.
PERT (Program Evaluation and Review Technique) and CPM (Critical Path Method) started as two separate methods developed in the 1950s, but they converged in practice — most tools, including this generator, build a single diagram and refer to it as a PERT chart, a CPM chart, or a PERT/CPM chart interchangeably.
Why Use a PERT/CPM Chart
A list of tasks in a spreadsheet doesn't show you which delays actually matter. A PERT/CPM chart does, by making dependencies and timing explicit.
Find the critical path
See exactly which chain of activities controls your project's total duration, instead of guessing from a task list.
Spot float before it's a problem
Activities with slack can absorb small delays or free up resources for the critical path without affecting the deadline.
Communicate dependencies clearly
A network diagram makes it obvious to stakeholders which tasks are blocking which, far more clearly than a flat list.
Prioritize schedule risk
Critical-path activities are the ones worth the closest monitoring, since any slippage there slips the whole project.
How to Make a PERT/CPM Chart
- List every activity required to complete the project.
- Estimate a duration for each activity, in whatever unit fits your project (days, weeks).
- Identify each activity's predecessors — the activities that must finish first.
- Enter the activities, durations, and predecessors into the generator above to draw the diagram and calculate early/late start and finish times.
- Read off the critical path — the chain of activities with zero slack.
Critical Path Formulas
The generator above calculates these automatically for every activity, but here's what it's doing under the hood.
Early Start (ES) = Max(Early Finish of all predecessors)
Early Finish (EF) = Early Start + Duration
Late Finish (LF) = Min(Late Start of all successors)
Late Start (LS) = Late Finish − Duration
Slack = Late Start − Early Start
An activity sits on the critical path when its slack equals zero — there's no room to delay it without delaying the whole project.
PERT Three-Point Estimate Formula
Classic PERT handles uncertain durations by combining three estimates per activity into one expected duration, weighted toward the most likely case.
Expected Duration = (Optimistic + 4 × Most Likely + Pessimistic) ÷ 6
Worked Example
A task could take as little as 4 days (optimistic), as long as 10 days (pessimistic), and most likely takes 6 days.
Expected Duration = (4 + 4 × 6 + 10) ÷ 6 = 6.33 days
Run that expected duration through the generator above as the activity's duration, the same way you would a single fixed estimate — the critical path math afterward is identical.
PERT/CPM Chart Example
The generator above is pre-loaded with a worked example: Design → Development → Testing → Deployment, with Procurement running in parallel. Design (3 days) has no predecessors, so it starts at day 0. Development (8 days) and Procurement (5 days) both depend on Design and can run at the same time. Testing (4 days) depends on Development, and Deployment (2 days) depends on both Testing and Procurement.
Design → Development → Testing → Deployment totals 3 + 8 + 4 + 2 = 17 days, while Design → Procurement → Deployment totals only 3 + 5 + 2 = 10 days. Since the first path is longer, it is the critical path, and Procurement has 7 days of slack to spare before it would start delaying the project.
PERT/CPM Chart vs Gantt Chart
Both are common ways to visualize a schedule, but they answer different questions.
| PERT/CPM Chart | Gantt Chart | |
|---|---|---|
| Best for | Seeing dependencies and the critical path | Seeing a timeline against actual calendar dates |
| Layout | Network of nodes connected by dependency arrows | Horizontal bars plotted against a date axis |
| Shows slack? | Yes, per activity | Not directly, usually needs extra annotation |
| Typical audience | Project planners scheduling and sequencing work | Stakeholders tracking progress against dates |
Many project managers build both: a PERT/CPM chart to plan the sequence and find the critical path, then a Gantt chart to communicate the resulting schedule against real dates.
Common Mistakes When Building a PERT/CPM Chart
- Missing dependencies — an activity left without a predecessor it actually needs will start earlier in the chart than it can in reality.
- Circular dependencies — two activities that depend on each other, directly or through a chain, make the chart impossible to calculate; the generator above flags this as an error.
- Treating every activity as critical — if nothing has slack, you have no flexibility left when something inevitably slips; check whether your dependencies are genuinely necessary or just how the plan happened to be drafted.
- Ignoring slack until it's gone — activities with slack can still become critical if they're delayed past their float; slack should be monitored, not forgotten.
- Padding every duration “just in case” — inflated estimates hide the real critical path and make the chart less useful for spotting genuine risk.
Who Uses PERT/CPM Charts
Construction & engineering
Sequencing trades and procurement where a delay in one phase (materials, permits) can block everything after it.
Software & product teams
Mapping which features or infrastructure work block a launch, and which can happen in parallel without risk.
Event & production planning
Coordinating vendors, venues, and approvals that all have to land in the right order before an event date.
Students & PMP/PMI exam prep
Practicing critical path calculations is a core part of project management coursework and certification exams.
PERT/CPM Glossary
- Node / Activity
- A single task in the project, shown as a box in the diagram above, with its own duration.
- Predecessor
- An activity that must finish before another activity can start.
- Early Start (ES) / Early Finish (EF)
- The soonest an activity could possibly start or finish, based only on its predecessors.
- Late Start (LS) / Late Finish (LF)
- The latest an activity can start or finish without delaying the overall project.
- Slack (Float)
- How much room an activity has to slip without affecting the project's end date.
- Critical Path
- The longest sequence of dependent activities from start to finish, and therefore the shortest possible duration for the whole project.
PERT/CPM Chart FAQ
What is a PERT/CPM chart?
A PERT/CPM chart is a project management network diagram that maps out a project's activities, their durations, and the dependencies between them. PERT (Program Evaluation and Review Technique) and CPM (Critical Path Method) are two closely related techniques, and in practice the diagrams they produce are built and read the same way — hence "PERT/CPM chart."
How do you make a PERT/CPM chart?
List every activity in the project along with its duration and which other activities must finish before it can start. Enter those into the generator above, and it lays out each activity as a box, draws arrows for the dependencies, and calculates early/late start and finish times to find the critical path automatically.
What is the critical path in a PERT/CPM chart?
The critical path is the longest chain of dependent activities from the start of the project to the end — it determines the shortest possible time the whole project can take. Activities on the critical path have zero slack, meaning any delay to them delays the entire project.
What does slack (or float) mean on a PERT/CPM chart?
Slack is how much an activity can be delayed without pushing back the project's finish date. It's calculated as late start minus early start (or equivalently, late finish minus early finish). Critical-path activities always have zero slack.
What's the difference between PERT and CPM?
CPM (Critical Path Method) uses a single, fixed duration estimate per activity. PERT (Program Evaluation and Review Technique) was originally designed to handle uncertainty by using three duration estimates per activity — optimistic, most likely, and pessimistic — averaged into a single expected duration. Most modern tools, including this one, use the simpler CPM-style single-duration approach and refer to the resulting diagram as a PERT/CPM chart interchangeably.