Free tool

Critical Path Calculator

Enter your activities, their durations and what each one waits for. Get early and late start and finish, total float and the critical path, with a bar chart. Edit the fit-out example or start your own.

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Runs in your browser
Nothing is saved or sent

Activities

Durations in calendar days. Predecessors are the IDs that must finish first, separated by commas. IDs are case-sensitive.

Activities for the critical path calculation
Row	ID	Activity name	Duration (days)	Predecessors	Remove
1
2
3
4
5
6
7
8

8 of 30 activities

Results

Project duration

44 days

Critical activities

4 of 8

Critical, in start order

ABEH

Schedule by day Critical Has float Float Milestone

Day 010203040

A Site survey and measurements

B Design and material approvals

C Strip-out and demolition

D MEP first fix

E Joinery procurement (long lead)

F Partitions and ceilings

G Floor and wall finishes

H Joinery install and handover

Critical path results for each activity
ID	Activity	Duration	ES	EF	LS	LF	Total float	Critical
A	Site survey and measurements	3	0	3	0	3	0	Critical
B	Design and material approvals	10	3	13	3	13	0	Critical
C	Strip-out and demolition	5	3	8	12	17	9	No
D	MEP first fix	4	8	12	17	21	9	No
E	Joinery procurement (long lead)	25	13	38	13	38	0	Critical
F	Partitions and ceilings	10	13	23	21	31	8	No
G	Floor and wall finishes	7	23	30	31	38	8	No
H	Joinery install and handover	6	38	44	38	44	0	Critical

Day 0 is the project start; an activity with ES 3 and duration 2 occupies days 3-4 and its successors can start on day 5 (its EF). Durations are calendar days, links are finish-to-start with no lag.

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What the critical path method does

The critical path method (CPM) finds the longest chain of dependent activities in a project. That chain sets the shortest possible project duration: if any activity on it slips by a day, the whole project finishes a day later. Activities off the critical path have float, also called slack: the number of days they can slip without moving the finish date.

That makes CPM a way to decide where to put your attention. Speeding up an activity that has float does not bring the finish date forward. Speeding up a critical one can.

How the calculation works

The calculator makes two passes over your activities and the links between them.

Forward pass. Each activity starts as soon as all of its predecessors have finished. Its early start (ES) is the latest early finish among its predecessors, or 0 if it has none, and its early finish (EF) is ES plus its duration. The largest EF is the project duration.
Backward pass.Working back from the project finish, each activity's late finish (LF) is the earliest late start among its successors, or the project duration if it has none. Its late start (LS) is LF minus its duration.
Float. Total float is LS minus ES. Activities with zero total float are critical; together they form the critical path.

Before either pass, the activities are put in dependency order. If that is impossible because a chain of links loops back on itself, the calculator names the loop so you can remove one of its links.

What the columns mean

ES (early start): The earliest day the activity can begin.
EF (early finish): ES plus the duration: the earliest day its successors can begin.
LS (late start): The latest day the activity can begin without delaying the project.
LF (late finish): The latest day it can finish without delaying the project.
Total float: How many days the activity can slip before the project finish moves.
Critical: Zero total float: any delay here delays the whole project.

How the days are numbered

Day 0 is the first day of the project. An activity with ES 3 and a duration of 2 days is worked on days 3 and 4, and its EF is 5, which is the day its successors can start. Durations are calendar days, so weekends and holidays are not skipped. A duration of 0 makes an activity a milestone: a point in time with no work.

Limits of this calculator

Up to 30 activities.
Finish-to-start links only, with no lags or leads.
Calendar days only, with no working-day calendars.
Nothing you enter is saved or sent anywhere. It is calculated in your browser and gone when you leave the page.

From a one-off calculation to a working schedule

A calculator gives you a snapshot. On a real project, dates move every week. In Milesto, the scheduling engine works out the critical path and float for your whole plan, next to progress, earned value and a Gantt chart, and all of it is on the Free plan.

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Run your whole plan in Milesto

Milesto's scheduling engine works out the critical path and float for your whole plan, next to progress, earned value and a Gantt chart. Start on the Free plan, no credit card needed.

Plan it in Milesto, free