Construction schedule with critical path and S-curve

What the tool does

You enter activities in a table like a spreadsheet: name, duration in working days, predecessors, value and the party responsible. The tool calculates early and late dates, total and free float and the critical path on working-day calendars, and draws a Gantt chart next to the table. As the work goes on you set a data date, enter actual dates and percent complete, and the remaining work is rescheduled from the data date. A planned curve from a baseline and an actual curve from your progress records show whether the project is ahead or behind.

Schedules are kept in this browser on this device. Nothing is uploaded.

Where it fits in a project

Schedulers and site engineers prepare the contract programme, update it every week or month, report progress to the client and hand short look-ahead schedules to subcontractors. Desktop scheduling software costs from a few hundred to a few thousand dollars per seat, and many people who receive a schedule cannot open the file at all, so the work is often redrawn in spreadsheets. This tool opens the common exchange files in a browser and writes them back.

Entering activities

• Type in the empty row at the bottom to add an activity. Arrow keys, Tab and Enter move between cells; typing starts editing.
• Predecessors are written as row numbers with an optional link type and lag in working days, for example 3, 5SS+2 or 7FF-1. Ctrl+L links the current row to the row above.
• Alt+Shift+Right and Alt+Shift+Left indent and outdent rows. A row with indented rows below it becomes a summary row that spans its activities.
• Rows copied from Excel can be pasted straight into the table. Leading spaces in the name column become indent levels.
• A bill exported from the PDF takeoff tool on this site can be opened as a project file: its work sections become summary rows and its items become activities with their values.

How dates are calculated

The forward pass takes the latest of the dates allowed by each predecessor: finish-to-start uses the predecessor's finish plus the lag, start-to-start its start plus the lag, finish-to-finish and start-to-finish fix the successor's finish. The backward pass mirrors this from the project finish. Total float is counted in working days between the early and late finish, and free float up to the earliest successor. These are the rules of the critical path method as set out in the open textbook by Hendrickson, Haas and Au; the tool is tested against the answers of its worked examples.

Each activity can use its own calendar. Calendars hold the working days of the week, non-working days and extra working days. Built-in public holidays come from official government sources for the years published so far, and any calendar can take holidays from an ICS file. Constraints (start or finish on, before or after a date) are applied as in common scheduling software, and a finish constraint that cannot be met shows as negative float.

The calculation options follow the definitions in the Primavera P6 help and use the same defaults: progress out of sequence by retained logic (the remaining work waits for unfinished predecessors) or progress override, lags counted on the successor's calendar or another one, and critical activities by a float limit or by the longest path.

Progress, baselines and the S-curve

Save the current dates as a baseline before work starts; later baselines can be added and shown in the chart. To update, set the data date, enter actual start and finish dates and percent complete, and record the progress. The planned curve spreads each activity's value over its working days in the baseline, and the actual curve sums value times percent complete at each record. With no values entered, activities are weighted by duration. The Excel file contains the weights by month and the cumulative planned percentages as formulas.

Being behind is shown in two ways, because they are not the same number: the difference in percentage points (plan 30 %, actual 27 % is 3 points) and the shortfall as a share of the plan (3 of 30 is 10 %). Korean public contracts use a 10-point limit in the general conditions and a 5 % limit in the ministry's construction management guidelines, and the two are read differently in practice, so both limits can be set and both values are shown.

Schedule check and look-ahead

The schedule check runs the 14 points of the DCMA schedule assessment: missing logic, leads, lags, share of finish-to-start links, hard constraints, high float and duration over 44 working days, negative float, invalid dates, activities without value, missed activities, a critical path test that delays a critical activity by 600 days and checks that the finish moves by as much, and the CPLI and BEI indices. Each line lists the rows behind it.

The look-ahead lists the activities that are in progress or start within the next weeks, by responsible party, and exports one sheet per subcontractor with a day-by-day bar, or an ICS file for their calendar.

Files

• MS Project XML: read and written in the public MSPDI format with calendars, outline, links, constraints, actual dates and baseline. Our files validate against Microsoft's schema. MS Project's own MPP format is not public: save as XML in MS Project first.
• Primavera P6 XER: read and written with calendars and their exceptions, WBS, activities, links, constraints and progress. After opening an MS Project or P6 file, the tool recalculates and tells how many activities have the same dates as in the file.
• Excel (XLSX): the schedule, weights by month with formulas, the progress curve with points and ratio, and the schedule check, written with ExcelJS.
• PDF: table and Gantt chart on A3 landscape pages, made with pdf-lib.
• CSV, ICS and the project file (JSON). CSV with a header row (name, duration, predecessors, level, code, value, responsible) can also be opened.

Limits

Measured on a desktop browser on 2026-10-19 with 10,000 activities: opening an XER file took about 1.2 seconds, recalculating after an edit about 0.5 seconds, and the schedule check about 0.6 seconds.

Durations and lags are whole working days; hours are not scheduled. Resource levelling, the actual dates option of P6 and splitting of activities are not available. Calculated dates are a tool for planning and must be checked against the contract and by the people responsible for the schedule.

Last checked 2026-10-19