Personal blog. I write about what I learn as I am learning it

A Critical Path Method Template That Does Both Passes

Most of what gets offered as a critical path method template is a Gantt chart with a column called Start that you fill in yourself. That is not the critical path method. The whole point of CPM is that you never type a date — you type durations and predecessors, and the dates are computed. If the template makes you enter the start, it has quietly handed the work back to you and kept the chart.

So this is what an honest one has to do, which half of it a spreadsheet handles well, which half it fights you on, and the file I ended up with. Numbers throughout come from the same 21-task renovation plan the template ships with.

Table of early start, early finish, late start, late finish and total float for 21 tasks, with the zero-float critical path rows shaded. — What the method actually produces. Four dates and a float per task; the shaded rows are the critical path.

Free Float vs Total Float, and the Third Kind

Free float vs total float sounds like the kind of distinction that exists to be asked about in an exam. It is not. They answer two genuinely different questions, they disagree on real plans, and the one your software prints by default is the one that will get you a phone call from a subcontractor.

I ran both numbers over a 21-task house renovation — the sample plan I use for all of this — and then a third number that no CPM tool prints, because it needs information the critical path method throws away. Here is all three, on the same plan.

Bar chart comparing total float, free float and crew-constrained float for the eleven tasks where the three measures disagree. — Eleven of the twenty-one tasks. Three measures of slack, and they rarely agree.

What a CPM Schedule Is, and How to Build One

A CPM schedule is not a chart. It is the output of an arithmetic procedure you could run on the back of an envelope: two passes over a list of tasks and the links between them. The bars you end up looking at are just a drawing of the answer. I think most of the confusion around the term starts there — people are shown the picture and told it is the method.

So here is the method itself, worked end to end on a real plan: a 21-task house renovation, five trades, no invented numbers. Every figure below came out of the same file, and you can run it yourself in a minute.

Gantt chart of a 21-task renovation showing critical tasks in orange and tasks with slack in blue, with daily resource load rows underneath. — The drawing at the end of the arithmetic. Orange bars have no room to move; blue ones do.

A Construction Schedule Template That Counts Crew

Download any free construction schedule template and you get the same thing: a task list, durations, start and finish dates, and a bar chart. What you almost never get is a column for how many people each task needs, and a row underneath that adds them up. Which is odd, because on a real job the binding constraint is rarely the calendar. It is that there are three electricians.

I built one with that row in it. It is free, it is a normal Excel file, and this is what the extra column changes.

Gantt Chart in Google Sheets: What Actually Ports

I built a scheduling template for Excel and then did the thing everybody does, which is open it in Google Sheets to see what survived. Most of a Gantt chart in Google Sheets works exactly as it does in Excel — the date maths, the lookups, the bars. One design decision in the file exists specifically because of what does not survive a trip between spreadsheet programs, and it is worth explaining, because it is the kind of thing that fails quietly rather than loudly.

This is the Google Sheets half of the template I wrote about when I built the Excel version. Same file, same six tabs — you can download the Excel version or copy the Google Sheets version, whichever saves you a step.

Bar chart of how many working days each task moves when the schedule is solved against a real crew limit. Eleven of twenty-one tasks move, by three to fourteen days; ten do not move at all. — Eleven of twenty-one tasks move once the crew limit is real. If the file arrives with every task reading as pinned, this chart comes back empty and the tool still reports success.