Join Chris Croft for an in-depth discussion in this video Drawing a Gantt chart, part of Project Management Simplified.
- Okay, so how do we construct a Gantt chart? Now I'm going to draw one out on here for you. You'll probably be using Excel or Microsoft project, something like that on your computer but I'm going to draw you out the process so you can see. The first thing we do is you put the time scale across the top. We know from the post-it note diagram that this project's going to take 34 weeks. I'm going to put my time scale across here, so zero, 10, 20, 30, 40, so we're going to finish at 34.
Later you would probably actually put the name of the months along here as well and then the first bit of the drawing is to put the critical path in. We've identified this as the longest path, the critical path. If you're using colors, you would normally draw this in red. First of all, the first task is choosing the country. That's got a duration of four, so I'm going to draw it with a length of four on here. Choose country, then we move on to getting the permit, that's got a length of 10 so that takes us to 14.
That's get permit then it's purchasing the site which is another 10 so that takes us from 14 to 24, purchase site. We've got modifying the building which is eight weeks so that takes us from 24 to 32. That's modify the building and then the final task we have to do is installing the furniture which is two more weeks and that takes us to 34.
It's a relief when they add up, so installing a furniture. Now, there is one more thing on our critical path we've got opening but opening is an event, it has zero duration so it's not an activity, it's an event. How do we show events on the Gantt chart? Well because it's got no duration, it's just a vertical line. That's easy and I'm going to call that open. That's the critical path. It's drawn coming down in steps and the reason for that is that if one of the task takes longer than you expected, you can show slippage like that without it going too messy.
The other reason it's in steps is it means that everything is on its own line so that you can add extra columns over here. You could put for example who's doing the task here and in fact quite often when you use computers, they have the name of the task over here and they just have lines but if you're drawing it by hand, I think it's clearer to actually put the names on the boxes like I've done here. That's why it comes down in steps. That's the drawing of the critical path. I think you'd agree that's pretty easy. Now we come to the slightly tricky part which is putting in the floating tasks and I'd like to stress a particular rule here.
When you draw the floating tasks in, always put vertical lines in for the constraints. I'll show you what I mean by that. I'm going to draw in finding a site. Now the constraints of this is it has to come off until we've chosen the country. It has to be done before we buy the site. We can put in two vertical lines from where those points are. After we've chosen the country, it's basically here isn't it? Then before we purchase the site, there's a line before this box here for purchasing the site.
Finding the site which has a duration of five has to be fitted in between those lines, so I'm going to draw that in with a length of five. That's find site. If you put those vertical lines in then you can see how much float that task has and you've got a choice about whether you want to float it earlier or later. I've put that one in the middle there. I could have decided to do it absolutely straight away and that perhaps would have been safer in case it took longer. What you shouldn't do is float it really late so it's right up against the finished point because then it's effectively become a critical task.
If it takes any longer than five, it's going to start to delay the critical path and then that's a big crime if your project is held up by a floating task going late. You should always keep a little bit of a safety buffer after each floating task. That's finding the site. Now, I've only got three more tasks to put in and I'm going to put in the manager next but the thing about the manager is I know what the starting constraint is. It's going to be after choosing the country but the finishing constraint is buying the furniture and getting the staff and they're not in there yet.
What I need to do is I need to put two in at the same time. It's not a problem. It's a concept known as sharing a float. The manager and the buying of the furniture, they come after choosing the country, so that's this same line here and they have to be done before we install the furniture which is actually here. The two of them are going to be between these two lines. They've actually got loads of float, there's loads of choice about when I'm going to do those. I'm actually going to choose to get the manager really early so I'm going to put in a box with a length of six, nice and early here.
That's the manager. I'm getting the manager earlier because he can then help me with all these other things that can come later and then I'm going to buy the furniture and I'm going to get a furniture really late. It's got a length of only two and I'll put it in right over here, furniture, buying furniture. The reason why I'm buying the furniture so late is there's no point in using up my cash early and I don’t want the furniture sitting around for ages before I install it. I've moved it up nearly against install.
I've got the little safety margin there just in case it takes me longer than two weeks. That's those two floating tasks in and that's the concept of sharing float. Clearly for manage it took a bit longer or it's done a bit later then there's less float that this one's got because they're sharing it between them. In this particular case, there's so much it's not going to be a problem. Now, the final floating task I'm going to put in is the staff and they come after the manager. They come after this point here. Always put the verticals in and they have to be in before we open which is here.
The staff are floating all the way between this point and here. They've got a length of eight. Now, shall I get the staff earlier or later? Well I think I'm going to get them relatively late because they're costing me quite a lot of money, put staff in there and there's no point of getting the staff really early when they're going to be sitting around costing me money. I'm going to get them relatively late. Now you probably noticed that one of the constraints of the staff, this line here is actually a moving point.
If the manager takes longer or comes in later, the staff again have less float. What we've got is a floating task hanging off another floating task. It's not a problem but it's a concept that's worth understanding. In fact if you understand the two concepts of sharing a float and floating task which depend on other floating tasks, you can build up any Gantt chart. You could draw the biggest Gantt chart in the world and it would be absolutely fine and it would be easy because it would be based on these simple building blocks.
That's my Gantt chart finished. I think you'll agree it wasn't difficult, it was quick to do and the reason why it was easy and quick to do is because we'd already got this diagram here. This was really the work and then based on this we've just been drawing it out to scale down here. I think you can see and this is really important message, it's almost impossible to jump straight to a Gantt chart and get it correct. If you jump straight to this, you would almost certainly have made mistakes because everything's hanging off the critical path.
You need to know the critical path before you draw your Gantt chart and that's why this diagram is so important because this diagram allows us to find the critical path. What I'd like you to try now is to do this for a real project that you've got. Make the post-it note diagram and then convert it into the Gantt chart and you'll be amazed how well it works and how easy it is to do.
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- Defining project scope
- Deciding how to list tasks
- Estimating costs and time
- Planning for risk
- Staying on budget