Art of Speaking Science

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Q: How many slides do I use for a ten minute presentation?
A: It depends. Read on for the details.

If you ask a general presentation skills “pundit” you are likely to hear the rule of thumb that says “one slide per minute”. Even some suggest up to as many as 3 slides per minute.

However, if you forced me to give you a rule of thumb for science and technology it’s probably slightly less than one slide per minute.

But, again, it all depends; allow me to explain.

I think those that suggest 2-3 slides per minute are assuming very simplistic content and/or considering the liberal use of progressive builds (a build is when you present only a portion of the slide then click to reveal remaining portions). I can’t imagine an effective scientific or technical presentation that would use that many slides.

I prefer to think about this issue differently.

In general, you should be able to talk at least 30 seconds about whatever slide you are presenting; if you don’t have at least 30 seconds then it needs to either be combined to another slide or used as part of a progressive build. Less than 30 seconds feels like you are “skipping something” or “glossing over details” when making a technical presentation.

Typically ideas in science and technology presentations are somewhat complex (certainly more complex than most business presentations) and deserve more than a minute of discussion. Typically technical speakers should spend approximately 1.5 to 2.5 minutes on a slide (a single main idea). If it takes you longer than 3.5 minutes to present one slide, then you probably need to add another slide or perhaps you need to break up the slide into a progressive build.

Unfortunately, when you consider the question from this perspective it doesn’t really help you with figuring out how much material you can present–which I think is the reason that most people ask this question to begin with. I have a different rule for that.

I suggest taking your overall amount of time and then subtract approximately 15%, then take the remainder and divide by the number of main ideas you would like to present. The result will tell you how much time you have to present each main idea within the body of your talk. So for example, if you are planning a 10 minute conference talk and you have 4 main ideas that you want to discuss, you’ll have about 2 minutes per main idea. Which means you’re likely to have 4 or 5 slides for your main ideas (again depending on the content and the number of builds) and perhaps 3-5 slides for the beginning and end portions. This means you’d end up with between 7-10 slides for a 10 minute presentation.

Let’s say you have a longer presentation, say a job talk, which is typically about 35 minutes of prepared material. Again, we subtract 15% for the “beginning and end”, leaving about 30 minutes, divide that by 4 (or 3 or 5 depending on how you have divided up the sections of your talk) and you get about 7.5 minutes per section. With longer talks, the next step is to determine how many main ideas you have to present per section and divide that into the time you have available for each section. So if you have 3 main ideas in the first section then you will have about 2.5 minutes per idea (probably 1 or 2 slides for each idea in the first section depending on complexity) Of course, you’ll need to do this with each section of the talk.

This above method of “calculation” I think is a much better way to making rough guess at the amount of slides you’ll need to cover your material. It gives you a way to begin your planning.

However, my final bit of advice is…don’t ever use “a slide per minute” rule or even my suggested calculation to determine the “right” amount of slides.  You’ll need to run through the presentation-live.  Of course, the actually timing will tell you if you ultimately have the right amount of material to fit the time slot.

I always say, “Use as many slides as you need to clearly and efficiently communicate your ideas within the given time frame”. The only way to know for sure if you have prepared the correct amount of material is to practice your presentation aloud using a timer. Only practice will tell you if your presentation delivery is within your time limits.

Some people run through the slides more quickly when they practice, others more slowly–I call this the n-factor (that is the impact of nervous energy on the delivery of the presentation).  You’ll need to consider in your personal n-factor as well.

Finally, to ensure that you are within your time, calculate three “timing” points within your presentation.  For example in a ten minute talk you should know what material will need to have been covered at three minutes into the talk, at six  minutes into the talk and at 8 minutes.  This way you can adjust your pace as your are moving through the materials.  If you are behind at three minutes, you know that you’ll need to start cutting some of the “nice to know” material as you move forward, then at six minutes, you check again.  If you are now ahead, you can slow down and take more time to discuss or take questions from audience.   The idea is to use the “timing” points to guide you.

I use this technique for every talk and it works well.  Just before I deliver a talk I prepare a piece of paper with my “timings.”  I start with the total time available and then determine  when I want to finish (always 5-10 before our time is completely up so there is time for questions).  Then I add 10 minutes to the start (rarely do groups gets started right on time, most wait at least 5 minutes for people to arrive).

Then, on a small piece of paper, I calculate the exact time that I need to be at my three “timing points”.  Then when I am talking, I simply look at the clock then look down at the paper to compare the times.  One small warning though–  double check your timings.  Once I accidentally calculated the points incorrectly and didn’t realize it until I was about a 2/3 of the way through the presentation!  Once I realized I had done it wrong, I had to cut a lot of material from the end of the presentation.

Do you want to learn more? Consider signing up for an online workshop, one-on-one coaching, or asking your organization to sponsor a large group seminar or workshop.  Afterall, you didn’t learn to ride a bike by reading a book–you needed an expert coach (your Dad) to show you how to do it and then you needed lots of encouragement and practice, right?