One of the most frequently asked questions every time I give training on presentations is, "How many slides are appropriate for a presentation, if we are only given 30 minutes?" "Is there a formula to calculate the number of slides based on presentation time?"

To answer that question, I start from this popular quote:
If I had more time, I would have written a shorter letter.

If only I had more time, I would have written a shorter letter.
The shorter a letter is to convey a message completely, the better. The same applies to presentations. The shorter a presentation can be to achieve its objectives, the better. The shorter the presentation, usually the fewer the number of slides.

But you must remember, a presentation is not the same as a slideshow. Presentations are not the same as our PowerPoint (Keynote, Google Slide) files. A presentation is an event or occurrence, when someone delivers an idea that is prepared to be spoken in front of an audience. In presenting the idea, the presenter can (but is not required) to use visual aids. Today, these visual aids often take the form of slideshows created using computer softwares.

In other words, an essential part of a presentation is an idea spoken in public. Slides are simply visual aids to strengthen the delivery of certain parts of the idea. So, the length of a presentation depends on the length of the message delivered, not the number of slides shown.

The essence of a presentation is the idea prepared to be delivered to the audience. The word "prepared" shows that there is a goal that the presenter wants to achieve when they speak the idea directly (live) while face to face with the other party. Therefore, a presentation is the right way if the idea delivered is intended to get the audience's support for your ideas or suggestions. If you just want to share information (for example about the status of a project), perhaps you can simply send a short report in an email to people who have an interest in the project. You save your time and effort. But more importantly, you respect others by not wasting their time.

“Not wasting the audience's time” is one of the two main guidelines in determining how long a presentation should be. Too often I witness pitches that fail because the presenter rambles on before getting to the part that is actually most important to the potential client or investor.

Your presentation must compete for a scarce resource: the audience's attention. Remember, their heads may be bored with the previous presentation, or full of things that occupy their attention more. Usually the audience will give their attention for only five, even two minutes, at the beginning of the presentation. After that, their attention will move to other things that are more interesting or important.

Another main guideline is “get the audience to support my proposal or idea”. Remember, the goal of a presentation is to get the audience's support or approval. Have you ever watched a presentation and when it ended, you didn't catch the main point of the presenter at all? If you often attend seminars or symposiums, especially those where the presenters are officials or academics, perhaps quite often. If we can't capture the presenter's main thoughts, how can we support them?

These two main guidelines can be compared to two sides of the same coin.

Kesimpulannya, pertanyaan “Seberapa banyak jumlah salindia yang harus saya siapkan?” keliru. Ada pertanyaan yang lebih baik yang jauh lebih membantu Anda dalam menyusun isi presentasi. Pertanyaan itu adalah:

What should the audience hear or know, so they can support my ideas in the shortest time possible?

By answering these questions, you will use the audience's perspective in designing the content of your presentation. You will be more aware of what they believe or feel now, what facts they don't know, what they want, need, or worry about. If you are more aware of these things, then you can choose relevant facts, data, illustrations and examples so that they are persuasive to them.