Interaction Cost: The Effort Users Pay

Nielsen Norman Group's interaction cost: the mental and physical effort users pay, how it relates to Tesler's law, and how to reduce it safely.

info Quick Definition
Interaction cost is the mental and physical effort a person has to make to get something done in a digital product. Every click, every decision, every piece of data to remember, and every step in a flow adds to that cost.

What is it, and what does it include?

Nielsen Norman Group defines interaction cost as the sum of the mental and physical effort users must deploy when interacting with a digital product to reach their goals. Its example is looking up where the word “ceremony” comes from in the Dictionary.com iPhone app: waiting for the splash screen, typing a few letters and picking the suggestion, waiting for the results, noticing the tabs, swiping between them, and scrolling to the origin section. A simple task involves many steps and substeps, and each one adds cost.

According to the article, physical effort includes clicking or tapping, typing, scrolling, and waiting for a page to load. Mental effort includes reading, comprehending, looking for information, switching attention between windows, and holding information in memory. How much each matters depends on the user and the device: a slow load is negligible on a fast desktop connection and painful on a phone with weak coverage.

The concept goes beyond clicks. A single-screen form can have a high cost if it asks you to remember a reference number from another part of the app. And a screen with two buttons can have a high cost if both options look equally plausible.

Why does this matter for a junior designer?

Because interaction cost is the most useful language for discussing a flow without arguing about taste. “This button is wrong” is an opinion. “This step adds a decision the user cannot make without looking up a detail” is an argument with an identifiable cost.

The concept also forces you to look at the whole flow. A junior who reviews each screen in isolation can miss a seven-step path in which every step seems reasonable.

What does Tesler’s law say?

Laws of UX covers Tesler’s law, also called the law of conservation of complexity: every system has a fixed amount of complexity that cannot be eliminated. Larry Tesler developed the idea at Xerox PARC in the mid-1980s. Every process has an irreducible core that either the system or the user must handle, and the advice is to deal with that complexity during design and development so users carry as little of it as possible. Tesler put it this way: an engineer should spend an extra week reducing an application’s complexity rather than have millions of users each lose a minute to it.

The same page includes a counterpoint from Bruce Tognazzini: people resist having complexity removed from their lives, so when an application is simplified, users start attempting more complex tasks. Reducing the cost of one task can expand what users dare to do; it is worth anticipating.

In practice, the law asks you to separate what can be removed (steps and data that are not needed) from what cannot (the inherent difficulty of the task) and to decide who carries the latter.

How does it relate to Fitts’s law and Hick’s law?

According to Laws of UX, Fitts’s law states that the time to reach a target depends on its distance and size. It is part of physical cost, and it explains why large buttons close to where attention already is are easier to use, especially on mobile. You can review it on the Fitts’s law page.

Hick’s law says that the more options a person faces, and the more complicated they are, the longer it takes to decide. It is part of mental cost: fifteen options take longer to choose from than five. Hick’s law has its own page.

Cognitive load, meanwhile, describes how much working memory a task occupies. It is a broader mental cost, and it connects with the article on cognitive load.

How do you reduce cost without losing control?

  1. Find the step with the highest cost. On each screen, ask what the user has to remember, decide, look up, or wait for.
  2. Remove accidental complexity. Asking for data the system already has, showing options nobody uses, or repeating unnecessary confirmations.
  3. Decide who pays for essential complexity. If a task is hard, does the system ease it with sensible defaults, or does it leave it to the user?
  4. Use defaults with judgment. A good default lowers cost when it is right. When it is wrong, the user has to correct it, which costs more.
  5. Review the whole flow, not only the screen. Count the steps and decisions from start to finish.

Nielsen Norman Group lists concrete measures for reading and tapping cost: bullet points and short sentences, large and easy-to-tap buttons and search fields, a search field where users can find it quickly, and descriptive headings in plain language without all caps.

The article on wizards and steppers shows how to split a long task into steps without the total cost growing.

When should you NOT reduce the cost?

  • When the action is irreversible or sensitive. An extra confirmation step before deleting data or paying is a cost that protects the user. Removing it for efficiency can be an expensive mistake.
  • When the user needs to review carefully. A contract approval flow should not be so fast that the user reads nothing.
  • When the reduction hides important information. Removing a step that showed shipping costs lowers interaction cost and raises distrust.

Example

Illustrative example: a restaurant booking app with three steps to reserve a table.

  • Before: the user picks a date, then a time, then the number of guests, and then has to type their name, phone, and email again, even though they gave them at sign-up. At the end, a message asks them to confirm the email address they already have.
  • Cost analysis: four decisions, two repeated pieces of data, and a confirmation screen with no value.
  • After: the app prefills name, phone, and email, suggests the time closest to the last booking, and keeps a confirmation only for changes to allergies or special needs.

The flow has the same result, but the cost drops visibly. And the allergy confirmation stays, because it protects the customer.

For the habits and motivation side of why someone returns to an app, the Fogg behavior model describes the relationship between motivation, ability, and prompt.

Common mistakes

  • Confusing interaction cost with the number of clicks.
  • Removing steps without checking what information is lost along the way.
  • Removing protective confirmations on irreversible actions.
  • Ignoring the mental cost of options while optimizing physical cost.
  • Asking for data the system already has.
  • Reviewing each screen separately instead of the whole flow.

Sources

Frequently asked questions

What is interaction cost?
It is the sum of the mental and physical effort users must deploy when interacting with a digital product to reach their goals. That is Nielsen Norman Group’s definition, which calls an interaction cost near zero the holy grail of usability.
Is interaction cost the same as the number of clicks?
No. Nielsen Norman Group emphasizes total effort, mental and physical, not just clicks. A screen with a single click can have a high cost if it requires remembering data or making hard decisions.
What does Tesler's law say?
That every system contains an amount of complexity that cannot be eliminated. Someone has to handle it: the system or the user. That is why it should be resolved in design and development, so the user carries as little of it as possible.
Should you always reduce interaction cost?
Not always. When a task is sensitive or irreversible, an extra confirmation step may be exactly what the user needs. Reducing cost without judgment can remove useful protections.
How does it relate to Fitts's law and Hick's law?
Fitts’s law describes the time to reach a target based on its distance and size, which is physical cost. Hick’s law describes decision time based on the number of options, which is mental cost.