Fitts's Law in the Wild: What YouTube Shorts Teaches About Effortless Interaction

Fitts’s law, from psychologist Paul Fitts’s 1954 work, predicts the time to acquire a target: it increases with the distance to the target and decreases with the target’s size — MT = a + b·log₂(2D/W). YouTube Shorts is the law applied with discipline. The dominant action, advancing to the next video, is a swipe of a few centimeters whose target is effectively the entire screen — distance near zero, size near infinite, so movement time approaches the floor. Secondary actions — like, comment, share — sit as large, generously-spaced buttons on the right edge, inside the resting thumb’s natural arc. Nothing core requires reaching the top of the phone. Full-bleed vertical video means zero precision is ever demanded. The compounding effect: each loop of the experience costs almost no physical effort, which is a large part of why one Short becomes forty. Your product’s repeated actions deserve the same math.

Fitts’s law is usually taught with abstract diagrams — point A, target B, log₂ somewhere in the middle. Then you open YouTube Shorts, watch twenty videos, realize your thumb never left its resting position, and suddenly the law isn’t abstract at all. Shorts is a masterclass in minimizing the cost of the next action. Let’s take it apart.

What does Fitts’s law actually say?

Paul Fitts’s 1954 model formalized something every designer feels intuitively: the time to hit a target depends on how far away it is (distance D) and how big it is (width W). Far and small is slow and error-prone; near and large is fast and forgiving. The equation — MT = a + b·log₂(2D/W) — matters less than its two design corollaries: shrink the distance to frequent actions, and grow the size of their targets.

Mobile adds a third variable Fitts never dealt with: the device is held, and the pointing instrument is a thumb with a limited arc. The “distance” that matters isn’t pixels — it’s whether the target sits inside the thumb’s comfortable sweep or requires a grip change. That context is where Shorts excels.

How does Shorts minimize distance?

The main action is a micro-swipe. Advancing to the next video — the action that defines the entire product — is a short vertical flick, repeated identically forever. Same direction, same starting position, every time. In Fitts’s terms, D approaches a constant minimum and the movement becomes ballistic: no aiming, no correction, no thought. Your thumb learns one gesture and performs it on autopilot.

The target is the whole screen. A swipe works anywhere on the video surface. W is effectively infinite, which makes the log term collapse — you cannot miss. Compare that to tapping a small “next” button: an aimed movement, orders of magnitude more demanding. Shorts eliminated aiming from its core loop entirely.

The thumb never leaves the zone. The like, comment, share, and subscribe controls stack vertically along the right edge — precisely where a right-handed thumb rests while holding a phone. Reaching them requires no grip change, no stretch, no second hand. Every secondary action lives within the same few centimeters as the primary one.

How does Shorts maximize effective target size?

Beyond the infinite swipe target, the tappable controls are physically large — well above the 44–48pt minimums in both platform guidelines — with generous spacing that prevents mis-taps even mid-motion. And the full-bleed vertical format means there are no small inline links, no tiny close buttons, no precision targets anywhere in the core experience. The interface demands exactly two motor skills: swipe anywhere, tap a large button. A user can perform both while walking, eating, or half-asleep — and regularly does.

Why does this matter so much for engagement?

Because interaction cost compounds. A session of Shorts is a loop of dozens or hundreds of micro-interactions. If each loop cost even two seconds of reaching and aiming, friction would accumulate into fatigue and sessions would end. At near-zero cost per loop, the only remaining cost is attention — and the content algorithm handles that. Fitts’s law doesn’t explain why the videos are compelling; it explains why nothing physical ever interrupts the compulsion. The interface disappears, and what remains is the feed.

This is the general principle: engagement loops are bounded by their cheapest repeated action. Every product with a core loop — swiping cards, scrolling a feed, processing a queue — lives or dies by the Fitts math of that one gesture.

What should you steal for your product?

Four transfers, no algorithm required:

1. Identify your repeated action and move it under the thumb. In your app, what’s the “next” gesture — advancing, approving, dismissing, saving? It belongs in the bottom-right arc (bottom-left for RTL-first Arabic products — mirror the zone, a detail most imported designs get wrong).

2. Replace aimed taps with gestures where repetition is high. Anywhere a user does the same thing fifty times a session, an aimed button is a design failure waiting to compound.

3. Audit everything at the top of the screen. Every core action placed top-left on a phone is a grip change you’re charging the user, dozens of times a day.

4. Size targets for motion, not for stillness. Users tap while walking and multitasking. Minimum sizes are for stationary thumbs; real targets need margin.

Fitts’s law is seventy years old, and YouTube Shorts — one of the most engineered consumer experiences on earth — still treats it as load-bearing physics. Your product should too.

Interaction physics like this is the invisible half of product design — and it’s measurable, testable, and fixable. That’s our product design practice. Let’s look at your core loop →